Chapter 4 & 5

What three factors do volcanologists monitor in order to determine whether magma is migrating toward Earth's surface?

1. Changes in the pattern of earthquakes produced by the movement of magma 2. Magma entering a near-surface magma chamber, which leads to inflation of the volcano 3. Changes in the amount or composition of gases released from a volcano

Is it possible for two igneous rocks to have the same mineral composition but be different rocks? Use an example to support your answer.

Yes, volcanoes can make volcanic ash and Pumice.

chapter 5

now

Describe two mechanisms by which mantle rock can melt without an increase in temperature. How do these magma-generating mechanisms relate to plate tectonics?

water-boundaries

What is the process that is thought to generate most granitic magmas?

A basaltic pond of magma becomes trapped below continental crust and partially melts this silica-rich crust into a granitic magma. partial melting of intermediate rocks can generate felsic (granitic) magmas.

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

An increase in temperature with depth. 20-30 degrees Celsius per kilometer. The temperature at which peridotite melts is higher than the geothermal gradient. Temperature increases with depth. Thus, since the peridotite melting point is higher than the geothermal gradient, the mantle is solid.

Describe the materials that compose composite volcanoes.

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

Name two plate tectonic settings in which you would expect magma to be generated.

Divergent and convergent

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).

The texture of obsidian

glassy

Identify the igneous rock textures described by each of the following statements. Openings produced by escaping gases

vesicles

Distinguish among a conduit, a vent, and a 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.

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.

Different processes produce magma in different tectonic settings. Consider situations A, B, and C in the diagram and describe the processes that would be most likely to trigger melting in each.

A: As hot mantle goes up, it moves into zones of low pressure. This drop in pressure initiates melting in the upper mantle. B: As an oceanic plate goes down in the mantle, water and other pressures lower the melting temperature to start up partial melting. C: Heating of crustal rocks above their melting temperature produces magma,

How does the crystallization and settling of the earliest formed minerals affect the composition of the remaining magma?

After the earliest formed minerals have settled, the resulting magma will be more felsic than the original magma.

Explain the process of decompression melting.

As hot mantle rock ascends, it continually moves into zones of lower pressure. This drop in confining pressure initiates decompression melting in the upper mantle.

Describe the composition of a cinder cone.

Basaltic; generally loose scoria. cinder cones (also called scoria cones) are built from ejected lava fragments that begin to harden in flight to produce the vesicular rock scoria

In addition to composite volcanoes, what other volcanic landform can generate a pyroclastic flow?

Calderas

How do cinder cones compare to shield volcanoes in terms of size and steepness of their flanks?

Cinder cones are much smaller and steeper than shield volcanoes.

How do 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.

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.

Which type of plate boundary generates the greatest quantity of magma?

DIVERGENT

Describe each of the following in terms of composition and texture: diorite, rhyolite, and basalt porphyry.

Diorite: Course grained rock that looks like gray granite. Contains little of no visible quartz crystals. Rhyolite: fine grained light colored silicates. Buff to pink color, to light gray. Basalt: small, light colored feldspar, glassy olivine grains

List these three magmas in order, from the most silica-rich to the least silica-rich, based on their composition: basaltic magma, rhyolitic magma, andesitic magma.

Felsic, intermediate, mafic

Classify each of the following rocks by their mineral composition (felsic, intermediate, or mafic): gabbro, obsidian, granite, and andesite.

Gabbro: Mafic Obsidian: Felsic Granite: Felsic Andesite: Intermediate

What is emitted from a fumarole?

Gases

Why glass is not considered a mineral?

Glass has a disorderly atomic structure.

List the six major igneous rock textures.

Glassy, Aphanitic, Phaneritic, Porphyritic, Vesicular, Pyroclastic.

List the four basic compositional groups of igneous rocks, in order from the group with the highest silica content to the group with the lowest silica content.

Granitic, Andesitic, Basaltic, Ultramafic

Was the magma likely a high-temperature magma or a low-temperature magma? Explain.

High temperature bc mafic magma, like olivine, is melted at high temperature

Name two minerals typically found in rocks with high silica content and two minerals found in rocks with relatively low silica content.

High: quartz and potassium feldspar Low: olivine, pyroxene

Compare the processes of assimilation and magma mixing.

Incorporation of foreign matter (surrounding rock bodies) into a magma Assimilation Two chemically distinct magmas intruding one another to produce a composition quite different from either original magma `Magma Mixing

A common misconception about Earth's upper mantle is that it is a thick shell of molten rock. Explain why Earth's mantle is actually solid under most conditions.

It is a solid layer than goes through a slow, continuous convection motion with iron and nickel. The increase in temperature with depth and pressure is between solid rock and 100% melted

During a field trip with your geology class, you visit an exposure of rock layers similar to the one sketched here. A fellow student suggests that the layer of basalt is a sill. You disagree. Why do you think the other student is incorrect? What is a more likely explanation for the basalt layer?

Lava flow that the atmosphere messed with A sill would not likely have vesicles, and thelava flow effected the bottom layer partially into sandstone and then cooled therefore the layer of shale on top is because of weathering

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.

How do light-colored igneous rocks differ in composition from dark-colored igneous rocks?

Light-colored igneous rocks are more granitic (felsic) in composition and contain more feldspar and silica. Dark-colored igneous rocks are more basaltic (mafic) in composition and contain more iron and magnesium.

What is magma? How does magma differ from lava?

Magma is a body of molten rock found at depth, including any dissolved gases and crystals. Lava is magma that reaches the Earth's surface. Magma is intrusive while lava is extrusive.

What is the source of magma for most intraplate volcanism?

Mantle plumes, or hot spots.

List and describe the three components of magma.

Melt is the liquid portion of magma and has the eight most common elements found in Earth's crust. Solid component is crystals of silicate minerals. Volatiles are the gaseous component of magma that forms a gas at surface pressures.

