Marshark Chapter Four

How much silica is in mafic melts?

45-52% silica

What is the silica content of intermediate melts?

52-66% silica

How much silica is in felsic melts?

66-76% of silica

What produces extrusive rocks?

Lava flows, as well as various types of explosive eruptions

Freezing of the magma that remains after much fractional crystallization has occurred, yields _______.

a felsic igneous rock

What is a large volume of igneous rock called?

a large igneous province (LIP)

How do molten rocks differ from one another?

in the terms of the proportions of chemicals that they contain

A description of igneous texture specifies whether the rock consists of...

interlocking crystals, stuck-together fragments, or solid glass

How much of a source rock can melt to produce magma?

only 2% of 30% of a source rock can melt to produce magma at a given location

How do plutons originate?

plutons may have originated as diapirs, meaning that they rose upward through the crust as buoyant, light-bulb shaped blobs of magma that pierced overlying rock and pushed it aside as they rose

Why is a wet melt less viscous than a dry melt?

reactions with volatiles also tend to break apart silicate molecules

What happens if a melt cools slowly?

successful crystals have time to grow large before the rock solidifies completely. As the large crystals are growing, new seeds and small crystals do form, but because the melt remains hot, they tend to dissolve into the melt again before they can grow large

How much silica is in ultramafic melts?

38-45% silica

What are the geologic settings that lead to melting?

-1) at hot spots -2) along volcanic arcs bordering oceanic trenches -3) along mid-ocean ridges -4) within continental rifts

Why does the texture of a non fragmental igneous rock reflects its cooling rate?

-Because mineral crystals grow when atoms diffuse (move) through melts and attach to crystal seeds. At high temperatures, seeds constantly form and dissolve because heat causes diffusion to occur rapidly. Only some are successful and grow into small crystals, but most end up dissolving again.

What is melting due to decompression?

-Because pressure prevents melting, even in a very hot rock, a decrease in pressure can trigger melting as long as the rock remains hot. -Decompression melting takes place where hot mantle rock rises slowly. As the rock moves up, its pressure becomes less (due to the decrease in overburden) while its temperature remains nearly unchanged (rock acts as an excellent insulator).

What are the two reasons why magma rises?

-Buoyancy -Magma pressure

How do porphyritic rocks form?

-First, the melt cools at depth slowly enough for phenocrysts to form. -Then the melt erupts, and the remainder cools quickly, so fine-grained groundmass forms around the phenocrysts

How does a rock have a fine-grained (aphanitic) texture?

-If the melt cools rapidly, but not rapidly enough to form glass -Many crystals in the resulting rocks will have grown from seeds, but none of those crystals will have had enough time to grow large

What is a batholith?

-Intrusions of numerous plutons in a region makes a vast composite body. -The resulting immense mass of igneous rock is called this

What is stopping?

-It is related to a pluton -It is a process during which magma assimilates wall rock and blacks of wall rock break off and sink into the magma. If a stopped block does not melt entirely, but rather becomes surrounded by new igneous rock, it becomes a xenolith, after the greek word xeno, meaning foreign

What are the types of melts?

-Mafic melts -Ultramafic melts -Felsic melts -intermediate melts

What is melting due to the addition of volatiles?

-Magma also forms at locations where volatiles mix with hot mantle rock. Volatiles are substances, such as water and carbon dioxide, that evaporate easily. -When volatiles mix with hot, dry rock, they react with minerals and break chemical bonds, so that the rock begins to melt in a process called flux melting. -In other words, adding volatiles decreases a rocks melting temperature

What is the end result of partial melting composition in magma?

-Magmas formed by partial melting are more felsic than the source rock from which they were derived because more silica enters the liquid, as melting begins, than remains behind in the still-solid source

What do LIPS produce?

-On land, the hot mafic lava that erupts has such low viscosity that it can flow tens to hundreds of km across, forming a vast sheet of basalt. -This process may repeat resulting in thick stacks of thick, broad lava flows, known as flood basalts.

How does subduction trigger melting in the oceanic crust?

