Chapter 6: Magma, Igneous Rocks, and Mineral Review Flashcards

Comprehensive vocabulary flashcards generated from lecture notes on magma formation, igneous rock types, textures, intrusive/extrusive settings, and general mineralogy.

Igneous Rock

Rock that forms when hot molten rock (magma or lava) cools and freezes solid.

Magma

Molten rock located underground.

Lava

Molten rock located aboveground.

Volcano

A vent from which melt from inside the Earth spews out onto the planet's surface.

Pyroclastic Debris

Fragmented material that sprayed out of a volcano and landed on the ground or seafloor in solid form, which can cement together to form an igneous rock.

Phenocryst

A large or conspicuous crystal in a porphyritic rock, distinct from the groundmass

Extrusive Igneous Rock

Rock formed by the freezing of lava above ground, after it flows or explodes onto the surface and comes into contact with the atmosphere or ocean.

Intrusive Igneous Rock

Rock formed by the freezing of magma underground.

Wall Rock

The rock surrounding an igneous intrusion.

Magma Chamber

A space below ground filled with magma.

Geotherm

A graph showing how pressure and temperature interact inside the Earth, with temperature plotted on the x-axis and pressure on the y-axis.

Decompression Melting

Melting that takes place in locations where hot mantle rock rises to shallower depths (mantle plumes, beneath rifts, and beneath mid-ocean ridges), causing a decrease in pressure.

Volatiles

Substances (such as $\text{H}_2\text{O}$ or $\text{CO}_2$) that evaporate easily and can exist in gaseous forms at Earth's surface.

Flux Melting

Melting that occurs when volatiles mix with hot, dry rock and break chemical bonds at the rock's melting point.

Heat-Transfer Melting

Melting that occurs when hot magma from the mantle is pushed to the surface and conducts heat to surrounding wall rock, causing it to melt.

Mafic Melts

Melts containing a high proportion of $\text{MgO}$ and $\text{Fe}_2\text{O}_3$ relative to silica, with a $\text{SiO}_2$ content of $46-52\%$.

Ultramafic Melts

Melts containing an extremely high proportion of $\text{MgO}$ and $\text{Fe}_2\text{O}_3$ relative to silica, with a $\text{SiO}_2$ content of $38-45\%$.

Felsic Melts

Melts containing relatively high proportions of $\text{SiO}_2$ ($67-76\%\text{ SiO}_2$) compared to $\text{MgO}$ and $\text{Fe}_2\text{O}_3$; also called silicic melts.

Intermediate Melts

Melts whose chemical composition lies partway between mafic and felsic, containing $53-66\%\text{ SiO}_2$.

Partial Melting

The process by which only $2-30\%$ of a source rock melts into magma because temperatures at sites of magma production do not get high enough to melt the entire rock.

Assimilation

The process of magma contamination in which blocks of wall rock fall into a magma chamber and dissolve.

Magma Mixing

The process where different magmas formed from different source rocks enter and mix within the same magma chamber.

Viscosity

A liquid's resistance to flow, which affects the speed of movement.

Fractional Crystallization

The process by which magma becomes progressively more felsic as it cools because mafic minerals crystallize first.

Lapilli

Pyroclastic particles measuring $2-64\,\text{mm}$ in diameter (marble-sized), consisting of frozen lava clots, pumice fragments, or ash clumps.

Ash

Tiny glass shards formed when volcanic spray immediately freezes and is carried by wind.

Pyroclastic Flow

A fast-moving avalanche that occurs when volcanic ash and debris mix with air and flow down the side of a volcano.

Intrusive Contact

The boundary between new intrusive igneous rock and the surrounding wall rock.

Dike

A tabular intrusion that cuts across pre-existing layering (bedding or foliation), radiating from a volcano or forming from rifting.

Sill

A tabular, horizontal sheet intrusion that injects between pre-existing rock layers.

Laccolith

A blister-shaped igneous intrusion formed when magma injects between underground layers and pushes the overlying layers up into a dome.

Pluton

An irregular or blob-shaped igneous intrusion ranging up to tens of kilometers wide.

Batholith

An immense intrusive complex formed by the intrusion of numerous plutons in a region.

Stoping

The process by which magma intrudes, where blocks of wall rock break off and sink into the magma.

Xenolith

A relict block of wall rock enclosed within intrusive rock after the magma freezes.

Phaneritic

A coarse-grained crystalline igneous texture in which individual mineral crystals are visible.

Aphanitic

A fine-grained crystalline igneous texture in which individual mineral grains are too small to see with the naked eye.

Porphyritic

An igneous rock texture consisting of large crystals (phenocrysts) surrounded by fine crystals (groundmass).

Pegmatite

A coarse-grained (phaneritic) igneous rock containing exceptionally large crystals up to tens of centimeters across, typically occurring in dike-shaped intrusions.

Vesicles

Open holes in igneous rock formed by gas bubbles preserved as magma cools and solidifies.

Obsidian

Vesicle-free felsic glass that is usually black or brown and fractures conchoidally.

Pumice

A felsic glass rock in which $75-90\%$ of its volume consists of vesicles, formed from quickly cooled frothy lava.

Scoria

A mafic igneous rock with approximately $30\%$ of its volume composed of large vesicles, generally more glassy than pumice.

Tuff

A fine-grained pyroclastic rock composed of volcanic ash or ash mixed with pumice lapilli.

Volcanic Agglomerate

An accumulation of volcanic bombs and other relatively large chunks of igneous rock.

Volcano Breccia

A pyroclastic rock consisting of fragments of volcanic debris that accumulated in the air or formed as solidifying lava broke up during flow.

Volcanic Arc

A chain of volcanoes developing along convergent plate boundaries, usually on the edge of the overriding plate.

Luster

The property describing the way a mineral surface scatters light.

Conchoidal Fracture

Smoothly curving, clamshell-shaped fracture surfaces produced when minerals (such as quartz) or glass break.