1)Physical geology

-examines the materials composing Earth to gain an understanding of the many processes that operate both on and beneath its surface

2)Historical Geology

-aim is to understand the origin of Earth and its development through time

-strives to develop an orderly, chronologic arrangement of the mutitude of physical and biological changes that have occurred in geological past

-the bedrock exposed at the ground surface

-much information for geological observations comes from these

-famous Canadian born geologist

-proposed theory for origin of coal

-drew attention to Canada's mineral wealth

-mid 1600 view that earth was only a few thousand years old, and that Earth's landscapes had been shaped primarily by great catastrophes.

-this is wrong

-theory put forth by James Hutton in 18th CE

-begining of modern geology

-states that the physical, chemical and biologic laws that operate today have also operated in the geologic past

"the present is key to the past"

-acceptance of this meant earth was very old

-

relative dating - means that events are placed in their proper sequence or order without knowing their age in years

-done by applying principles such as law of superposition .... which states that in layers of sedimentary rock or lava flows, the youngest layer is on top and the oldest is on the bottom

-method used before radiometric dating

-principle of fossil succession - states that fossil organisms succeed one another in a definite and determinable order, and therefore any time span can be recognized by its fossil content

-fossils are essential to the development of geologic time scale

12-5 bya was the big bang

-debris formed stars and galaxies... our solar system... milkey way.. formed from one of these galaxies

nebular hypothesis 

-suggests that the bodies of our solar system evolved from an enormous rotating cloud (nebula)... composed mostly of hydrogen and helium

-for some reason, cloud contracted

-gravitational energy formed heat of the sun

-flat disk shape... other things gravitated together as well

-meteorites are matter than were not incorporated into planets

core-

dense iron rich area at center of the earth... created when temperatures were hot enough to melt iron and nickel

crust

-melting formed buoyant masses of molten rock that rose toward the surface, where they solidified to produce a primitive crust

-rich in oxygen and oxygen-seeking elements

mantle

-earth's largest layer, which is located b/w the core and crust... dominated by iron, magnesium, and oxygen seeking elements

-escaping gases by these formation processes created earth's early atmosphere

-the idea that the continents move abot the face of the planet

-proposed by Alfred Wegener

-became a paradigm after 50 years of data collection

plate techtonics- the paradigm that finally emerged, which provided geologists with the first comprehensive model of Earth's internal workings

-supercontinent that began to break up about 200million years ago

continental shelf-

-a gently sloping platform of continental material extending seaward from the shore

-this was used, rather than shorelines, to prove pangea

-overlap accounter for by stream sediment deposits

 -fossils such as Glossopteris and Mesosaurus contribute as evidence

-rock types and mountain ranges also fit together (pieces and picture of puzzle)

-Finally, Wegener found climatic evidence, in the remains of the effects of ancient ice shets, to prove the hypotheis

-in the paleozoic era, large tropical areas existed in teh Northern Hemisphere... and are now the major coalfields of eastern North America, Europe, and Siberia

lithosphere- earth's rigid outer shell (sphere of rock)

-according to geologists, this lithosphere is broken into numerous slabs called plates, which are in motion and continually changing shape and size

-seven major ones are recognized

-North American

-South American

-Pacific

-African

-Australian-Indian

-Antarctic

-many intermediate and small sized plates exist

 -lithospheric plates move at a rate of approx 5cm per year

-this movement is ultimately driven by the unequal distribution of heat within Earth, reulting in mantle convection

-first attempts to outline plate boundaries were made by using the locations of earthquakes

3 kinds of boundaries:

1)DIVERGENT BOUNDARIES

-plates move apart, resulting in upwelling of material from the mantle to create new seafloor

-sea floor spreading occurs mainly at mid-ocean ridges, bu can also ocur under rift valleys on continents

-called sea floor spreading

-this mechanisms has created the floor of the Atlantic Ocean over the past 160 million years

-as that part of the plate moves farther away from the mid-ocean ridge with seafloor spreading, it becomes older, cooler, and thicker

2)CONVERGENT BOUNDARIES

-plates move together, resulting in the descent (consumption) of oceanic lithosphere into the mantle, or two collide to make a major mountain system

-total surface area of the planet remains relatively constant (not growing)

-subduction - process by which the leading edge of an oceanic slab is bent downward as two plates slowly converge

-these occur at subduction zones

3)TRANSFORM FAULT BOUNDARIES

-these are plates that slide past each other without the production or destruction of lithosphere

-most located along oceanic ridges, though some slice through the continents, causing devastating earthquakes

crust

-comparatively thin, rocky outer skin... oceanic or continental crust

-oceanic is roughly 7km thick and composed of basalt

-continental crust averages 35-40 km thick but sometimes exceeds 70km in mountainous regions

-consists of many rock types

-upper crust has composition similar to granite, and lower continental crust more similar to basalt

 -rocks in oceanic crust are slightly denser than continental, and are much much younger (average 180my compared to 4+by)

mantle

-82+% of earth's volume

-is a oslid rocky shell that extends to a depth of 2900 km

-predominant rock is peridotite (density 3.3g/cm³)

core

-iron-nickel alloy with minor amounts of oxygen, silicon and sulphur

-at teh extreme pressure found in the core, this iron rich material has an average density of 11, and a maximum density of 14 g/cm³)

-gradual increase of temp, pressure, and density with depth

-center may exceed 6700C

-5 main layers, based on physical properties: lithosphere, asthenosphere, mesosphere, outer core, inner core

1)lithosphere

-consists of the crust and uppermost mantle... forms a cool, rigid, and brittle shell

-average thickness 100km, up to 250km

-thinner under oceans

2)asthenosphere

-upper mantle to depth of 660km

-lithosphere is detached and moves independently from asthenosphere

3)mesosphere/lower mantle

-below the zone of weakness in the uppermost asthenosphere, increased pressure counteracts the effects of higher temperature, and the rocks gradually strengthen with depth

-b/w 660-2900km

-very hot, solid rocks, but still capable of very gradual flow

4/5)inner/outer core

-iron-nickel alloy

-outercore is a liquid layer 2270 km thick

-convective flow of metallic iron within this zone that generates Earth's magnetic field