-Some minerals in oceanic crust contain volatile compounds. at shallow depths, these volatiles are bonded to other elements within mineral crystals. But when subduction carries crust down into the hot asthenosphere, the crust warms up and it becomes so hot that volatiles separate and diffuse into the overlying hot ultramafic rock (peridotite) of the asthenosphere as molecules of H20 or CO2. the addition of these volatiles causes flux melting of peridotite. -only part of the original ultramafic rock actually melts, since silica preferentially goes into the melt. So the process yields mafic magma, some of which rises to form basaltic sills and dikes in the crust, and some travels all the way to the surface to extrude as basaltic lava.

What factors control the cooling time of intrusive magma?

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

What effects the speed at which a liquid moves?

-The resistance to flow. -This is viscosity

Why don't the molecules of molten rock not lie in an orderly crystalline lattice?

-They do not lie in an orderly crystalline lattice because molten rock is a liquid -rather, they occur in clusters or chains that can move with respect to one another

What do wet melts include?

-They include up to 15% dissolved of volatiles, including water, carbon dioxide, nitrogen, hydrogen, and sulfur dioxide. -These volatiles come out of the earth at volcanoes in the form of gas. -Usually, water makes up about half of the gas erupting at volcanoes.

How much does the oceanic crust cover the earths surface?

-This layer of basalt and gabbro covers 70% of the earth's surface and forms at mid-ocean ridges (divergent boundaries)

Why does the rate of cooling change?

-When melts rise, they eventually cool because the temp of the earth decreases upward. -The time it takes for a magma to cool depends on how fast it can transfer heat into its surroundings. -Ex: lava in an extrusive environment cools quickly. Magma in an intrusive environment cools slowly because wall rock acts as insulation

What is the movement of a plume in the plume hypothesis?

-When the hot rock of a plume reaches the base of the lithosphere, decompression causes partial melting within the hot rock

What are tabular intrusions?

-also known as sheet intrusions -they are roughly planar and have a fairly uniform thickness -they form when magma injects into cracks underground

What is assimulation?

-as magma sits underground before solidifying completely, it may incorporate chemicals dissolved from the wall rock or from blocks that detached from the wall and sank into the magma -heat provided by magma partially melts wall rock; the new magma may then mix with the original magma. Also, blocks of wall rock can dissolve in the original magma, and that wall rock may chemically react with the magma

Why does igneous activity happen at mid-ocean ridges?

-as seafloor spreading takes place, oceanic lithospheric plates drift away from the ridge. -Hot asthenosphere rises to keep the resulting space filled. -As asthenosphere rises, it undergoes partial melting due to decompression. -Partial melting of mantle peridot yields mafic magma, which rises into the crust and collects in a magma chamber. Some cools slowly in the magma chamber to form massive gabbro, while some intrudes upward to fill vertical cracks that appear as newly formed crust splits apart. Magma that cools in the cracks forms basalt dikes, and magma that rises as far as the seafloor, and extruded as lava, forms pillow-basalt flows

How do you describe an igneous rock?

-crystalline texture -fragmental texture -glassy texture

Melting occurs only in special locations where conditions lead to:

-decompression -addition of volatiles -or heat transfer

Where do melts cool?

-extrusive igneous settings -intrusive ignous settings

How does the shape and size of a magma body control the cooling time of intrusive magma?

-heat escapes from magma at an intrusion surface, so the greater the surface area for a given volume of intrusion, the faster it cools. (pancake shaped magma cools faster than melon shaped magma). -And because the ratio of surface area to volume increases as size decreases, a body of magma that is small cools faster than a larger one

Why is a pegmatite (a very coarse-grained igneous rock) an exception to the standard cooling rate-grain size relationship?

-it doesn't form by slow cooling -this rock, which can contain crystals, typically occurs in dikes. Because dikes generally cool relatively quickly, the coarseness of this rock may seem surprising. -Pegmatite becomes coarse because it forms from water-rich melts in which atoms can diffuse so rapidly that large crystals can grow very quickly

How does a volcanic island arc grow?

-it grows on oceanic crust where one oceanic plate beneath another. -Beneath volcanic arcs, a variety of intrusions-plutons, dikes, and sills-develop to be exposed only later by erosion.

How does a volcanic arc form?

-it is a chain of volcanoes -forms on the overriding plate adjacent to the deep-ocean trenches that mark convergent plate boundaries

What is pumice?

-it is a felsic volcanic rock that contains abundant (75-90% of the rocks volume) tiny vesicles, each surrounded by a thin screen of glass. -It has more open space than solid glass (looks sponge like). -Pumice forms from quickly cooling, volatile rich frothy lava

What is a sill?

