Geology Chapter 4

volcano

is a vent or opening from which that melt originates inside the earth emerges onto the planet's surface and/or rises into the air during an episode known as a volcanic eruption.

melt

incandescent molten rock

Felsic (or Silicic) Melts

Have a fairly high proportion of silica, compared to magnesium and iron oxide.

Porphyritic Rocks

Have large crystals (called phenocrysts) surrounded by a mass of fine crystals (called groundmass).

Dry melts

do not contain volatiles.

The Presence of Circulating Groundwater

(Factors that control the cooling time of magma) Water passing through magma absorbs and carries away heat

The Depth of Intrusion

(Factors that control the cooling time of magma) Magma intruded deep in the crust is surrounded by hot wall rock, and thus cools more slowly than does magma intruded into cold wall rock near the ground surface.

The Shape and Size of the Magma Body

(factors that control the cooling time of magma) Heat escapes from magma at an intrusion's surface, so the greater the surface area for a given volume of intrusion, the faster it cools.

Compositional Categories of Magma

-Felsic (or Silicic): 66-76% [weight percent of silica] -Intermediate: 52-66% [weight percent of silica] -Mafic: 45-52% -Ultramafic: 38-45%

The conditions that lead to melting, and therefore, to igneous activity, can develop in four geologic settings:

1. at hotspots 2. along volcanic arcs bordering oceanic trenches 3. along mid-ocean ridges 4. within continental rifts

Xenolith

A body of rock within an intrusion.

Volcanic Arc

A chain of volcanoes forms on the overriding plate adjacent to the deep-ocean trenches that mark convergent plate boundaries.

Silica

A compound of silicon and oxygen (SiO2), and all molten rocks (magma or lava) contain this.

Diapir

A light-bulb-shaped blob that flows upward and pushes aside soft wall rock as it rises.

Obsidian

A mass of solid, felsic, glass. It tends to be black or brown. Because it breaks conchoidally, sharp-edge pieces split off its surface when you hit a sample with a hammer.

Stoping

A process during which blocks break off from a magma chamber wall and sink into the magma within, where they become partly or entirely assimilated.

Bowen's Reaction Series

A schematic description of the order in which minerals form during the cooling and solidification of magma and of the way newly formed minerals react with the remaining magma to form yet another series of minerals.

Sill

A tabular intrusion that injects between layers, and thus is parallel to layering. (Occur near the surface of the earth)

pyroclastic debris

Accumulation of fragments such as clots or droplets of lava, glass shards, larger fragments of pumice, and other broken-up chunks of recently formed igneous rock.

Pegmatite

An exception to the standard cooling rate and grain size relationship is pegmatite, a very coarse-grained igneous rock that contains crystals up to tens of centimeters across.

Magma Chamber

An irregularly shaped area inside the crust where magma accumulates.

Plutons

Are blob-shaped intrusions that range in size from tens of meters across to tens of kilometers across.

Rifts

Are places where horizontal stretching of continental lithosphere takes place. As a result of this stretching, the lithosphere also thins vertically.

Tabular Intrusions (or Sheet Intrusions)

Are roughly planar and have a fairly uniform thickness.

Volatiles

Are substance such as water (H2O) and carbon dioxide (CO2), that evaporate relatively easily.

Color of an Igneous rock

As a rough guide, the color of an igneous rock indicates its composition - mafic rocks tend to be black or dark gray, intermediate rocks tend to be lighter gray or greenish gray, and felsic rocks tend to be light tan to pink or maroon.

Assimilation

As magma sits underground before solidifying completely, it may incorporate chemicals dissolved from the wall rocks, or from blocks that detached from the wall and sank into the magma.

Phaneritic

Coarse-grained rocks that have crystals large enough to identify with the naked eye.

Mafic Melts

Contain a relatively high proportion of magnesium and iron oxide compared to silica

Wet Melts

Contain volatiles, include up to 15 % dissolved volatiles including water, carbon dioxide, nitrogen, hydrogen, and sulfur dioxide.

Continental Arcs

Grow along the edge of a continent, where oceanic lithosphere subducts beneath continental lithosphere.

Fragmental Igneous Rocks

Form from pyroclastic debris and consist of chunks and/or shards that are packed together, welded together, or cemented together after they have solidified.

4.4 Summary

Geologists divide igneous rocks into three general categories, based on texture - crystalline rocks have interlocking crystals, glassy rocks consist of a mass of glass with no crystalline structure, and pyroclastic rocks have a fragmental texture. Crystalline rocks, in turn, can be classified based on composition and on whether they are fine or coarse grained. The color of a rock may provide a clue to its composition.

intrusive igneous rock

Geologists refer to underground melt as magma, and to the rock formed by solidification of magma as this.

Large Igneous Provinces (LIPs)

Huge volumes of igneous rock.

Vessicles

In some cases, a rapidly cooling lava freezes while it still contains a high concentration of gas bubbles - these bubbles remain as open holes.

Batholith

Intrusions of numerous plutons in a region makes a vast composite body that may be hundreds of kilometers long and over 100km wide. The resulting immense mass of igneous rock is called a batholith.

