Understanding the Rock Cycle, Magma, and Igneous Rocks

What is a rock?

A rock is typically a mixture of several different materials, although a few types may include only one type of mineral.

What is a metamorphic rock?

Metamorphic rocks form when igneous or sedimentary rocks are heated and squeezed, causing unstable minerals to form new minerals.

What is the rock cycle?

The rock cycle involves the transformation of rocks through processes like melting, cooling, weathering, erosion, and sedimentation.

What eight elements make up the majority of magmas?

Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium, Magnesium.

What factors determine the composition of a magma?

The rock it was formed from and the conditions of that melting.

What are the two main mechanisms by which rocks melt?

Decompression melting and Flux melting.

What mineral forms at the highest temperature in Bowen's Reaction Series?

Olivine.

Does mafic magma have a relatively high or low silica content?

Mafic magma has a relatively low silica content.

Name three intrusive igneous bodies.

Stocks, batholiths, *****, sills, or pipe-like bodies.

What processes must take place to transform rocks into sediment?

Weathering, erosion, and deposition.

What processes normally take place in the transformation of sediments to sedimentary rock?

Sediments are buried, compacted, and cemented under pressure.

What are the processes that lead to the formation of a metamorphic rock?

Burial and heating of rock within the crust.

What is the significance of the term 'reaction' in Bowen's Reaction Series?

As temperature decreases, early-formed minerals may react with remaining magma to form new minerals.

Why is it common for plagioclase crystals to be zoned?

Calcium-rich plagioclase forms early, while sodium-rich varieties form as temperature drops.

What must happen within a magma chamber for fractional crystallization to take place?

Early-forming minerals sink to the bottom, separating from the remaining magma.

Explain the difference between aphanitic and phaneritic textures.

Aphanitic textures have crystals too small to see without a microscope; phaneritic textures have large enough crystals to see with the naked eye.

Explain the difference between porphyritic and pegmatitic textures.

Porphyritic rocks have two distinct crystal sizes from two cooling stages; pegmatitic rocks have consistently coarse crystals larger than 1 cm.

What is an extrusive rock with 40% Ca-rich plagioclase and 60% pyroxene?

Basalt.

What is an intrusive rock with 65% plagioclase, 25% amphibole, and 10% pyroxene?

Diorite.

What is an intrusive rock with 25% quartz, 20% orthoclase, and 50% feldspar?

Granite.

What is the difference between a concordant body and a discordant body?

A concordant body (sill) is parallel to layering; a discordant body (****) cuts across layering.

Why does a **** commonly have a fine-grained margin?

The margins cool quickly upon intruding into cold rock, forming small crystals.

What is the difference between a batholith and a stock?

A batholith has an exposed area greater than 100 km²; a stock has an exposed area less than that.

Describe two ways in which batholiths intrude into existing rock.

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What are batholiths?

Large intrusive igneous bodies that can intrude into existing rock by melting, stoping, or pushing the country rock aside.

Why is compositional layering common in mafic plutons?

Compositional layering forms when early-crystallizing minerals sink in non-viscous mafic magma.

What are the three types of rocks?

Igneous, sedimentary, and metamorphic.

How is the rock cycle driven?

By Earth's internal heat and surface processes driven by solar energy.

What is magma?

Molten rock that forms from partial melting of existing rock.

What are the two main processes of magma formation?

Decompression melting and flux melting.

What is the composition of mafic rocks?

Rich in iron, magnesium, and calcium, with about 50% silica.

What is the composition of felsic rocks?

Rich in silica (~75%) with lower levels of iron, magnesium, and calcium.

What happens as magma cools?

Silica tetrahedra polymerize into chains, increasing viscosity.

What does the Bowen reaction series predict?

The order of crystallization of magma as it cools.

How are igneous rocks classified?

Based on mineral composition and texture.

What characterizes felsic igneous rocks?

Less than 20% ferromagnesian silicates and varying amounts of quartz and feldspars.