-it is a tabular intrusion that injects between layers, and thereof, parallels layering

What is a pyroclastic rock?

-it is when pyroclastic debris becomes consolidated into a solid mass, due either to still-hot clasts welding together during accumulation, or to cementation by minerals precipitated from groundwater long after accumulation -Pyroclastic rocks have a fragmental texture

What does studying a rocks composition reveal?

-it tells us about the original source of the magma and the way in which the melt evolved before finally solidifying

What are composed of fine grained rock?

-lava flows, dikes, and sills -this is because of the the relationship between cooling rate and texture

What are the sources of melting?

-melting due to decompression -melting due to the addition of volatiles -melting due to heat transfer

What rifts?

-places where continental lithosphere stretches horizontally -As a result of this stretching, the lithosphere thins vertically and the asthenosphere undergoes decompression

what is glassy texture?

-rocks made of a solid mass of glass or of tiny crystals surrounded by glass are glassy igneous rocks. -Glassy rocks typically fracture conchoidally (fractures are curbed, like a clam shell)

What does the viscosity of a given melt depend on?

-temperature -volatile content -silica content

What are mafic melts?

-they contain relatively high proportion of magnesium oxide and iron oxide -Ma stands for magnesium, and fic comes the latin word for iron

What is fragmental texture?

-they form from pyroclastic debris and consist of chunks or shards that are packed together, welded together or cemented together after they have solidified. -these are fragmental igneous rocks

How do LIPS form?

-they may form when the bulbous head of a mantle plume first reaches the base of the lithosphere. -More partial melting can occur in a pume head than in normal asthenosphere, because temperatures are higher in a plume head. -Thus, a very large quantity of particularly hot mafic magma forms in the plume head, and when this magma reaches the surface, huge amounts of lava spew out of the ground. -If the plume head lies beneath a rift, thinning of the lithosphere causes even more decompression, which in turn leads to even more melting

What are vesicles?

-they occur when rapidly cooling lava freezes while it still contains a high concentration of gas bubbles-these bubbles remain so open holes result

What is the texture of plutons?

-they tend to be composed of coarse-grained rock -Plutons that intrude into hot wall rock at great depth cool particularly slowly and tend to have larger crystals than plutons that intrude into cool wall rock at shallow depth

What is a crystalline texture?

-when a melt solidifies, minerals in some rocks grow and interlock like pieces of a jigsaw puzzle. -The crystals interlock because the rock does not solidify instantly. Rather, different crystals grow at different rates and at different times, and as the crystals grow, they interfere with each other

At oceanic hotspots, mafic magma erupts at the surface and solidifies to form____.

Basalt

What do early-formed minerals tend to be?

Early-formed minerals tend to be relatively mafic, so their growth preferentially removes iron and magnesium from the magma. As a consequence, the remaining magma becomes more felsic

How are igneous rocks formed from Rocks that forms from lava that freezes above ground, in contact with air or water, after it erupts or extrudes, is extrusive igneous rock?

Ex: rock that has solidified within a lava flow, as well as rock made from cemented-together fragments of pyroclastic debris (pyro means fire) that blasted out of a volcano and into the air

In continental volcanic arcs, some mantle derived basaltic magma gets trapped at the base of the continental crust, and some resides in magma chambers deep in the crust. How does fractional crystallization play a part in this?

Fractional crystallization, as well as assimilation, may cause the magma to become more progressively felsic. But in addition, this very hot mantle-derived magma transfers heat into the adjacent continental crust, which causes partial melting of the continental crust. Some of this new melt becomes assimilated in the rising melt, and some rises on its own. And, as the magma cools, it undergoes fractional crystallization. Because much of the continental crust has a mafic to intermediate composition, the resulting magma is intermediate to felsic. As this magma rises, it either cools higher in the crust to form plutons in magma chambers, or it rises to the surface to erupt as lava. These processes (heat transfer melting, assimilation, and fractional crystallization) contribute to the abundance of granite, rhyolite, diorite, and andesite at continental arcs.

How does a rock have a glassy texture?

If a melt cools very rapidly, it can become solid before seeds have grown into crystals

How does mafic lava erupt?

It can erupt in fountains, move long distances, and form thin lava flows

How do geologists distinguish among four major compositional types of molten rocks?

