Ch. 9

How is magma generated along convergent plate boundaries?

As old oceanic crust is subducted, it descends into the lower crust and mantle. This process destroys the old crust, which melts and generates magma beneath the surface.

Distinguish among conduit, vent, and crater.

A conduit is the tube through which magma flows from the magma chamber to the vent. The vent is where magma is emitted from the volcano. A crater is a bowl-like depression at the top of the volcano.

How is a crater different from a caldera?

A crater is a small depression at the top of a volcano. A caldera is a crater that has a diameter greater than 1 kilometer (0.6 miles).

Describe the materials that compose composite volcanoes.

Andesitic magma flows alternating with layers of ash and pyroclastic material.

Describe the process of decompression melting.

As hot mantle rock rises closer to Earth's surface, it enters a zone where the pressure is reduced. The drop in confining pressure allows the rock to melt without an external heat source because its melting temperature is now changed under different pressure conditions.

Volcanism at divergent plate boundaries is most often associated with which rock type? What causes rocks to melt in these settings?

Basalt. As the rock rises to the surface, the decrease in confining pressure generates decompression melting.

Describe the composition of cinder cones.

Basaltic; generally loose scoria.

Compare and contrast batholiths, stocks, and laccoliths in terms of size and shape.

Batholiths are the largest and can be as much as 100 km wide. They tend to be wide and bulbous. Stocks are smaller plutonic structures similar in shape to batholiths. Laccoliths are the smallest of the three and tend to be mushroom shaped.

Pyroclastic flows are associated with what volcanic structure that is not a cinder cone?

Calderas

What is meant by the term country rock?

Country rock is the existing rock that is invaded by magma.

Describe the formation of Crater Lake. Compare it to the calderas found on shield volcanoes such as Kilauea.

Crater Lake formed when a composite cone volcano erupted and the force of the explosion collapsed the crater into a caldera. Rainfall filled the depression, generating a lake. Calderas on shield volcanoes tend to form more gradually due to magma loss from a shallow magma chamber.

At which of the three types of plate boundaries is the greatest quantity of magma generated?

Divergent

What is emitted from a fumarole?

Gases

Where are the best-known shield volcanoes in the United States? Name some examples in other parts of the world.

Hawaii. Canary Islands, Galapagos, and Easter Island.

How do lava tubes form?

Lava tubes can form in pahoehoe flows when the inner part of the flow remains at a high temperature, allowing lava to flow through, while the outer portion cools and hardens.

Over what time span does a typical cinder cone form?

Less than one year for most, less than a month for half.

What is the source of magma for most intraplate volcanism?

Mantle plumes, or hot spots.

Explain how the viscosity of magma influences the explosiveness of a volcano.

More viscous magmas tend to generate more explosive eruptions.

Are pyroclastic materials a significant component of shield volcanoes?

No

Where do most shield volcanoes form - on the ocean floor or on the continents?

Ocean floor

What is scoria? How does scoria differ from pumice?

Scoria is a basaltic pyroclastic rock that is dark and riddled with vesicles from magma gas. It is darker and denser than pumice and does not float as pumice might.

What zone on Earth has the greatest concentration of composite volcanoes?

Ring of Fire that circles the Pacific Ocean rim.

How do the eruptions that created the Columbia Plateau differ from eruptions that create large composite volcanoes?

The Columbia Plateau was generated from fissure eruptions, or extrusion of basaltic magma from fissures in Earth's crust. These magmas spread far and wide. Large composite volcanoes are made from andesitic magmas and are generally the result of much more violent eruptive events.

Briefly compare the May 18, 1980, eruption of Mount St. Helens to a typical eruption of Hawaii's Kilauea volcano.

The eruption of Mount St. Helens was sudden and violent, generating a blast, ash, and projectiles. Kilauea generates relatively quiet gentle outpourings of more fluid lavas.

What is the geothermal gradient? Describe how the geothermal gradient compares with the melting temperatures of the mantle rock peridotite at various depths.