 ----

-the inner core is a sphere with a radius of 1216km

-higher temperature, though is a solid due to pressur

-a dynamic mass of water that is continually on the move, evaporating from the oceans to the atmosphere, precipitating on the land, and returning to the ocean again

-ocean 70% of surface

-97% of earth's water

--a life-giving gaseous envelope

-is thin and tenuous when compared to geosphere

-50% occurs below altitude of 5.6km

-provides air and protects us from the sun's intense heat and dangerous ultraviolet radiation

BIOSPHERE

-includes all life on earth

-concentrated near the surface in a zone that extends from the ocean floor upward for several km into the atmosphere

GEOSPHERE
-beneath atmosphere and oceans

-the boundary between the continents and the deep ocean basins lies along the continental slope, which is a steep dropof that extends from teh outer edge of the continental shelf to the floor of the deep ocean

-using this as the dividing line, we find that about 60% of eath's surface is represented by ocean basins and the remaining 40% by continents

-most prominent topographic features are linear mountain belts

-two prominent belts: 1)the curum-Pacific belt (region surounding pacific ocean) and 2)the Alps through tot he Himalayas

-older mountain ranges are also found on the continents (ie. appalachian and the urals)

shields- stable continental interiors, which are extensive and relatively flat expanses composed largely of crystalline material

-as varied topography as the continents

-mid oceanic ridge system.... similar to seam of a baseball.. more than 70000km around the globe (atlantic ridge and east pacific rise)

-is is layer upon layer of volcanic rock

trenches- extremely deep depressions that can exceed 11000 metres below sea level

-these are relatively narrow and represent only a small fraction of the ocean floor

-cycle that involves the process by which one type of rock changes to another

basic cycle

-magma is molten decomposed rocks

 -when magma cools and solidifies, the process is called crystallization, and formes igneous rock

.....which becomes....

-sediment-unconsolidated particles created by the weathering and erosion of rock, by chemical precipitation from solution in water, or from the secretions of organisms, and then trasported by water, wind or glaciers.... most ends up in ocean basin

-sediments undergo lithification and become sedimentary rock

-metamorphic rock -created when sedimentary or igneous rock is subject to high temperatures and great pressures

- additional pressure and heat will yield magma

Alternative Paths

-igneous rock may reform into metamorphic

-metamorphic and sedimentary rocks can experience weathering

1)original horizontality

2)Lateral Continuity

3)Law of Superposition

 4)Crosscutting relationships

5)Law of Inclusions

-sediments are usually deposited in horizontal layers

-used to find folded and tilted rock layers

-Layered rocks form when particles settle from water or air. The Law of Original Horizontality states that most sediments, when originally formed, were laid down horizontally. However, many layered rocks are no longer horizontal. Because of the Law of Original Horizontality, we know that sedimentary rocks that are not horizontal either were formed in special ways or, more often, were moved from their horizontal position by later events, such as tilting during episodes of mountain building.
 

-rock layers usually are deposited over a wide area

-breaks in continuity suggest either erosion or a fault has occurred

Law of Superposition

superposition:

-younger rock layers are deposited on top of older rock layers. Used to derive the relative ages of rocks

graded bedding- decreasing grain size upward through the bed indicating deposition from a waning current

-Using relative dating principles and the position of layers within rock, it is possible to reconstruct the sequence of geologic events that have occurred at a site.
 

Geologic maps are prepared by geologists to provide detailed information about the type and age of rocks exposed at the Earth's surface. They also contain information about the underlying structures formed when rocks respond to stresses below the surface - this process is called deformation.

Geologic maps can be useful in identifying geologic structures and locating natural resources such oil, gas and groundwater.
They also help identify potential areas prone to landslides.

Geologic Mapping shows the distribution of geologic materials and geologic structures that are visible at the Earth's surface,
and show information about geologic materials, geologic time, fossils, and the geologic history of a particular area

 

Describing and mapping the orientation or attitude of a rock layer or fault surface involves determining the features

Strike (trend)
The compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane
Generally expressed as an angle relative to north

Dip (inclination)
The angle of inclination of the surface of a rock unit or fault measured from a horizontal plane
Includes both an angle of inclination and a direction toward which the rock is inclined

Dip is the angle that the rock layer is tilted from a horizontal plane.
measured as an angle and a direction, and varies from 0o (horizontal) to 90o (vertical).
 

1)homogeneous

2)naturally occurring (vs synthetic minerals such as zirconia)

3)inorganic (does not contain organic chemicals)

4)solid (maintains shape)

5)Definite chemical composition (possible to write chemical formula)

6)ordered internal molecular structure (called crystal lattice)

-elements (basic building blocks .... 112 are known.. 92 naturally occuring)

-atoms

-Metallic bonding is the bonding within metals
-Weaker and less common than ionic or covalent bonds
-The metallic bond accounts for many physical characteristics of metals, such as strength, malleability, ductility, conduction of heat and electricity, and lustre

In metallic bonding instead of sharing electrons between two atoms the electrons in the outer shells are shared amongst all the atoms in a lattice with all the atoms positively charged
These atoms are attracted to the negatively charged 'cloud' of electrons.
 

-mass # is sum of neutrons and protons

-an isotope is an atom that exhibits variation in its mass number 

-some isotopes have unstable nuclei that emit particles and energy in a process known as radioactive decay

-two or more minerals with the same chemical composition, but different crystalline structures and properties

-diamond and graphite are good exampls

-the transformation of one polymorph to another is called a phase change

-external expression of the orderly internal arrangement of atoms

-crystal growth is often interrupted because of competition for space and rapid loss of heat

Lustre

-appearance of a mineral in reflected light

-two basic categories: metallic & non-metallic

-other terms, such as vitreous, silky, or earthy are used to further describe lustre

Colour

-generally an unreliable diagnostic property to use for mineral indentification

-often highly variable for a given mineral due to slight changes in chemistry

-exotic colourations of some minerals produce gemstones

Streak

-colour of a mineral in its powdered form

-helpful in distinguishing different forms of the same mineral

Hardness

-resistance of a mineral to abrasion or scratching

-all minerals are compared to a standard scale called the Mohs scale of hardness

Cleavage

-Tendency to break along planes of weak bonding
-Produces flat, shiny surfaces
-Described by resulting geometric shapes (Number of planes, and angles between adjacent planes)