What characterizes mafic igneous rocks?

More than 50% ferromagnesian silicates, primarily pyroxene.

What is a phaneritic texture?

A texture where crystals are visible to the naked eye.

What is a porphyritic texture?

A texture with large crystals in a matrix of smaller crystals due to two stages of cooling.

What is a pegmatitic texture?

A texture with very large crystals formed from extremely slow cooling or water presence.

What are the types of intrusive igneous bodies?

Irregular (stocks and batholiths), tabular (***** and sills), or pipe-like.

What is the size difference between batholiths and stocks?

Batholiths have exposed areas greater than 100 km², while stocks are smaller.

What is the orientation of sills and *****?

Sills are parallel to layering, while ***** cut across layering.

What is a chilled margin?

A feature of a pluton that intruded into cold rock.

What is andesite?

A volcanic rock of intermediate composition.

What does aphanitic mean?

An igneous texture characterized by crystals that are too small to see with the naked eye.

What is a batholith?

An irregular body of intrusive igneous rock that has an exposed surface of at least 100 km².

What is the Bowen reaction series?

The scheme that defines the typical order of crystallization of minerals from magma.

What does concordant mean in geology?

Parallel to pre-existing layering or foliation within a rock.

What is country rock?

The original rock of a region, into which younger rock (typically igneous) has been intruded.

What is decompression melting?

Melting of rock resulting from a reduction in pressure without a significant reduction in temperature.

What is diorite?

An intermediate intrusive igneous rock.

What does discordant mean?

A geological feature that is not parallel to any existing layering in the country rock.

What is a ****?

A tabular intrusive igneous body that is discordant to any existing layering in the country rock.

What is extrusive rock?

Igneous rock that cooled at the surface.

What does felsic mean?

Silica rich (>65% SiO2) in the context of magma or igneous rock.

What is flux melting?

Melting of rock that is facilitated by the addition of a flux (typically water) which lowers the rock's melting point.

What is fractional crystallization?

The sequential crystallization of minerals from magma, and the physical separation of early-forming crystals from the magma.

What is gabbro?

A mafic intrusive igneous rock.

What does igneous mean?

A rock formed from the cooling of magma.

What is intrusive rock?

An igneous rock that has cooled slowly beneath the surface.

What is a laccolith?

A concordant intrusion in which the central part has formed an upward dome.

What does mafic mean?

Silica poor (<45% SiO2) in the context of magma or igneous rock.

What is a mantle plume?

A plume of hot rock that rises through the mantle and reaches the surface where it spreads out.

What is metamorphism?

The transformation of a parent rock into a new rock as a result of heat and pressure.

What is partial melting?

The process during which only specific mineral components of a rock melt in response to changing conditions.

What does phaneritic mean?

A rock texture in which the individual crystals or grains are visible to the naked eye.

What is a phenocryst?

A relatively large crystal within an igneous rock.

What is a pipe in geology?

A cylindrical body of igneous rock, typically resulting from a feeder conduit to a volcano.

What is a pluton?

A body of intrusive igneous rock.

What does polymerize mean?

The formation of molecular chains within a fluid that lead to an increase in the fluid's viscosity.

What does porphyritic mean?

An igneous texture in which some of the crystals are distinctively larger than the rest.

What is rhyolite?

A felsic volcanic rock.

What is sedimentary rock?

Rock that has formed by the lithification of sediments.

What are sediments?

Unconsolidated particles of mineral or rock.

What is a sill?

An igneous intrusion that is parallel to existing layering in the country rock.

What is a stock?

An irregular pluton with an exposed area less than 100 km².

What is stoping?

The fracturing and incorporation of fragments of country rock as a magma body moves upward through the crust.

What does vesicular mean?

An igneous texture characterized by holes left by gas bubbles.

What is a xenolith?

A fragment of country incorporated into igneous rock, commonly as a result of stoping.