It depends on the proportion of silica relative to the sum of magnesium oxide and iron oxide in the melt

What is silica?

It is a compound of silicon and oxygen (SiO2)

What is a lava flow?

It is a downslope motion of lava stream

What is the boundary between wall rock and any igneous intrusion?

It is a intrusive contact

What is a volcano?

It is a vent or opening from which melt that originates inside the earth emerges onto the plant's surface or rises into the air during an episode called a volcanic eruption

What is an igneous rock?

It is any rock formed by the solidification of melt

What are the two main categories of igneous rock?

It is based on where it solidified: 1) Rock that forms from lava that freezes above ground, in contact with air or water, after it erupts or extrudes, is extrusive igneous rock 2) Rocks that have formed by the solidification of magma is an intrusive rock

What is lava?

It is melt that has emerged at the surface as lava

What is magma?

It is underground melt

What is a laccolith?

It is when an intrusion starts to inject between layers by then domes upward and yields a blister-shaped intrusion

Why do we see intrusive igneous rocks?

Mountain building slowly uplifts belts of crust. Erosion by water, wind, and ice can gradually strip away the thick, overlying rock and expose the intrusive rock that has formed below

What is the source of heat that can cause magma to form?

Much of this heat is a relic of our planet's formation: the compression of mass into a smaller volume, the sinking of iron to form the core, the impact of meteorites and the decay of radioactive elements

What does partial melting at rifts produce?

Partial melting due to decompression produces basaltic magma, which rises into the crust. Some magma makes it to the surface and erupts as basalt. However, some become trapped in crustal magma chambers and transfer heat to the crust. The resulting partial melting of the crust yields felsic magmas that erupt explosively as ryhdilte. As a result, volcanic rocks in a rift generally include lava flows of basalt and layers of rhyolitic tuff

What do all molten rocks (magma/or lava) contain?

Silica

How are igneous rocks formed by the solidification of magma is an intrusive rock?

The majority of igneous rocks are formed by this solidification of magma underground, after it has pushed its way, or intruded, into pre-existing wall rock

Why does only 2% of 30% of a source rock can melt to produce magma at a given location (partial melting)?

The temperature at sites of magma production simply never get high enough to melt the entire source rock, and magma tends to migrate away from the site of melting before all of the original rock has melted

What are intermediate melts?

Their composition is partway between that of mafic and felsic melts

What are the subcategories of crystalline rocks based on?

They are based on the size of crystals

What are "dry" melts?

They are melts that do not contain volatiles

What are "wet" melts?

They do contain volatiles

Where does melting due to decompression occur?

Upward movement causes decompression melting in mantle plumes, beneath rifts, and beneath mid-ocean ridges

Where does melting due to the addition of volatiles occur?

Volatiles seep into hot asthenosphere in the region just above subducting oceanic lithosphere, causing volcanism at convergent plate boundaries

What feature is associated with subduction?

Volcanic arcs (continental and oceanic)

What is melting due to heat transfer?

When very hot magma from the mantle rises into the crust, the heat it brings raises the temperature of the surrounding crustal rock. In some cases, the rise in temperature may be sufficient to cause the crustal rock to begin melting, a process known as heat-transfer melting

How do you classify ignous rocks?

according to their texture and composition

Why are felsic rocks commonly glassy?

because the high concentration of silica inhibits the easy growth of crystals

a cool felsic lava has relatively high viscosity and____.

bulbous mound

How does magma rise due to buoyancy?

buoyancy force drives magma upward because molten rock is less dense than surrounding solid rock. Pressure within the magma also contributes to driving magma upward

Why does the earth remain hot inside?

even after 4.56 billion years the earth remains hot because heat produced by decaying radioactive elements in the crust, as well as heat released by the core as it slowly solidifies, replaces much of the heat lost to space by radiation at the earth's surface

At continental hot spots, part of the mafic magma erupts to form basalt, but some transfers heat to the continental crust, which itself then partially melts, producing____.

felsic magmas that erupts to yield rhyolite

What is a dike?

it is a tabular intrusion that cuts across preexisting layering (bedding or foliation)

What is partial melting?

it is the process in which only part of an original rock melts to produce magma

What is the extrusive realm?

it lies above ground

What is the intrusive realm?

it lies underground

How does Felsic to intermediate lava erupt?

it may form a mound-like lava dome around the volcanoes vent

What does studying a rocks texture reveal?