Fracture

-absence of cleavage when a mineral is broken

Specific Gravity

-ratio of the weight of a mineral to the weight of an equal volume of water

-average value approximately 2.7

Other Properties

Magnetism
Reaction to hydrochloric acid
Malleability
Double refraction
Taste
Smell
Elasticity
 

-nearly 4000 minerals have been identified on earth

rock forming minerals

-common minerals that make up most of the rocks of earth's crust

-only a few dozen members

-composed mainly of the 8 elements that make up over 98% of the continental crust (O..46%, Si..28%, Al, Fe, Ca, Na, K, Mg,) (everything else: 1.7%)

-Quartz

-Feldspar (potassium, plagioclase)

-Olivine

-Pyroxene

-Amphibole

-Mica

-Calcite

-most important mineral group

-comprise most of the rock-forming minerals

-very abundant dur to large amounts of Si, and O2 in crust

-Basic building block is the Si-O tetrahedron

-four oxygen ions surounding a much smaller Si ion (SiO₄^4-)

Common Silicate Minerals

Dark Ferromagnesian Silicate Minerals

Olivine

-High temperature Fe-Mg silicate; individual tetrahedral linked together by iron and magnesium ions
-Forms small, rounded crystals with no cleavage
 

Pyroxene

-Single chain structures involving iron and magnesium; 2 directions of cleavage at 90 degrees
-Augite is the most common mineral in the pyroxene group ~ black, opaque
 

Amphibole Group

-double chain structures involving a variety of ions

-2 perfect cleavages: angles about 60/120

-hornblende is the most common mineral in the amphibole group

Light Non-Ferromagnesian Silicate Minerals

Mica Group

-sheet structures that result in one direction of perfect cleavage

-biotite is the common dark coloured mica mineral

-muscovite is the common light coloured.. pearly lustre

Clay Minerals

-clay is a general term used to describe a variety of complex minerals

-clay minerals all have a sheet or layered structure

-most originate as products of chemical weathering

Feldspar Group

-most abundant mineral group

-3D framework of tetrahedra; two directions of perfect cleavage at near 90-degree angles

-Orthoclase (potassium feldspar  ~ KAlSi₃O₈)

-Plagioclase (Na and Ca feldspar... replace them with K in KAlSi₃O₈)

Several Major groups exist including:

-oxides (magnetite, hematite [Fe2O3])

-ores of iron

-ores of other metals

Carbonates

-one of largest groups of minerals

-most common ... calcite 

-Calcite (CaCO₃)

-Dolomite Ca,Mg(CO₃)₂

-Potash (K₂CO₃)

-economic uses... road aggregate, building stone, cement, fertilizer

Sulphides

-the majority of ore minerals from which metals are formed

-pyrite (FeS₂)

-Galena (PbS₂)

Sulfates

-insoluble in water often associated with salts

-Gypsum (CaSO₄ + 2H₂O)

-Barite (Ba SO₄)

Native Elements

-Copper (Cu)

-Gold (Au)

-Sulfur (S)

Phosphates (PO₃)

-apatite

Halides

-Halite - table salt (NaCl)

-Chlorine (Cl)

-Fluorine (Fl)

Formation

1)Solidification from a melt

-freezing from a liquid (magma)

-as magma cools within the earth, minerals have time to grow to erfect crystals without interruption and available space

2)Precipitation from a Solution

-atoms, molecules, or ions dissolved in water bond together and then separate out of the water

-a spectacular example is a geode, a roughly spherical cavity in a rock in which crystals precipitate out of water solutions passing through thte rock

3)Solid State Diffusion

-develop during the formation of metamorphic rocks

-during the process atoms or ions already within the rock undergo rearrangement so that as new crystals gradually form, old ones gradually disappear

Destruction

-melting (heating)

-dissolving (contact with solvent)

-other chemical reaction

-crystals form when they undergo a process of solidification
-Under ideal conditions, a single crystal may form where all of the atoms in the solid fit into the same crystal structure
-many crystals form simultaneously during solidification, leading to a polycrystalline solid

-The crystal form is the external expression of a mineral that reflects the orderly internal arrangement of atoms
 

-cubic

-tetragonal

-orthorhombic

-triclinic

-hexagonal

-rhombohedral

-monoclinic

-Crystals in the cubic, or isometric, system have three mutually perpendicular axes of equal length.

Common forms in the cubic system:
-the tetrahedron (4 faces),
-the cube (6 faces),
-the octahedron (8 faces),
-the dodecadehedron (12 faces),
-the trapezohedron (24 faces),
-and the hexoctahedron (48 faces).
-cubic crystal forms include diamond, the garnets, pyrite, and spinel.

-may be a pure chemical element ~ diamond is pure carbon
-a relatively simple chemical compound (quartz is silicon dioxide, SiO2)
-a more complex mixture of various compounds and elements (e.g. garnet family includes a highly variable mix of iron, magnesium, aluminum, and calcium silicates).

-gemstones may be formed in single or multiple discrete crystals (such as diamond),
-in massive collections of microscopic crystals (cryptocrystalline ) (such as chalcedony), or in amorphous (non-crystalline) masses (such as opal).
-the great majority of familiar gem materials are oxides or silicates
 

-rocks formed from it can be intrusive(plutonic) or extrusive

-magma consists of three components:

-a liquid portion, called melt, that is composed of mobile ions

-solids, if any, are silicate minerals that have already crystallized from the melt

-volatiles, which are gases disolved in the melt, including water vapour, CO2, and SO2

-cooling of magma results in the systematic arrangement of ions into orderly patterns

-silicate minerals form in a predictable order

Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals

Factors affecting crystal size

Rate of cooling
Slow rate promotes the growth of fewer, but larger crystals
Amount of silica (SiO2) present
Amount of dissolved gases

TYPES of Texture

Aphanitic (fine-grained) texture
Rapid rate of cooling of lava or magma
Microscopic crystals ~ <1mm
May contain vesicles (holes from gas bubbles) and thus rocks that contain them have a vesicular texture
Basalt, rhyolite, andesite

Phaneritic (coarse-grained) texture
Slow cooling
Visible grains 1-10mm
Granite, diorite, gabbro
 

