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Geology 110 Volcanoes and Other Igneous Activity
Third chapter in the 1st Exam
Undergraduate 3

Additional Geology Flashcards




What factors affect the viscosity of lava?


Silca- Composition

Dissolved Gasses 

Water Vapor



Viscosity in lava is chiefly influenced by temperature, and the presence of gases. The higher the temperature of a given lava flow, the lower the viscosity. A particular lava may initially flow rapidly, but as temperature decreases, viscosity will increase, and alter the type of flow. 
The presence of dissolved gases called volatiles also plays a key role in lava flow, and type of eruption.
The highly viscous (Aa type) lava can flow only for a short distance. The resulting rocks are irregular blocky type.
The low viscous (Pahoehoe type) lava flows for considerable distance (Flood basalt, for example). 


Name and describe several different lava flow?

A'a lava flows have a rough surface made up of broken blocks of lava. These broken pieces of lava are called clinkers. It looks like little lava spines are sticking up from the lava as it moves. The lava is like paste because it is so thick and the clinkers travel on top of the lava flow. Below the top layer of the lava flow there is a core of slow viscous lava. The clinkers are burried by the front of the lava flow as the lava moves.

Pahoehoe lava is usually the first kind of lava to erupt from a volcano. The lava that flows from Pahoehoe flows is very viscous, that means that it is thin and liquid. It has a smooth surface that dries pretty quickly and becomes thicker and flows more slowly than the still hot lava below the surface. As the surface of the flow dries, it turn black and becomes crusty. The top crust is not safe to walk on because it is weak and can collapse.

Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like channel.

Block – The very last stage of a lava flow will create block lava. When the lava has cooled off to the point where it is just barely moving, it creates solid blocks of black basalt with hardly any air bubbles trapped inside. This lava is very dense and heavy because there is no air in it.

Cinders – When a volcano first erupts, there is a lot of built-up pressure, so it tends to explode violently sending tons of lava into the air. The lava cools and solidifies while still in the air, and comes to the ground as solid rock. Cinders tend to be fairly small pieces of volcanic rock with lots of holes in them from all the gas it held. It is a very lightweight stone because of all the air holes.

Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in diameter. They form the upper part of 'Layer 2' of normal oceanic crust.

Label, draw, and describe the three different types of volcanoes.


Shield volcanoes are large volcanoes that are built almost entirely of fluid lava flows. It has broad sloping sides and is usually surrounded by gently sloping hills in a circular or fan shaped pattern, that looks like a warrior's shield.

The volcano is produced by the action of the gas (steam or water vapor) with heat from the earth's core. This action melts rock turning it into magma. The pressure from the heat of the gas pushes the magma upwards till it explodes. Molten magma shoots upward from deep below the ocean floor and breaks through the drifting plates to form shield volcanoes. Lava flows gently and continuously out of the central volcanic vent or group of vents. This lava is very runny, and can't be piled up into steep mounds. It gradually accumulates and cools around the volcano. The eruptions are characterized by low explosivity lava-fountaining that forms cinder cones and spatter cones at the vent. The volcanoes are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava. The lava spread widely over great distances, then cools as thin gently dipping sheets. Lavas also erupt from vents along fractures (rift zones) that form on the flanks of the cone. Some of the largest volcanoes in the world are Shield volcanoes.

Shield volcanoes may be produced by hot spots which lay far away from the edges of tectonic plates. Shields also occur along the mid-oceanic ridge, where sea floor spreading is in progress and along subduction related volcanic arcs.

In northern California and Oregon, many shield volcanoes have diameters of three or four miles and heights of 1,500 to 2,000 feet. A good example of a shield volcano is the Island of Hawaii. The Big Island is formed of five coalesced volcanoes of successively younger ages. The Hawaiian Islands are composed of linear chains of volcanoes including Kilauea and Mauna Loa on the island of Hawaii.