A scientist creates diamonds using high-pressure laboratory experiments. These diamonds are not considered minerals because which of the following requirements has been violated?

Minerals must occur naturally.

Would you ever expect to find quartz and olivine in the same rock? Why or why not?

No because quartz is only found in felsic rocks while olivine is only found in mafic rocks

If all silicate minerals melted at exactly the same temperature, would magmas of different compositions exist? How is partial melting important for generating the different kinds of rocks on Earth?

No, since magma differentiation occurs because of partial melting (minerals melt at different temperatures). If all minerals melted at the same temperature you would get the same magma composition of the parent rock. As rock begins to melt, the minerals with the lowest melting temperatures are the first to melt. If melting continues, minerals with higher melting points begin to melt, and the composition of the melt steadily approaches the overall composition of the rock from which it was derived. Most often, however, melting is not complete. The incomplete melting of rocks is known as partial melting, a process that produces most magma. When a rock undergoes partial melting, it forms a melt that is enriched in ions from minerals with the lowest melting temperatures, while the unmelted portion is composed of minerals with higher melting temperatures. Separation of these two fractions yields a melt with a chemical

olivine Did the magma from which this rock formed likely originate in the mantle or in the crust? Explain.

Olivine is a common dark (ferromagnesian) silicate mineral which means that it melted in the upper mantle. Therefore, the magma from which this rock formed likely originated in the mantle.

Contrast 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.

Briefly describe why partial melting results in a magma having a composition different from the rock from which it was derived.

Partial melting generates a magma that is nearer the felsic (granitic) end of the compositional spectrum than the parent rock from which it was derived.When a rock undergoes partial melting, it will form a melt that is enriched in ions from minerals with the lowest melting temperatures, while the unmelted portion is composed of minerals with higher melting temperatures.

What name is given to the region having the greatest concentration of composite volcanoes?

Ring of Fire that circles the Pacific Ocean rim.

How are granite and rhyolite different? In what way are they similar?

Same: They are both Granitic. Different: Granite is Intrusive, Rhyolite is Extrusive.

What is scoria? How is scoria different 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 type of volcanic structure is Shiprock, 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.

Igneous rocks are composed mainly of which group of minerals?

Silicate minerals

How does the rate of cooling influence crystal size? What other factors influence the texture of igneous rocks?

Slow cooling promotes the growth of large crystals. Rapid cooling generates small crystals.

List the two criteria by which igneous rocks are classified.

Texture and mineral composition.

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.

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

The lava will be basaltic and have low viscosity.

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.

In what way does tuff and volcanic breccia differ from other igneous rocks such as granite and basalt?

They do not imply mineral composition.

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.

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.

List at least three volcanic hazards besides pyroclastic flows and lahars.

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

Are volcanoes fed by highly viscous magma more likely or less likely to be a greater threat to life and property than volcanoes 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 causes rock to melt at lower temperatures, just as putting rock salt on icy sidewalks induces melting.

Explain how the crystals in pegmatites are able to grow so large.

Water makes ion migration easier in the later stages of magma crystallization

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.

Consists of crystals that are too small to be seen without a microscope

aphanitic texture

A huge expanse of granitic rock forming a mountainous terrain tens of kilometers wide

batholith

Distinguish among batholiths, stocks, and laccoliths in terms of size and shape.

batholiths - largest intrusive igneous bodies, mammoth linear structures, can cover a large area but thin stocks - smaller plutons laccoliths - igneous rock forcibly inserted between sedimentary strata

Minerals must be solid, have an orderly atomic structure, have a recognized chemical composition, be naturally occurring, and __________.

be generally inorganic in origin

A vertical wall-like feature a few meters wide and hundreds of meters long

dike

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

dikes - discordant & tabular discordant bodies that form when magma is forcibly injected into fractures and cut across bedding surfaces and other structures in the host rock. sills - concordant are nearly horizontal, concordant bodies that form when magma exploits weaknesses between sedimentary beds or other rock structures

What type of magma must erupt to produce an eruption column?

felsic

Mount Whitney, the highest summit (4421 meters [14,505 feet]) in the contiguous United States, is located in the Sierra Nevada batholith. Based on its location, is Mount Whitney likely composed mainly of granitic, andesitic, or basaltic rocks?

granite

Exceptionally large crystals, most exceeding 1 centimeter in diameter-

hese may be phenocrysts, such as K- feldspar, in phaneritic, porphyritic granite; many crystals in pegmatites exceed l centimeter in diameter. In any case, these coarse mineral grains point to a long, sustained period of grain growth in the magma or to specific compositional factors, such as high water content, that greatly facilitate growth of large-size silicate mineral grains from a magma.

A dome-shaped mountainous structure flanked by upturned layers of sedimentary rocks

laccolith

A relatively thin layer of basalt sandwiched between horizontal layers of sedimentary rocks exposed along the walls of a river valley

limestones

How are most basaltic magmas thought to have formed?

partial melting of ultramafic rocks tends to yield mafic (basaltic) magmas (peridotite)

What is meant by the term country rock?

preexisting rock through which intrusive igneous bodies intrude When magma rises through the crust, it forcefully displaces preexisting crustal rocks

5 mineral rules

solid naturally occuring generally inorganic characteristic chemical composition orderly crystalline structure

A matrix of fine crystals surrounding phenocrysts=

the finer-grained matrix is the groundmass and the texture is porphyritic

What does a porphyritic texture indicate about the cooling history of an igneous rock?

the magma sat and cooled a bit below the Earth's surface, thus giving time for the large crystals to grow, before erupting onto the surface and cooling very quickly

Question 1 Most of this rock is composed of tiny ash fragments. What name is given to rocks of this type?

tuff