Porphyritic texture
Mixture of grain sizes caused by mixed cooling history; slow cooling first, followed by a period of somewhat faster cooling
Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass
 

 Pyroclastic texture
Various fragments of rock & ash ejected during a violent eruption and welded together
Tuff, volcanic breccia
Textures often appear more similar to sedimentary rocks
 

Pegmatitic texture
Exceptionally coarse-grained (pegmatites)
Very large crystals ~ >2cm
Form in late stages of crystallization of granitic magmas

Vesicular texture
contains tiny holes called vesicles which formed due to gas bubbles in the lava or magma
very porous, may resemble a sponge
commonly low density; may float on water
pumice~ white to gray; glassy/dull, vesicles
Scoria~ brown, black or dark red, vesicles

 

-are composed primarily of silicate minerals

Dark (ferromagnesian) silicates

-olivine

-pyroxene

-amphibole

-biotite mica

Light (nonferromagnesian) silicates

-quartz

-muscovite mica

-feldspars

Granitic

-composed of light-coloured silicates

-designated as being felsic (feldspar and silica) in composition

-contains high amounts of silica (SiO2)

-major constituents of continental crust

Basaltic 

-composed of dark silicates and Ca-rich feldspar

-designated as being mafic (magnesium and ferrum (iron)) in composition

-more dense than granitic rocks

-comprise the ocean floor as well as many volcanic islands

Andesitic (intermediate)

-contains at least 25% dark silicate minerals

-associated with explosive volcanic activity

Ultramafic

-rare composition that is high in magnesium and iron

-composed entirely of ferromagnesian silicates

***silica content in crustal rocks exhibits a considerable range

-a low of 45% in ultramafic

-over 70% in felsic rocks

-silica content influences a magma's behaviour

-granitic magma has high silica content, and is thus extremely viscuous... liquid exists at temperatures as low as 700C

Granite

-Phaneritic, intrusive
-Over 25 percent quartz, about 65 percent or more feldspar
-May exhibit a porphyritic texture
-Very abundant as it is often associated with mountain building
-The term granite covers a wide range of  mineral compositions

Rhyolite

--Extrusive equivalent of granite
-May contain glass fragments and vesicles
-Aphanitic texture
-Less common and less voluminous than granite
 

Obsidian
-Dark-coloured
-Glassy texture

Pumice
-Volcanic
-Glassy texture
-Frothy appearance with numerous voids

 

Andesite
-extrusive
-aphanitic texture
-gray to black volcanic rock with between about 52 and 63 weight percent silica (SiO2).
-often resembles rhyolite

Diorite
-Plutonic equivalent of andesite
-Coarse-grained
-Intrusive
-Composed mainly of intermediate feldspar and amphibole
 

Basalt
-Dark green to black in colour
-Aphanitic texture
-Composed mainly of pyroxene and calcium-rich plagioclase feldspar
-Most common extrusive igneous rock
-oceanic crust, Hawaiian islands

Gabbro
-Intrusive equivalent of basalt
-Phaneritic texture consisting of pyroxene and calcium-rich plagioclase
-Makes up a significant percentage of the oceanic crust
 

composed of fragments ejected during a volcanic eruption

-Varieties:

-Tuff: ash-sized fragments

-Volcanic breccia - particles larger than ash

Highly debated topic
Generating magma from solid rock
Produced from partial melting of rocks in the crust and upper mantle

Role of Temperature
-Temperature increases within Earth’s upper crust (called the geothermal gradient) average between 20oC to 30oC per kilometre

-Increase of temperature is the most typical mechanism for formation of magma within continental crust.
-Such temperature increases can occur because of the upward intrusion of magma from the mantle.
-Rocks in the lower crust and upper mantle are near their melting points
-Any additional heat (from rocks descending into the mantle or rising heat from the mantle) may induce melting
 

Role of Pressure
-an increase in confining pressure causes an increase in a rock’s melting temperature; conversely, reducing the pressure lowers the melting temperature
-decompression melting occurs because of a decrease in pressure.
-crystallization temperatures of most rocks increase with increasing pressure in the absence of water
-Decompression melting creates the ocean crust at mid-ocean ridges. Decompression melting caused by the rise of mantle plumes is responsible for creating ocean islands like those of the Hawaiian Islands
 

Role of volatiles
The change of rock composition most responsible for creation of magma is the addition of water
Water lowers the solidus temperature of rocks at a given pressure
Volatiles (primarily water) cause rocks to melt at lower temperatures
This is particularly important where oceanic lithosphere descends into the mantle
For example, at a depth of about 100 kilometers, peridotite begins to melt near 800°C in the presence of excess water, but near or above about 1500°C in the absence of water

Bowen's Reaction Series

-N.L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points

-difference in crystallization temperature for the different kinds of minerals plays a major role in the differentiation of rock composition as the magma cools 

-During crystallization, the composition of the liquid portion of the magma continually changes
-Composition changes due to removal of elements by earlier-forming minerals
-The silica component of the melt becomes enriched as crystallization proceeds
-Minerals in the melt can chemically react and change
 

Processes Responsible for changing a Magma's Composition:

Magmatic differentiation
Separation of a melt from earlier formed crystals to form a different composition of magma (fractional crystallization)
Assimilation
Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma
 Magma mixing
-Involves two bodies of magma intruding one another
-Two chemically distinct magmas may produce a composition quite different from either original magma

-Geologists classify magmas based on the relative proportion of silica to the sum of magnesium and iron oxides composing the magma
-Four types of magmas have been identified

1. Silicic magmas – rich in silica (70%), with relatively little magnesium & iron
-Rhyolite (aphanitic, extrusive)
-Granite (phaneritic, intrusive)

2. Intermediate magmas – 55% silica
-andesite
-diorite       

3. Mafic magmas – rich in magnesium & iron with less silica (50%)
basalt
gabbro

4. Ultramafic magmas – have even more magnesium & iron and less silica (40%)
komatite
peridotite

*at 1200C... all minerals are molten

*at 1000, Olivine, Pyroxene, Ca-rich melt

*at 800, amphibole, Ca/Na-plagioclase melt

*at 600 quartz, K-feldspar, Na-plagioclase, micas melt

formation of...

