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Igneous Rocks
Basic Terms, defintions and explanations
76
Geology
Undergraduate 1
10/28/2013

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Term
How do igneous rocks form?
Definition
From the crystallization of minerals
Term
What are Intrusive Igneous Rocks?
Definition

Igneous rocks that form inside Earth

(will eventually come to Earth's surface)

Term
What are extrusive Igneous Rocks?
Definition
Igneous rocks that form at Earths surface
Term
What is the rock cycle?
Definition
The interaction of plate tectonic and climate systems. (Rocks can skip phases in the cycle, there is no particular order.)
Term
What is Magma?
Definition
underground melted rock
Term
What is a magma chamber?
Definition
An intrusion of magma into pre-existing rock where it may accumulate into an irregularly shaped mass
Term
What is lava?
Definition
melted rock at Earth's surface
Term
What reasons to geologist study igneous rocks? (4)
Definition

-Info about Earth's mantle

-Radiometric dating (tool used to find the age of Earth)

-Ore deposits

-other economic uses (building stones, granite tables)

Term
Where do magmas form?
Definition
Upper part of asthonosphere and lower crust (100-150 ft deep)
Term
Why do magmas form? (3 terms)
Definition
temperature, pressure, volatiles
Term
One cause of magma formation is decompression melting. What is decompression melting?
Definition
melting of rock due to a decrease in pressure (forms magma inside Earth)
Term

Where does decompression melting occur?

Name examples of where this can occur.

Definition
Where hot mantle rock rises to shallower depths in the Earth. This occurs in matle plumes, beneath rifts and beneath mid ocean ridges.
Term
Magma is formed from additon of volatiles. How so?
Definition
When volatiles mix with hot, dry rock they help break chemical bonds so that the rock begins to melt. Adding volatiles decreases a rock's melting temperature. (known as flux melting)
Term
What are volatiles?
Definition
Substances such as water and carbon dioxide that evaporate easily and can exist in gaseous forms at the Earth's surface
Term
Where does flux melting occur?
Definition
where volatiles enter hot mantle; at subduction zones
Term
Magma is formed from the melting as a result of heat transfer from rising magma. Explain the process.
Definition

When magma from the mantle rises up into the crust it brings heat with it. This heat raises the temperature of the surrounding crustal rock and the rise in temperature may be enough for the crustal rock to begin melting. (known as heat transfer melting)

Example: Imagine injecting hot fudge into ice cream; the fudge transfers heat to the ice cream, raises it's temperature and causes it to melt

Term
What is heat transfer melting?
Definition
the transfer of heat from a hotter material to a cooler one
Term
How do magmas form? (3)
Definition
decompression melting, addition of volatiles, heat transfer
Term
What is magma made of?
Definition
Silicon, oxygen, aluminum, calcium, sodium, potassium, iron, and magnesium
Term
Because magma is a liquid, how are its molecules arranged?
Definition
grouped in clusters or short chains, relatively free to move with respect to one another
Term
What are dry magmas?
Definition
Magmas that do not conatin volatiles
Term
What are wet magmas?
Definition
Magmas that contain volatiles
Term
There are four types of distinguished magmas based on the proportion of silica relative to other metal oxides. What are the four types?
Definition
Mafic magma, ultramafic, Felsic and intermediate magma
Term
What proportion of chemicals to silica does mafic magma contain?
Definition
a relatively high proportion of iron exide and magnesium oxide relative to silica (low silica, 45-52% silica)
Term
What proportion of chemicals to silica does ultramafic rock contain?
Definition
An even higher proportion of magnesium and iron oxides relative to silica. (38-45% silica) (contains least amount of silica of the four types of magmas)
Term
What proportion of chemicals to silica does felsic magma contain?
Definition
a high proportion of silica relative to magnesium and iron oxides (66-76% silica) (highest amount of silica of the four magma types)
Term
what proportion of chemicals to silica does intermediate magma contain?
Definition
It is partway between mafic and felsic. (which is where its name comes from) (52-66% silica)
Term
What factors influence magma composition? (5)
Definition

Source rock composition

partial melting

contamination/assimilation

Magma mixing

fractional crystallization

 

Term
What is source rock composition?
Definition
The composition of a melt reflects the compostion of the solid from which it was derived. Not all magmas form from the same source rock, so not all magmas have the same composition.
Term
What is the process of partial melting?
Definition
Only 2%-30% of an original rock can melt to produce magma at a given location, the temperature at sites of magma production never get high enough to melt the entire original rock before the magma has a chance to migrate away from its source.
Term
What is partial melting?
Definition
the process by which only part of an original rock melts to produce magma
Term
Magmas formed by partial melting are more felsic than the original rock from which they were derived. give an example.
Definition

Partial melting of an ultramafic rock produces a mafic rock.

-mafic rock produces and intermediate rock

-intermediate rock produces a felsic rock

Term
Magma composition is influenced by Contamination/assimilation. How is this process defined?
Definition
Magma incorporates parts of the rock it passes through. .
Term
What is magma mixing? Explain an example.
Definition

Different magmas formed in different locations from different sources may enter a magma chamber

Example: mixing a felsic magma witha  mafic magma in equal proportions produces an intermediate magma.