The geothermal gradient is the increase in temperature with depth into the upper crust. The temperature at which peridotite melts is higher than the geothermal gradient at every depth. However, the difference between these two is greatest at the surface, they approach similar temperatures at 150 km depth, and then the temperatures become more different again.

Describe pyroclastic flows and explain why they are capable of traveling great distances.

These are hot gases mixed with ash and large fragments of lava. They are fast moving flows that are driven by gravity down the side of a volcano with little friction between the solid portions and the ground because of the high gas content.

What is a lahar?

These are volcanic mud flows that form when volcanic debris becomes saturated with water and flows quickly down the volcano.

How do volcanic bombs differ from blocks of pyroclastic debris?

Volcanic bombs are emitted as molten rock and cool after being ejected. Pyroclastic debris blocks are ejected as solid materials.

Are volcanoes in the Ring of Fire generally described as quiescent or explosive? Name an example that supports your answer.

These volcanoes are generally explosive. Volcanoes such as Mt. St. Helens are in this ring.

List at least three volcanic hazards other than pyroclastic flows and lahars.

Volcano-triggered tsunami, volcanic ash stalling aircraft engines, dangerous gases breathed by humans and other animals.

Define viscosity and list three factors that influence the viscosity of magma.

Viscosity is a fluid's resistance to flow. Three factors that affect magma viscosity are temperature, composition, and presence of dissolved gases.

Why is a volcano that is fed by highly viscous magma likely to be a greater threat to life and property than a volcano supplied with very fluid magma?

Volcanoes with viscous magma tend to produce violent eruptions that include clouds of hot gas and ash. The blast from the eruption can also cause collapse of part of the surrounding area.

What role do water and other volatiles play in the formation of magma?

Water and other volatiles lower the melting temperature of magmas. Therefore, rocks will melt more readily with less pressure.

List the main gases released during a volcanic eruption. What role do gases play in eruptions?

Water vapor, carbon dioxide, nitrogen, and sulfur dioxide. These gases escape as the confining pressure over a magma is released, releasing the gases into the atmosphere.

Contrast the composition of a typical lava dome and a typical fissure eruption.

A lava dome will tend to be felsic to intermediate while a fissure eruption will emit basaltic lava.

Describe dike and sill, using the appropriate terms from the following list: massive, discordant, tabular, concordant.

Both dikes and sills are tabular. They are produced when magma intrudes upon rock with fractures or weak zones. Dikes are discordant, or not aligned with the existing layers where sills are concordant, or aligned with the existing layers.

How do the size and steepness of slopes of a cinder cone compare with those of a shield volcano?

Cinder cones are much smaller and steeper than shield volcanoes.

How does the composition and viscosity of lava flows differ between composite volcanoes and shield volcanoes?

Composite volcanoes tend to have intermediate to felsic lava that is very viscous whereas shield volcanoes have basaltic lava that is low in viscosity.

The eruption of what type of magma may produce an eruption column?

Felsic

List these three magmas in order from most silica rich to least silica rich, based on their compositions: mafic (basaltic), felsic (rhyolitic), intermediate (andesitic).

Felsic, intermediate, mafic

Describe pahoehoe and aa lava flows.

Pahoehoe lava flows smoothly and sometimes generates smooth surfaces that look ropy or braided. Aa flows are rough with jagged blocks and spines of volcanic ejecta.

What is a parasitic cone, and where does it form?

Parasitic cones form on the flanks of volcanoes and tend to form in mature volcanoes where magma erupts from fissures along the flank or base of the volcano.

What type of volcanic structure is Shiprock, in New Mexico, and how did it form?

Shiprock is a volcanic neck. It consists of igneous rock that crystallized in the vent of a volcano. The less-resistant rock surrounding this rock eroded away, leaving the volcanic neck standing high on the landscape.

Relate lava tubes to the extent of lava flows associated with shield volcanoes.

Since shield volcanoes are not very steep, their lava flows fast and far from the vent by way of lava tubes. Therefore shield volcanoes tend to have lava tubes associated with them.

Describe the composition and viscosity of the lava associated with shield volcanoes.

The lava will be basaltic and have low viscosity.