it tells us about the rate at which it cooled, as we've seen, and therefore about the environment in which it formed

What do extrusive rocks include?

lava flows and pyroclastic layers

If an originally mafic magma freezes before much fractional crystallization has occurred, ______ rock forms.

mafic ignous rock forms

How does the depth of intrusion control the cooling time of intrusive magma?

magma intruded deep in the crust, where hot wll rocks surrounds it, cools more slowly than does magma intruded into cold wall rock near the ground surface

What is magma mixing?

magmas formed in different locations from different sources may come into contact underground. In some cases, the originally distinct magmas mix to yield new, different magmas

What feature is associated with hot-spots?

mantle plumes

Why does the earth and crust remain in solid form?

most of the crust and the mantle remains in solid form because the immense pressure (produced by the weight of overlying rock) prevents molecules from moving freely relative to one another, which would allow rock to turn into liquid

What does mafic lava have?

relatively low viscosity

Why are mafic melts less viscous than felsic melts?

relatively more silicon-oxygen tetrahedra occur in felsic melt. These tetrahedra link together to make long molecules that can't move past each other easily.

How does a continental arc form?

s grow on continental crust in locations where oceanic lithosphere subducts beneath continental lithosphere

Most ignous rocks present at the earth's surface are a product of what?

seafloor spreading

What is the process of fractional crystallization?

sequential crystal formation

What are new ideas leading to the alternate view of plutons?

t plutons form by intrusion of several superimposed dikes or sills, which coalesce to become a single, massive body. If high temperatures are sustained for a long time, diffusion can take place. The rock may gradually recrystallize and its composition may evolve. The overall space for plutons may form because of crustal stretching or because of uplift of the land surface.

Volatile-rich felsic lava tend...

tend to erupt explosively and form thick ash and debris deposits

Mafic lavas tend...

tend to have low viscosity and spread in broad, thin flows.

What does the composition of a melt reflect (source-rock composition)?

the composition of a melt reflects the composition of the solid form which it was derived from

What is the FALSE statement?

the crust, the earth's outer shell, floats on a sea of molten rock

What did geologists know by the end of the 19th century?

the earth's interior was mostly solid, for if it weren't, spinning on its axis would flatten the earth

What is the plume hypothesis?

the plume itself does not consist of magma. Rather, it's composed of solid rock that is hot enough to be relatively soft so that it can flow plastically, at rates a few centimeters a year

Why is hotter melt less viscous that a cooler melt?

thermal energy breaks bonds and allows atoms or molecules to move more easily

What are plutons?

they are blob-shaped intrusions that range in size from tens of meters across -They typically form when magma in a magma chamber solidified

What is intrusive igneous settings?

they are intrusions inside the earth

What are glass shards?

they are the broken up walls of vesicles in pumice

What are fine-grained (aphanitic) rocks?

they are too small to identify with the naked eye

What aree felsic melts?

they have a fairly high proportion of silica

What are ultramafic melts?

they have an even higher proportion of magnesium oxide and iron oxide

What are coarse-grained (phaneritic) rocks?

they have crystals large enough to identify with the naked eye

What are porphyritic rocks?

they have larger crystals (called phenocrysts) surrounded by a mass of fine crystals (called groundmass)

How does space for dike intrusions occur?

they take place in regions where the crust stretched horizontally, as happens in a rift. So, as the magma forces its way into a vertical crack, the walls of the crack can move apart sideways

Very hot mafic lava has relatively low viscosity and can flow to form _____.

thin sheets

How does space for sill intrusions occur?

this intrusion occurs near the surface of the earth, so pressure from magma can push the rock above the sill upward, leading to vertical movement of the earth's surface

How does magma rise due to pressure?

this magma pressure develops due to the weight of overlying rock and to the injection of new magma. If magma pressure increases, magma gets pushed into the overlying crust, or even to the earth's surface

What does felsic to intermediate lava have?

very viscous

How does the presence of circulating ground water control the cooling time of instrusive magma?

water passing through magma absorbs and carriers away heat. The presence of groundwater in wall rock, therefore, can accelerate cooling

What is an extrusive igneous setting?

when volcanoes erupt stems of low-viscosity lava that cover land in broad motions, the lava then freezes, and it forms relatively thin lava flow. Other volcanoes erupt viscous masses of lava that pile into rubbly domes