Mafic Magma (basaltic)

-most originate from partial melting of ultramafic rock in the mantle

-basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones

-As basaltic magmas migrate upward, confining pressure decreases which reduces the melting temperature
-Large outpourings of basaltic magma are common at Earth’s surface
-Basaltic  magmas are lower in silica and therefore less viscous than other magmas
-Basaltic magmas flow over long distances
 

Andesitic Magmas

-interactions b/w mantle-derived basaltic magmas and more silica-rich rocks in the crust generate magma of andesitic composition

-andesiti magma may also evolve by magmatic differentiation

Felsic Magmas (granitic)

-Most likely form as the end product of crystallization of andesitic magma
-Granitic magmas are higher in silica and therefore more viscous than other magmas
-Because of their viscosity, they lose their mobility before reaching the surface
-Tend to produce large plutonic structures
 

Movement..

-Hot magma is less dense than surrounding rock
-Volatiles in magma decrease the magma’s density
-Buoyancy drives magma upwards
-Weight of overlying rocks create a pressure at depth that literally squeezes magma upward
 

dike- nearly vertical magma intrusion

sill-horizontal intrusion (a laccolith is a sill that domes upward)

plutons-  irregular or blob shaped intrusions

-range in size from tens of meters across to tens of km

1)composition of magma

2)temperature of magma

3)dissolved gases in magma

-these 3 factors control viscosity, which in turn controls nature of eruption

Viscosity is a measure of a material’s resistance to flow (e.g., Higher viscosity materials flow with great difficulty)
Factors affecting viscosity
Temperature  -  Hotter magmas are less viscous (just as hot maple syrup is more fluid)

Composition  -  Silica (SiO2) content
        Higher silica content = higher viscosity
        (e.g., felsic lava such as rhyolite)
Lower silica content  = lower viscosity or more fluid-like behaviour (e.g., mafic lava such as basalt)

Dissolved Gases
Gas content affects magma mobility
Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure
The violence of an eruption is related to how easily gases escape from magma
 

    In Summary
Fluid basaltic lavas generally produce quiet eruptions, allowing the gases to migrate upward and escape with ease
Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions
 

Lava Flows
Basaltic lavas are much more fluid
Types of basaltic flows
Pahoehoe lava (a twisted or ropey texture)
Pillow lava forms when lava enters the ocean
Aa lava (rough, jagged blocky texture)
Gases
One to six percent of a magma by weight
Mainly water vapour and carbon dioxide, but also nitrogen and sulphur dioxide

Pyroclastic Materials – “fire fragments”
 Types of pyroclastic debris
-Ash and dust  -  fine, glassy fragments
-Lapilli  -  walnut-sized material
-Particles larger than lapilli
-Blocks  -  hardened or cooled lava
-Bombs  -  ejected as hot lava
-Pumice  -  vesicular ejecta from silica-rich magma
-Scoria -  vesicular ejecta from mafic magma

Shield volcano
-Broad, slightly domed-shaped
-Gently sloping flanks (2 – 10 degrees)
-Composed primarily of basaltic (low silica) lava
~Pahoehoe
~aa
-Generally cover large areas; low viscosity
-Produced by mild eruptions of large volumes of lava
-Mauna Loa on Hawaii is a good example

Cinder Cones (also called scoria cones)
-Built from ejected lava (mainly cinder-sized) fragments, mainly pyroclastics that build up around the vent
-Steep slope angle (30 – 40 degrees)
-Rather small size
-Short duration of activity
-Frequently occur in groups
-Paricutin in Mexico is a dramatic example
 

Composite cone (Stratovolcano)
-Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)~ Ring of Fire
-Large, classic-shaped volcano (hundreds of metres high & several kilometres wide at base)
-Composed of interbedded lava flows and layers of pyroclastic debris
-Eruptive behavior: explosive
-volcano has alternating pyroclastic layers and lava flows

-slopes intermediate in steepness

-intermittent eruptions over long time span

-mostly andesite

Nueé Ardente: A Deadly Pyroclastic Flow
Most violent type of activity (e.g., Mount Vesuvius and Mount St. Helens)
A nueé ardente:
Fiery pyroclastic flow made of hot gases infused with ash and other debris
Move down the slopes of a volcano at speeds up to 200 km per hour
May produce a lahar, which is a volcanic mudflow
 

Calderas
-Steep-walled collapse depressions at the summit (Crater Lake, Oregon)
-Size exceeds 1 km in diameter
<1km called collapse pit
-Hawaiian-Type Calderas
Collapse on top of a shield volcano; subterranean drainage from central magma chamber
-Yellowstone-Type Calderas
Collapse of large area independent of any existing volcanic structures

Fissure Eruptions and Lava Plateaus
Fluid basaltic lava extruded from crustal fractures called fissures
-Greatest volume of volcanic material extruded through fissures
-e.g., Columbia River Plateau; such flood basalts also occur in south-central BC ~ fluid mafic magma
-Deccan Traps, India  formed 66 mya
-Iceland, Laki eruptions of 1783 formed a rift 25km long that generated >20 other vents forming composite cones outpouring a total volume of lava in excess of 12 cubic km
 

Lava Domes
-Silica-rich lavas; highly viscous
-Bulbous mass of congealed lava
-Most are associated with late stages of an explosive eruption of gas-rich magma
Chiefly andesitic composite cones
Volcanic dome now growing in the vent of the 1980 eruption of Mount St. Helens
 

Volcanic pipes and necks
-Pipes are short conduits that connect a magma chamber to the surface
May extend to depths of 200km; ultramafic magmas (samples of the mantle) – primary magma
Volcanic pipes are diamond bearing structures of SA and NW Territories
-Magma solidified within the volcanic pipe, more resistant that surrounding pyroclastics ~ leaving a volcanic neck
 

Nature of Intrusions

Shape
-tabular (sheetlike) e.g. sills, dikes
-Laccolith similar to dike except with a dome shaped bulge
-massive e.g. batholiths

Orientation with respect to the host (surrounding) rock
Discordant – cuts across sedimentary rock units, or other existing structures (dike)
Concordant – parallel to sedimentary rock units (sill)
Columnar joints – as rocks cool develop shrinkage fractures ~ produce elongate pillar-like columns
 

