Geology Ch. 4 & 5 Questions

Describe how magmas are produced at continental rifts. Why can you find both basalt and rhyolite in such settings?

-splitting a continent in 2 and creating a new Mid Ocean Ridge -Some of the magma erupts to form basalt; but some transfers heat to the continental crust, which then partially melts itself, producing felsic magmas that erupt to form rhyolite.

What is the difference between a sill and a dike and how do both differ from a pluton?

A dike is a tabular intrusion that cuts across pre-existing layer ( its vertical). A sill is a tabular intrusion that injects parallel layering (horizontal). A pluton is different because it is just a blob intrusion that ranges in size from tens of meters across tens of km across.

Describe the activity in the mantle that leads to hot-spot eruptions.

A plume is formed.

What factors control the viscosity of a melt?

Temperature: hotter magma is less viscous, volatile content: volatile atoms tend to break apart bonds and may accumulate to form bubbles, and silica content: mafic magmas are less viscous than felsic magmas.

What are the sources of heat in the Earth? How did the first igneous rocks on the planet form?

The Earth is heated by planetismal accretion, gravitational compression, differentiation into the crust, mantle, & core, and radioactive decay. The first igneous rocks formed from freezing magma

Why does melting take place beneath the axis of a mid- ocean ridge?

As the asthenosphere rises it undergoes decompression which leads to melting.

Describe the three different kinds of material that can erupt from a volcano.

Basaltic lava: low viscosity, very hot, flows quickly, creates pahoehoe and a'a, can flow long distances. Andesitic lava: flow is too viscous to flow far, tends to break up as it flows. Rhyolitic/Felsic lava: so viscous that it may pile up in a dome shaped mass, this can cause pressure to build up and eventually lead to explosive eruptions.

Why are there so many different types of magmas?

Because it depends on the source, how it interacts with its surroundings (assimilation), and whether crystals sink as they form (partial melting and magma mixing).

Why do magmas rise from depth to the surface of the Earth?

Because it is less dense than surrounding rock and because the weight of the overlying rock creates pressure that squeezes magma upward.

Why do some volcanic eruptions consist mostly of lava flows, while others are explosive and do not produce flows?

Because of its viscosity. If it has high viscosity (felsic) then gas and volatiles cannot escape easily and pressure is built up causing an explosive eruption.

Explain, how steps can be taken to protect people from the effects of eruptions?

Danger assessment maps, evacuation, and diverting flows.

Describe the three processes that lead to the formation of magmas.

Decrease in pressure: a decrease in pressure permits melting because atoms are able to break free of solid mineral crystals. This occurs when hot mantle rock rises to shallower depths in the Earth. Addition of volatiles: when volatiles mix with mantle rock, they help break chemical bonds, volatiles also decrease a rock's melting temperature. Heat transfer from rising magma: surrounding crustal rock is melted.

How does grain size reflect the cooling time of a magma?

Extrusive rocks cool rapidly and have small grains. Intrusive rocks cool much slower and have more time to grow larger grains.

What process in the mantle may be responsible for causing hot- spot volcanoes to form?

Hot mantle plumes. Magma from deep within the mantle melts through the crust which is what causes hot spot volcanoes to form.

How do continental-rift eruptions form flood basalts?

Huge sheets of low viscosity lava erupt and form huge sheets, these sheets can layer on top of one another forming plateaus.

What does the mixture of grain sizes in a porphyritic igneous rock indicate about its cooling history?

It cooled slowly at first and then was ejected from the volcano causing it to cool rapidly.

What is a large igneous province (LIP)? How might the formation of LIPs have affected the Earth system?

LIP's are huge volumes of igneous rock. . LIPs have a profound affect on the environment and any changes in ocean chemistry, global temperature, and atmospheric clarity triggered by an LIP eruption could even cause the extinction of a life form

Describe the way magmas are produces in subduction zones.

Magma upwells because of the stretching and plate tectonics. it cools at the surface creating igneous rock.

Describe the differences between a pyroclastic flow and a lahar.

Pyroclastic flow is a fast moving avalanche that occurs when hot volcanic ash and debris mix with air and flow down the side of a volcano. Lahar is a thick slurry formed when volcanic ash and debris mix with water, either in rivers or from rain or melting snow and ice on the flank of a volcano.

Describe the differences among shield volcanoes, stratovolcanoes, and cinder cones. How are these differences explained by the composition of their lavas and other factors?

Shield volcanoes are broad domes, generally form from low viscosity basaltic lava flows. Effusive eruptions only. Cinder cones are cone-shaped piles of tephra, they are typically symmetrical and have deep craters at their summit. Pyroclastic eruptions only. Stratovolcanoes are also known as composite volcanoes, they are large, cone shaped and consist of alternating layer soft lava, tephra, and debris. Ex. Mt. Fuji. Alternate between effusive and pyroclastic eruptions.

What factors control the cooling times of a magma within the crust?

The depth of the intrusion, the shape and size of a magma body: the greater the surface area for a given volume of intrusion, the faster it cools, and the presence of circulating ground water: water passing through magma absorbs and carries away heat

Identify some of the major volcanic hazards, and explain how they develop.

Threat of lava flows, pyroclastic flows, ash and lapilli, the blast, landslides, lahars, earthquakes, tsunamis, and gas (CO2).

Contrast an island volcanic arc with a continental volcanic arc. What is a hot-spot volcano?

island: basaltic lava continental: erupt both basaltic lava and rhyolitic pyroclastic debris hot-spot volcano: an isolated volcano not caused by movement at a plate boundary, but rather by the melting of a mantle plume

How is the process of freezing magma similar to that of freezing water? How is it different?

it is similar because in both processes the temperature is decreasing and both substances are solidifying. They are different because magma can be made of many different types of rock and water is just two elements.

How do geologists predict volcanic eruptions?

recurrence intervals, earthquake activity, changes in heat flow, changes in shape, increases in gas and steam emission