Term
What is fractional crystallization?
Definition
Partial melting backwards; different minerals crystallize at different temperatures. (Bowen's Reaction Series)
Term
What is Bowen's Reaction Series?
Definition
The order of crystal formation from magma as cooling occurs.
Term
Why does magma rise? (2)
Definition
Buoyancy and Speed (Viscosity)
Term
Explain how magma rises through buoyancy.
Definition

Buoyancy drives magma upward because magma is less dense than the surrounding rock. Magma also rises because the weight of overlying rock creaes pressure at depth that literally squeezes magma upward.

Example: when you step into a puddle barefoot and mud sqeezes up between your toes

Term
What is viscosity? How does it affect magma/lava?
Definition
resistance to flow, it affects the speed with which magmas or lavas flow
Term
Magmas with low viscosity flow ______ than those with high viscosity.
Definition

easier

Example: Water flows more easily than molasses (syrup)

Term
What three factors does viscosity depend on?
Definition
temperature, volatile content, and silica content.
Term
Hotter magma is ____ _____ than cooler magma. Because thermal energy breaks bonds and allows atoms to move more easily
Definition
less viscous
Term
Magmas or Lavas containing more volatiles are ____ _____ than dry (volatile-free) magmas. Because the volatiles also tend to break apart silicate molecules and mayu also form gas bubbles.
Definition
less viscous
Term
Mafic magmas are ___ _____ than felsic magmas because silicon-oxygen tetrahedra tend to link together in magma to create long melecular chains that can't move past each other easily and there are more of these chains in a felsic magma than in a mafic magma.
Definition
less viscous
Term
What three factors control the cooling time of magma that freezes below the surface in the intrusive realm?
Definition

the depth of intrusion

the shape/size of a magma body

the presence of circulating groundwater

Term
Igneous rocks crystallize at what range of temperature?
Definition
650-1200 degrees Celcius
Term
What is a "dike?"
Definition
a tabular intrusion that cuts across pre-existing layering vertically(bedding)
Term
What is a "sill?"
Definition
a tabular intrusion that injects parallel to layering (horizontally)
Term
What is a pluton?
Definition
blob-shaped intrusions that range insize from tens of meters across to tens of kilometers across
Term
What are batholiths?
Definition
The intrusion of numerous plutons in a region that creates a vast composit body that my be several hundred kilometers long and over 100 km wide
Term
Where are batholiths in western North America? (4)
Definition
Coast Ranges batholith, Idaho, Sierra Nevada, Peninsular batholith
Term
What are xenoliths?
Definition
foreign rock (gaps of rock between magma)
Term
How can igneous rocks be classified? (2)
Definition
Texture and mineral/chemical composition
Term
What is aphanitic? Example
Definition
minerals to small to see. Ex. rhyolite
Term
What is phaneritic? Example
Definition
Minerals large enough to see with unaided eye (granite)
Term
What is glassy? example
Definition
no obvious minerals (obsidian)
Term
What is pyroclastic? example
Definition
fused, glassy volcanic rock fragments and ash from explosive volcanic eruption (Tuff)
Term
What is vesicular? example
Definition
Many holes or pits in rock surfaced caused by escaping gas (vesicular basalt)
Term
What is porphyrytic? example
Definition
two distinct mineral sizes (porphyritic basalt)
Term
How is texture of an igneous rock measured?
Definition
by using visual grain size depending on crystallization history
Term
Textures of igneous rocks are mainly controlled by the cooling rate. Slow cooling form ____ sized crystals and are _______ igneous rocks.
Definition

Large crystals

Intrusive

Term
What are the three types of textures of igenous rocks?
Definition
Crystalline, Fragmental and Glassy
Term
Fast cooling igenous rocks form _____ sized crystals and are ________ igneous rocks.
Definition

small crystals

extrusive

Term
Why must texture and composition of igneous rocks be studied?
Definition
Every intrusive rock has an extrusive rock that goes with it.
Term
Which two intrusive and extrusive Felsic Rocks go together?
Definition
Granite------Rhyolite
Term
Which two intrusive and extrusive Intermediate Rocks go together?
Definition
Diorite-------Andesite
Term
Which two intrusive and extrusive Mafic Rocks go together?
Definition
Gabbro------Basalt
Term
Which two intrusive and extrusive Ultramafic Rocks go together?
Definition
Peridotite-------Komatiite
Term
What is pumice?
Definition
a volcanic extrusive rock so light that paper can hold it up
Term
Obisidan and Volcanic Ash are more examples of extrusive igneous rocks
Definition
Term
All igneous rocks result from what?
Definition
Magma Differentiation
Term
Where does igneous activity occur? (4)
Definition
subduction zones, continental rifts, hot spots, mid-ocean ridges
Term
Where are intermediate and felsic rocks formed through subduction zones? (2) And what type of igneous rocks are formed in these areas? (3)
Definition

Andes, Sierra Nevada

Andesite, Granite, Diorite

Term
Where are Basalt and Rhyolite formed through hot spots?
Definition
Hawaii, Yellowstone, Iceland
Term
What two types of igneous rock is formed from mid-ocean ridges?
Definition
Basalt and Gabbro
Term

What are flood basalts?

Where has this occured?

Definition

thin basaltic lava that can flow very far

East African valley, Basin and Range Province, Clumbia River Plateau

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