Batholith

-largest intrusive body

-surface exposure of 100+ square km

-frequently form the cores of mountains

Global distribution of igneous activity is not random

1. Most volcanoes are located within or near ocean basins – “Ring of Fire”
-Assoc. with convergent plate margins
-Composite volcanoes that emit volatile-rich magma of andesitic composition

2. Emit very fluid mafic magmas at divergent plate margins
-Confined to deep ocean basins
-Active submarine volcanoes dot the ocean floor

3. Intraplate volcanoes (within the plate)
-Hawaiian-type volcanoes ~ ascending plumes of hot mantle rock
-on oceanic plates erupt lava of mafic composition
-on continents range from mafic to felsic in composition
 

Mechanical Weathering Includes:

-unloading

-frost action

-root wedging

-temperature change

Chemical Weathering

Carbonation
Calcium Carbonate + Water + Carbon Dioxide ---> Calcium Carbonate (soluble)

Hydrolysis
during hydrolysis, the feldspar in granite changes to clay mineral which crumbles easily, weakening the rock and causing it to break down

Oxidation
Iron + Oxygen -->  Iron Oxide (crumbles)

Solution
process by which minerals in the rocks dissolve directly in water

Hydration
Unhydrated calcium sulphate + Water -->   Hydrated calcium sulphate (expands)

-Sediment is deposited in flat, horizontal layers with the oldest layers on the bottom and the younger layers laying on and over the older layers
-Geologists use this knowledge to read layers of sedimentary rock like the pages in a book
-They can date layers by the fossils that are found in them
If a layer has a fossil in it that is known to be 50 million years old the layer itself must be at least 50 million years old and the layers below it have to be older than 50 million years.

-Sedimentary rocks are products of mechanical and chemical weathering
-They account for about 5 percent (by volume) of Earth’s outer 16 kilometres
-Contain evidence of past environments
-Provide information about sediment transport
-Often contain fossils
 

Sedimentary rocks are important for economic considerations because they may contain
-Coal
-Petroleum and natural gas
-Sources of iron, aluminum, and manganese and fertilizer
 

-rock types are based on the source of the material

-detrital sed. rocks- transported sediment as solid particles

-chemical sed. rocks-  sediment that was once in solution

The chief constituents of detrital sedimentary rocks include:
-Clay minerals
-Quartz
-Feldspars
-Micas
Particle size is used to distinguish among the various types of detrital rocks

Common detrital sedimentary rocks (in order of increasing particle size)

Shale and other Mudrocks
-Mud-sized particles in thin layers that are commonly referred to as laminae
-Deposited as a result of gradual settling from quiet, nonturbulent environments
-Shale exhibits fissility (splits into thin layers) and mudrock does not
-Siltstone consists of silt-sized particles as well as mud
-Most common sedimentary rock, but often

inconspicuous because they weather so easily

 Sandstone
-20% of sedimentary rocks are sandstone; second only to mudrocks in abundance
-Composed of sand-sized particles
-Forms in a variety of environments and transported by wind and water
-Sorting, shape, and composition of the grains can be used to interpret the rock’s history
-Quartz is the predominant mineral because it is so durable

Conglomerate and breccia
-Both are largely composed of particles greater than 2mm in diameter
-Indicative of very turbulent currents or steep slopes
-Conglomerate consists primarily of rounded gravels
-Breccia is composed largely of large angular particles
 

Consist of Precipitated material that was once in a solution

-Precipitation of material occurs in 2 ways:

inorganic and organic processes (biochemical origin)

Limestone
-Most abundant chemical rock; 10% of all sedimentary rocks
-Composed chiefly of the mineral calcite (CaCO3)
-Marine biochemical limestones form as coral reefs, coquina (broken shells; also called bioclastic grainstone), and chalk (microscopic organisms)
-Inorganic limestones include travertine and oolitic limestone (comprises small spherical grains or ooids formed in high wave-energy environments)

Dolostone
-Typically formed secondarily from limestone when magnesium replaces some calcium

Chert
-Made of microcrystalline silica (SiO2)
-Occurs as nodules in limestone and as tabular layers (siliceous organisms like diatoms and radiolarians often provide silica related to chert origin)
-Varieties include flint and jasper (banded form is called agate)
 

Evaporites
-Evaporation triggers deposition of chemical precipitates
-Examples include rock salt (NaCl) and rock gypsum (CaSO4.2H2O), sylvite (KCl; potash, which is used as a fertilizer)
 

Coal
-Different from other rocks because it is composed of organic material
-Stages in coal formation (in order)
1. Accumulation of plant material (often in swamps)
2. Partial decomposition into peat
3. Shallow burial forming lignite
4. Deeper burial forming bituminous coal
 

clastic

-discrete fragments and particles of rocks and mineral grains; all detrital rocks have a clastic texture

non-clastic

-the precipitation of minerals from ocean water or from the breakdown of the shells and bones of sea creatures; pattern of interlocking crystals; may resemble an igneous rock

SIGNIFICANCE

1. The only family of rock containing an abundant record of life forms and the changes of life forms throughout geologic time

2. The only family of rock in which natural gas and petroleum form and from which these are extracted in abundance

3. Used for constructing buildings, tomb stones, and some limestones are used as drive way coverings and an important source of lime

-these are areas where sediment is accumulating

-these areas are unique and determine nature of sediment

Continental
Dominated by erosion and deposition associated with streams & rivers
Glacial
Wind (eolian)

Marine
Shallow (to about 200 metres); includes land-derived sediment and where influx is small, carbonate skeletal debris and coral reefs accumulate
Deep (seaward of continental shelves; deeper than 200 m); includes biogenic ooze
 

...another in notes?

-Different sediments often accumulate adjacent to one another at the same time
-Each unit (called a facies) possesses a distinctive set of characteristics reflecting the conditions in a particular environment
-The merging of adjacent facies tends to be a gradual transition

A facies is a body of rock with specified characteristics ...
-distinctive rock that forms under certain conditions of sedimentation
-reflects a particular process or environment
-distinguished by what aspect of the rock or sediment is being studied

-based on  characteristics such as grain size and mineralogy ~ lithofacies
-based on fossil content ~ biofacies
sedimentary facies reflect depositional environment, each facies being a distinct kind of sediment for that area or environment. 
 