Cinder cone volcanoes are the most common kind of volcanoes.They are steep sided cones of basaltic fragments and are smaller and simpler than composite volcanoes. Streaming gases carry liquid lava blobs into the atmosphere that fall back to earth around a single vent to form the cone. The volcano forms when ash, cinders and bombs pile up around the vent to form a circular or oval cone.

Cinders are melted volcanic rock that cooled and formed pebble-sized pieces when it was thrown out into the air. They are ejected from a single vent and accumulate around the vent when they fall back to earth.

Bombs are melted volcanic rock that cooled and formed large pieces of rock when it was thrown out into the air before landing on the ground.

Cinder cones are chiefly formed by Strombolian eruptions. They grow rapidly and soon reach their maximum size. Cinder cones can occur alone or in small to large groups or fields. Most have a bowl-shaped crater at the summit. The longer the eruption, the higher the cone. They rarely exceed 250 meters in height and 500 meters in diameter, although some may rise to as high as 650 meters or more. If gas pressure drops, the final stage cinder cone construction may be a lava flow that breaks through the base of the cone. If a lot of water in the environment has access to the molten magma, their interaction may result in a maar volcano rather than a cinder cone. The shape of a cinder cone can be modified during its life. When the position of the vent alters, aligned twin cones develop. Nested, buried or breached cones are formed when the power of the eruption varies. A great example of a cinder cone is Paricutin in Mexico. In Iceland, Surter I and Surter II cinder cone volcanoes created an island named Surtsey.


Composite volcanoes form when runny lava escapes through a fissure and flows a long way. Composite volcanoes are tall cone-shaped mountains that are typically steeply-sided, symmetrical cones of large dimensions. The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash etc. are added to its slopes.

Composite volcanoes erupt in different ways at different times. These volcanoes are built in layers by multiple eruptions, sometimes recurring over hundreds of thousands of years, sometimes over a few hundred. Andesite magma (the most common but not the only magma type), tends to form composite cones. During some eruptions, cinders, bombs and blocks form a mountain or add height to one that earlier volcanic eruptions had built. During other eruptions, lava flows cement these rocks together. Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lava either flow through breaks in the crater wall or from fissures on the flanks of the cone. Lava, solidified within the fissures, form dikes that act as ribs which help to strengthen the cone. They may rise as much as 8,000 feet above their bases. Depending on the type of volcanic material it is composed of, some can grow to such heights that their slopes become unstable and are susceptible to collapse from the pull of gravity.

When volcanic activity ceases, erosion begins to destroy the cone. After thousands of years, the cone is stripped away and the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) become exposed, and it too is slowly reduced by erosion. Finally, all that is left is the plug or "volcanic neck" and dike complex projecting above the land surface.

Some composite volcanoes occur in chains and are separated by several tens of kilometers. There are many composite volcano chains on earth, notably around the Pacific rim, known as the "Rim of Fire". (Mount Saint Helens & Mount Rainer)

Label, draw, and describe the three different igneous intrusions. 





Magma that solidifies within fractures or cracks in the earth's crust forms sheet-like or tabular intrusions. If the intrusions cut across pre-existing features they are termed dikes. A dike may form a wall-like feature on the surface as it is typically more resistant to erosion than surrounding rocks. Intrusions that are oriented parallel to pre-existing

features (e.g. beds) are termed sills.

Describe several pyroclastic type materials as covered in the text. 

The different types of pyroclastic material include volcanic ash, gases, rock fragments, rock lava, lapili, dust, bombs and cinders. Blocks, pumice and tephra from the volcano are also classified under pyroclastic materials.

What caused the year without summer?
It is believed that the anomaly was caused by a combination of a historic low in solar activity with a volcanic winter event, the latter caused by a succession of major volcanic eruptions capped by the 1815 eruption of Mount Tambora, in the Dutch East Indies (Indonesia), the largest known eruption in over 1,300 years, which occurred during the concluding decades of the Little Ice Age.
How do Xenoliths form?

A xenolith is a large piece of country rock that has been broken off and/or surrounded by an igneous intrusion. As the magma intrudes the country rock it may completely surround large pieces. These pieces are then called xenoliths.

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