geology test 2

Felsic magma

(or silicic magma) contains about 66% to 76% silica

(c) Basalt, a fine-grained igneous rock; (d) granite; (e) obsidian

...

Ultramafic magma

38% to 45% silica.

Mafic magma

45% to 52% silica.

Geologists distinguish subcategories of crystalline igneous rocks according to the size of the crystals.

Coarse grain Fine grain porphyritic rocks

Decompression melting at a mid-ocean ridge.

Decompression melting happens when asthenosphere rises beneath the axis of a mid-ocean ridge, as sea-floor spreading occurs.

Decompression melting during rifting. (Not to scale)

Decompression melting happens when rifting thins the lithosphere, causing the underlying asthenosphere to rise. (Volume of asthenosphere rises to shallower depth and partially melts.)

Magma mixing.

Different magmas formed in different locations from different sources may enter a magma chamber. When this happens, the originally distinct magmas mix to create a new, different magma. For example, thoroughly mixing a felsic magma with a mafic magma in equal proportions produces an intermediate magma.

Volatiles

Elements or molecules, such as water (H2O) and carbon dioxide (CO2), that evaporate easily and can exist in gaseous forms at the Earth's surface.

Pressure prevents melting an increase in pressure can permit melting.

False - we need a decrease in pressure to achieve melting.

The Major Types of Magma

Felsic magma Intermediate magma Mafic magma Ultramafic magma

Magma rises because..

First, magma rises because it is less dense than surrounding rock. Second, magma rises because the weight of overlying rock creates pressure at depth that literally squeezes magma upward.

Flux melting (addition of volatiles)

Flux melting occurs when volatiles escape from subducted oceanic crust into the overlying asthenosphere.

IGNEOUS ROCK TEXTURES

Glassy texture: Interlocking texture: Fragmental texture:

Heat-transfer melting

Heat-transfer melting occurs when heat rises into the base of the crust from basaltic magma that has pooled at the Moho or has intruded into the lower crust. The heat causes the lower crust to partially melt, producing felsic magma that rises.

xenolith

If a stoped block does not melt entirely, but rather becomes surrounded by new igneous rock named after the Greek word xeno,meaning "foreign"

Melting as a result of the addition of volatiles.

Magma also forms at locations where chemicals mix with hot mantle rock.

The Formation of Magma

Melting as a result of a decrease in pressure (decompression) Melting as a result of the addition of volatiles. Melting as a result of heat transfer from rising magma.

flux melting

Melting due to addition of volatiles.

Several factors control magma composition.

Source rock composition. Partial melting. Assimilation. Magma mixing. Fractional crystallization.

magma forms

Specifically, if the pressure affecting hot mantle rock decreases while the temperature remains unchanged.

Fractional crystallization.

The composition of a melt reflects the composition 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.

Three factors control the cooling time of magma that intrudes below the surface.

The depth of intrusion: The shape and size of a magma body: The presence of circulating groundwater:

Assimilation.

This process of contamination takes place when wall rocks fall into the magma and then partially melt, or when heat from the magma partially melts the walls of the chamber.

"Dry" magmas contain no volatiles.

True

"Wet" magmas, in contrast, include up to 15% dissolved volatiles such as water, carbon dioxide, nitrogen (N2), hydrogen (H2), and sulfur dioxide (SO2).

True

As it forms, magma migrates away from the site of melting, leaving behind the solid that can't melt.

True

Because pressure prevents melting, a decrease in pressure can permit melting.

True

Beneath typical oceanic crust, temperatures comparable to those of lava (650°-1,100°C) generally occur in the upper mantle.

True

Even though the upper mantle is very hot, its rock stays solid because it is also under high pressure from the weight of overlying rock.

True

Eventually the flow cools through and through, and a new layer of basalt rock has formed.

True

Geologists distinguish among different types of intrusions on the basis of their shape.

True

Hotter mafic lavas have relatively low viscosity and flow in thin sheets over wide regions, but cooler felsic lavas are highly viscous and clump at the volcanic vent.

True

In a porphyritic rock, the larger crystals are called phenocrysts,while the mass of finer crystals is called groundmass.

True

In effect, adding volatiles decreases a rock's melting temperature.

True

Magma forms only in special places where preexisting solid rock melts.

True

Pressure at great depth, simplistically, prevents atoms from breaking free of mineral crystals.