Types of sedimentary structures
1.Strata, or beds (most characteristic of sedimentary rocks)
2.Stratification
-occurrence of sedimentary rocks in layers (strata or beds) and can be considered the most common feature related to all sedimentary rocks

Bedding planes that separate strata (originally horizontal) e.g. sandstone, shale sequence

Cross-bedding (inclined layers)
relatively thin layers inclined at an angle to the main bedding
formed by currents in water or wind

Graded beds - (rapid deposition from water containing many particle sizes; coarse material settles first)- turbidity current
 

Mud cracks – mud shrinkage upon exposure to air
 

Ripple marks – small waves of sand that are asymmetric (current ripples) or symmetric (oscillation ripples)

small waves of sand that develop by the action of moving water or air

 Hummocky cross stratification (HCS):
Storms give rise to unique sediment deposition in the seas
A common type of sedimentary structure found in sandy storm deposits is HCS
Formed by large storm waves in shallow marine environment
Reflects wave and current action generated by storms

-By definition, fossils are the traces or remains of prehistoric life now preserved in rock
-Fossils are generally found in sediment or sedimentary rock (rarely in metamorphic and never in igneous rock)

-Fossils form when organisms die and become buried by sediment or
-When they travel through or over sediment and leave their mark
-Some fossils are found in volcanic ash deposits – bury an organism; preserve a footprint
-Fossils can survive very low grade metamorphism
 

Two Categories:

- Body fossils – preserve evidence of the tissue and hard parts of an organism
-Trace fossils – preserve evidence of an organism’s activity (tracks and burrows)

1.Frozen or dried bodies:
-Most fairly young ~ 1000s of years old
-Wooly mammoths that became incorporated in the permafrost of Siberia
-In desert climates ~ organisms become desiccated e.g. Egyptian mummies

2. Fossils preserved in amber or tar
-Fossils landing on the bark of trees become trapped in the sticky sap or resin the trees produce – golden syrup envelops the insects and over time hardens into amber; amber can preserve insects, feathers for >40m yrs.
-Tar similarly serves as a preservative e.g. the La Brea Tar Pits in Los Angeles ~ large numbers of animals become mired in the tar and sunk into it. Their bones have been preserved  

3. Preserved or replaced bones, teeth, and shells
-Bones & shells consist of durable minerals which may survive in rock. Minerals may recrystallize or replaced by new minerals that precipitate from ground water ~ shape of bone or shell remains in the rock

4.Petrified (permineralized) organism
-‘turn to stone’ ~ plant material transformed into rock
-Cellulose within plant’s cell walls survive even after burial
-Groundwater carries dissolved silica ~ microcrystalline quartz precipitates inside the cell walls and the plant transforms into a solid mass of chert
-Fine details of the plant structure are still preserved
-Petrified wood

5. Molds and casts
-As sediment compacts a shell it conforms to the shape of the shell. If the shell later disappears a cavity called a mould remains
-The sediment that had filled the shell also preserves the shell’s shape – cast

6.Carbonized Impressions
-Flattened molds created when soft or semi soft organisms – leaves, insects, shell-less invertebrates, sponges, feathers, jellyfish – get pressed between layers of sediments
-Chemical reactions remove the organic chemicals leaving only a thin film of carbon on the surface of the impression
 

Soft-tissue rarely preserves, but occasionally rapid burial and low oxygen allow this to happen
-Middle Cambrian Burgess Shale from Field, B.C. preserves soft tissue by carbonization; these fossils are very important in providing a picture of a community shortly after the explosion of life earlier in the Cambrian

-Soft tissue of insects is often preserved in amber

Soft tissue may be preserved as imprints
-Ediacaran fossils from Newfoundland are preserved in such a way; these fossils are very important as they represent the earliest large multicellular organisms

....record the activity of organisms

-as footprints

-feeding traces

-as burrows and borings

-as fossil excrement (coprolite) and gizzard stones (gastrolith)

Fossil Preservation

1.Death in an anoxic (oxygen-free) environment
- oxidation reactions do not take place
-bacterial metabolism takes place slowly
- scavenging animals not abundant
~such environments: bottom of stagnant lakes, deep ocean, in tar or amber

2. Rapid Burial
E.g. sudden storm buries an oyster bed with a thick layer of silt ~ become part of the sedimentary rock
River suddenly floods and buries a dinosaur carcass in silt ~ the carcass becomes fossilized
 

3.The presence of hard parts
-Organisms without hard parts commonly won’t be fossilized
-Paleontologists know much more about the fossil record of bivalves that they do of the jellyfish or spiders
 

4. Lack of diagenesis or metamorphism
-Lack of these two may destroy fossils that had been present
-Note high-grade metamorphism will destroy fossils
 

-Abundance of fossils that show what shell-less invertebrates who inhabited the deep-sea floor about 510mya looked like

-At the time of deposition, this area was near
 the equator, and was the continental margin
of North America. A 100m high near-vertical
cliff of limestone occurred at the edge
of the shelf.

The Burgess Shale was deposited at the base
of this cliff, probably in anoxic conditions,
as indicated by the lack of bioturbation
(burrows, trackways, etc.) and the
abundance of pyrite (often indicating the
presence of H2S). All the organisms within
the Burgess Shale have been transported to
this location, probably by small mudflows that
flowed over the edge of the cliff. This accounts
for the variable orientation of the fossils and
 their superb preservation  

The Burgess Shale fossils merit special interest for several reasons:
Their age - from the Cambrian period, 505 million years ago, shortly after an astonishing burst of biodiversity occurred in the ancient oceans.
Their exquisite preservation - amazingly fine details of the structure of the animals are seen in the fossils, which tell paleontologists much more about what the ancient animals looked like, and how they lived.
The fossils reveal important clues to the nature of evolution - all of the major types of animals (phyla) known today are represented in the Burgess Shale, plus others that cannot be placed in our modern classification system.
 