Pumice

Is a felsic rock that consists of tiny vessicles, each of which is surrounded by a thin screen of glass. With far more open space than solid glass, it can look like a sponge, and some specimens can actually float on water. it forms from quickly-cooling, volatile-rich frothy lava.

Scoria

Is a mafic volcanic rock with abundant vesicles (more than about 30%). Generally, the bubbles in scoria are bigger than those in pumice, and the rock looks darker overall.

Dike

Is a tabular intrusion that cuts through pre-existing layering (bedding or foliation).

Tachylite

Is a vessicle-free mass consisting of more than 80% mafic glass. This rock is relatively rare compared to obsidian.

Tuff

Is one type of rock composed mostly of volcanic ash; larger fragments of pumice may be mixed in with the ash.

4.3 Summary

Magma rises because it's buoyant and because of pressure due to overlying rock. The rate of melt movement is affected by viscosity, which depends on composition and temperature. When molten rock enters a cooler environment, it freezes. The rate of cooling depends on the environment and on the shape of the magma body.

Why does magma rise?

Magma rises for two reasons. First, bouyancy drives magma upwards because molten rock is less dense than the surrounding solid rock. Second, magma rises because the weight of the overlying rock generates pressure at depth that squeezes magma upward.

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 a new, different magmas. For example, mixing felsic magma with mafic magma could produce intermediate magma.

Heat-transfer melting

Occurs when rising magma brings heat up with it and melts overlying or surrounding rock.

Peridotite

Of the major types of igneous rocks, this is the most common on earth, for it makes up the mantle, but it is relatively rare on the Earth's surface.

Island Arcs

Protrude from the ocean at localities where one oceanic plate subducts beneath another oceanic plate.

Partial Melting

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

Continuous Reaction Series

Refers to the sequence from calcium-rich to sodium-rich-plagioclase, in that each step yields a different version of the same mineral.

Discontinuous Reaction Series

Refers to the sequence of olivine, pyroxene, amphibole, biotite, k-feldspar/muscovite/quartz, in that each step yields a different kind of silicate mineral.

extrusive igneous rock

Rock that forms by the freezing of lava above ground, in contact with air or water, after it erupts or extrudes.

Glassy Igneous Rocks

Rocks made of a solid mass of glass, or of tiny crystals surrounded by glass. Glassy rocks typically fracture conchoidally (fractures are curved, like a clam shell). -In general form when melts cool very rapidly

lava flow

Some lava moves down the slope of a volcano as a syrupy red-yellow stream. (produce extrusive rocks)

Igneous texture

Specifies whether the rock consists of interlocking crystals, stuck-together fragments, or solid glass.

Decompression Melting

Takes place where mantle rock rises slowly, for as the rock moves up, its pressure becomes less (due to the decrease in overburden) while its temperature remains nearly unchanged (for rock acts as an excellent insulator).

Intrusive contact

The boundary between wall rock and any igneous intrusion.

Source Rock Composition

The composition of a melt reflects the composition of the solid from which it was derived. Not all melts form from the same source rock, so not all have the same composition.

Aphanitic

The crystals of fine-grained rocks are too small to identify with the naked eye.

4.5 Summary

The formation of igneous rocks can be understood in the context of plate tectonics. Flux melting happens in the mantle above subducting plates, and decompression melting takes place in the mantle within hot spots, and under both mid-ocean ridges and rifts. Melting of the mantle produces basalt. Other types of igneous rocks most commonly form where rising mantle magma interacts with continental crustal rock and heat-transfer melting takes place.

Viscosity

The resistance to flow (of a liquid affects the speed with which the liquid moves). -The viscosity of a given melt depends on temperature, volatile content, and silica content. -Hotter melt is less viscous than cooler melt -mafic melt is less viscous than felsic melt because relatively more silicon-oxygen tetrahedra occur in felsic melt.

Intermediate Melts

Their composition is part way between that of mafic and felsic melts.

Flood Basalts

Thick stacks of thin, broad lava flows.

Fractional Crystallization

This general process, by which different minerals grow in sequence so the melt composition changes progressively, as cooling takes place, is known as this.

4.2 Summary

Though the Earth is hot inside, the crust and mantle are solid, except in special places where pre-existing rock undergoes melting. Melting can be triggered by a decrease in pressure, addition of volatiles, and.or injection of hot magma from deeper below. Geologists classify magma based on its composition, specifically, the proportion of silica it contains.

Magma

Underground melt

volcanic ash

Very fine particles that blasted out of a volcano.

igneous rock

We refer to any rock formed by the solidifying of a melt as an igneous rock. Igneous rock is common at earth's surface, making up the entire oceanic crust and much of the continental crust.

Laccolith

When a intrusion starts to inject in between layers but then domes upward, it yields a blister-shaped intrusion known as this.

Crystalline Texture

When a melt solidifies, minerals in some rocks grow and interlock like pieces of a jigsaw puzzle.

Pyroclastic Rock

When pyroclastic debris becomes consolidated into a solid mass, due either to still-hot clasts welding together during accumulation, or to the cementation by minerals precipitating from groundwater long after accumulation, it becomes this type of rock.

Flux Melting

When volatiles mix with hot, dry rock, they cause chemical bonds to break, so that the rock begins to melt. -Occurs where volatiles enter hot mantle; this happens at subduction zones.

lava

melt that has emerged to the surface