True

Since felsic minerals tend to melt at lower temperatures, magmas formed by partial melting tend to be more felsic than the original rock from which they were derived—for example, partial melting of an ultramafic rock produces a mafic magma. Variations in the degree of partial melting affect the magma composition.

True

When volatiles mix with hot rock, they help break chemical bonds so if you add volatiles to a solid, hot dry rock, the rock begins to melt.

True

Melting as a result of heat transfer from rising magma.

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, in some cases, the rise in temperature may be sufficient to melt part of the crustal rock.

stoping

a process during which magma assimilates wall rock, and blocks of wall rock break off and sink into the magma

volcano

a vent at which melt from inside the Earth spews onto the planet's surface

volcanic ash

consists of fine particles of glass that form when a spray of lava erupts into the air and freezes instantly also a part of the pyroclastic debris.

Intermediate magma

contains about 52% to 66% silica.

Decompression melting at the top of a plume. (Not to scale)

decompression causes the melting and melt from mantle rises.

pyroclastic debris

extrusive igneous rock includes both solid lava flows, formed when streams or mounds of lava solidify on the surface of the Earth, and deposits of this type of debris.

Magmas with low viscosity

flow more easily than those with high viscosity, just as water flows more easily than molasses.

glassy igneous rocks

glassy texture of rocks made of a solid mass of glass, or of tiny crystals surrounded by glass

Porphyritic rocks

have larger crystals surrounded by a mass of fine crystals.

Extrusive igneous rock

igneous rocks form in two environments. If lava erupts at the Earth's surface and freezes in contact with the atmosphere or the ocean it forms this type of rock.

batholiths

intrusion of numerous plutons in a region creates a vast composite body that may be several hundred kilometers long and over 100 km wide; such immense masses of igneous rock

laccolith

intrusions start to inject between layers but then dome upward, creating a blister shaped intrusion

Plutons

irregular or blob-shaped intrusions that range in size from tens of meters across to tens of kilometers across

igneous rock

made by the freezing of a melt, has formed.

sill

nearly horizontal, tabletop-shaped tabular intrusion

decompression melting

occurs where hot mantle rock rises to shallower depths in the Earth.

lava

once the melt reaches the Earth's surface

Partial melting.

refers to the process by which only part of an original rock melts to produce magma.

Mafic magma gets its name because...

relatively high proportion of MgO and FeO or Fe2O3. The "ma" in the word "mafic" stands for "magnesium," and the "-fic" stands for "iron" (from the Latin ferric).`

Fractional crystallization.

remaining magma becomes progressively more felsic as it cools.

intrusive igneous rock

rock made by the freezing of magma underground, after it has pushed its way (intruded) into preexisting rock of the crust

extrusive igneous rock

rock that forms by the freezing of lava above ground, after it spills out (extrudes) onto the surface of the Earth and comes into contact with the atmosphere or ocean

Coarse grained (phaneritic)

rocks have crystals large enough to be identified with the naked eye

Fine-grained (aphanitic)

rocks have crystals too small to be identified with the naked eye.

Glassy texture:

rocks made of a solid mass of glass, or of tiny crystals surrounded by glass

interlocking texture:

rocks that consist of mineral crystals that intergrow when the melt solidifies, and thus fit together like pieces of a jigsaw puzzle

Fragmental texture:

rocks with a fragmental texture, consisting of igneous fragments that are packed together, welded together, or cemented together after having solidified

fragmental igneous rock

rocks with a fragmental texture, consisting of igneous fragments that are packed together, welded together, or cemented together after having solidified

crystalline igneous rocks

rocks with an interlocking texture

Tabular intrusions

sheet intrusions, are planar and are of roughly uniform thickness.

Mafic magmas are less viscous than are felsic magmas

silicon-oxygen tetrahedra tend to link together in the magma to create long chains that can't move past each other easily.

magma

some of the melt while under the ground

dike

tabular intrusions cut across rock that does not have layering, a nearly vertical, wall-like tabular intrusion

Magma viscosity depends

temperature, volatile content, and silica content.

volcanic eruption

the transfer of melt from inside the Earth onto its surface

melting heat-transfer melting

transfer of heat from a hotter material to a cooler one.

Melting as a result of a decrease in pressure (decompression).

variation in temperature with depth on a graph by a curving line, the geotherm.

magmas or lavas containing more volatiles are less viscous than dry (volatile-free) magmas

volatile atoms also tend to break apart bonds and may accumulate to form bubbles.