TEMPERATURE
-the most important agent

-chemical reactions and recrystallization result in new minerals that are stable for the new conditions

TWO sources of heat:

1)intrusive igneous body baking surrounding rocks

2)earth's internal heat when brought to great depths

PRESSURE

-increases with depth

confining pressure- (uniform stress) applies forces equally in all directions; harder and denser metamorphic rocks

-rocks may also be subjected to directed pressure (differential stress), which is unequal in dif directions

DIFFERENTIAL STRESS

-three basic types.... compressional, tensional, and shear stress

-rocks are brittle at the surface... and fracture when subjected to differential stress

-at depth, where temperatures are higher, rocks are ductile and minerals can flatten and elongate forming spectacular folds

Foliation... typical metamorphic texture caused by differential stress

-is any planar arrangement of mineral grains or structural features within a rock

-thre factors influnce foiation development

1.rotation of platy and/or elongate mineral grains into a new orientation

2.changing the shape of equidimensional grains into elongate shapes aligned in preferred orientation

3.Recrystallization of minerals to form new grains growing in direction of preferred orientation

CHEMICALLY ACTIVE FLUIDS

-mainly water with other volatile components; become more reactive with higher temperature

-enhance migration of ions; acts as catalyst to promote textural changes

-aid in recrystallization of existing minerals

-play role in changing overall composition of rock

sources of chemically active fluids:

-pore spaces of sedimentary rocks

-fractures in igneous rocks

-hydrated minerals such as clays and micas become dehydrated

-most metamorphic rocks have the same overall chemical composition as the parent rock from which they formed

-mineralogy and the textures of metamorphic rocks are related to the cariations in the degree of metamorphism (low to high grade)

INDEX MINERALS

-indicative of degree of metamorphism

-stable under certain pressure-temperature comditions

-good indicators of the metamorphic conditions under which they form 

low grade - chlorite (around 200C)

high grade - sillimanite (around 600C)

These minerals appear in a set order : chlorite, biotite, garnet, staurolite, kyanite, and sillimanite

-minerals will be foliated or nonfoliated depending on pressure conditions

Burial Metamorphism

-associated with very thick sedimentary strata:
~step beyond diagenesis – deep sedimentary basins, ocean trenches & passive margins
-Geothermal heat causes recrystallization of minerals ... low grade metamorphic minerals – zeolites; rocks become denser, but not distorted
~increased confining pressure but minimal directed pressure
~very mild metamorphism; bedding & grain size variations preserved
-depth varies from one location to another depending on the prevailing geothermal gradient
-low-grade metamorphism begins at about 8 km & Temp.  200C
 

Contact Metamorphism
-rise in temperature due to magma intrusion
-zone of alteration forms called aureole
-confining pressure low; does not involve directed pressure
-minerals within the aureole ~ randomly oriented ~ nonfoliated
-Temperature increase transform:
Sedimentary rocks - mudrocks, shale, and mafic igneous rocks such as basalt ~ metamorphic rock called hornfels (dark-colored, homogenous)

Clay + heat = hornfels (dark-coloured,  nonfoliated rock with platy micas)
Basalt +heat = hornfels (dark-coloured, nonfoliated rock with elongate crystals of amphibole)
Sometimes large metamorphic minerals (porphyroblasts) impart a spotted appearance ~ porphyroblastic texture
Quartzite and Marble often (not always) form by contact metamorphism from sandstone and limestone respectively

Regional Metamorphism

 (dif slide)

Subduction Zone Metamorphism

-Technically a form of regional metamorphism
-Special conditions of high pressure, but low temperature occur
-Na-rich blue-coloured amphibole called glaucophane forms
-Exhibits foliation ~ called blueschist
-Continued subduction drags blueschist into the mantle ~ eclogite (high P & T) – predominant minerals garnet & pyroxene
 

-Produces the greatest quantity of metamorphic rock
-Increased temperatures and pressures
-Associated with mountain building at convergent plate boundaries; most intense metamorphic activity
-Substantial differential stress
-Deep burial; increased confining pressure
-Consist of folded and faulted metamorphic rocks intruded with igneous rocks
-rocks that best display regional metamorphism ~ lots of platy &/or elongate mineral grains ~ attain a preferred orientation
-foliated rocks – characteristic of regional metamorphism 

REGIONAL METAMORPHISM OF SHALE

Slate - low grade metamorphism
-slaty cleavage; black, red, green slate
Phyllite – low grade; slightly higher T & P
-glossy sheen; muscovite &/or chlorite crystals
Schist – (texture-strongly foliated) medium grade
-platy mins. (micas); accessory mins. garnet, staurolite, sillimanite
Gneiss – high grade
-Banded appearance: feldspar-rich layers with bands of dark ferromagnesian minerals

REGIONAL METAMORPHISM OF BASALT

Foliation is less pronounced compared with mudrock-derived metamorphics
Greenschist – low grade
-Ferromagnesian minerals (olivine and pyroxene) are hydrated to form chlorite
-Phyllite-like foliation; green colour
Amphibolite – medium grade
-Chlorite & other minerals lose water and converted to amphibole; foliation produced by preferred orientation of needle like crystals
 Granulite – high grade
-Amphiboles are further dehydrated to produce pyroxene and garnets
-Little preferred orientation to minerals due to their shape (blocky)
-Very coarse-grained granular texture

metaconglomerate

-the pebles and cobles become flattened and stretched out

-recrystallization of the pebbles and cobbles, and surrounding grains occurs

-metaconglomerates are high grade metamorphic rocks

THE UPPER LIMIT OF REGIONAL METAMORPHISM

Migmatite

-rocks begin to partially melt at transition zone b/w metamorphis and igneous conditions

-light-coloured silicates like quarts and K-feldspar melt at lower temperature

-igneous bands are folded intensely

-dark layers maintain metamorphic origin

***depending on composition, regional metamorphic rocks may be nonfoliated

-includes quartzile and marble which lack platy and elongate minerals

-may be difficult to discern from similar rocks produced by contact metamorphism

Hydrothermal Metamorphism

-high temp/mod pressure rocks altered by hydrothermal fluids

Cataclastic metamorphism (forms mylonite)

-mechanical deformation; shear stress- mylonites

Shock/Impact Metamorphism 

-meteors or very large volcanic explosion

-ultra high pressures  produce minerals only stable at high pressures... coesite and stishouite  polymorphs of quartz