GEOL 101 – Chapter 4: Magma, Igneous Rocks and Intrusive Activity

Form as molten rock (magma) cools and solidifies

Parent material of igneous rocks Forms from partial melting of rocks Magma at surface is called Lava

Magma at surface

Dissolved gases in the melt that vaporize at surface pressure

The cooling of magma which results in the systematic arrangement of ions into orderly patterns

Magma that crystallizes at depth These rocks are observed at the surface following periods of uplifting and erosion of overlying rocks. ** Inside the earth**

The solidification of lava or volcanic debris **Outside the Earth**

Light-colored silicates Composed almost entirely of quartz and potassium feldspar Termed felsic (feldspar and silica) in composition High silica (SiO2) content Major constituent of continental crust

Dark silicates and calcium-rich felspar Termed mafic (magnesium and ferrum, for iron) in composition Higher density than granitic rocks Comprise the ocean floor and many volcanic islands

Contain 25% or more dark silicate minerals (amphibole, pyroxene, and biotite mica)

Rare composition of mostly olivine and pyroxene Composed almost entirely of ferromagnesium minerals Peridotite is an example

The overall appearance of a rock based on the size, shape, and arrangement of mineral grains.

Rapid rate of cooling Microscopic crystals

Slow cooling Large, visible crystals

Some minerals can grow large before others form from the magma The magma can move to a different environment which causes the remaining minerals to form quickly Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)

Large crystals

Smaller crystals

Rocks contain voids left by gas bubbles in the lava Common feature of an extrusive igneous rock

Very rapid cooling Ions are frozen in place before they can unite in an orderly crystalline structure Once used as arrowheads, they used in scalpels (less scarring)

Forms from the consolidation of individual rock fragments ejected during explosive eruptions

Exceptionally coarse-grained Form in late stages of crystallization of magmas Rocks with this texture are called pegmatites

Coarse-grained (phaneritic) One of the best known igneous rocks Very abundant Natural beauty, especially when polished 10% - 20% quartz, roughly 50% potassium feldspar Small amounts of dark silicates Some granites have a porphyritic texture Contain elongated feldspar crystals

Extrusive equivalent of granite Composed essentially of light-colored silicates Typically pink or light gray in color Less common and less voluminous than granite

Dark-colored, glassy rock Forms when silica-rick lava cools quickly at Earth's surface Usually black to reddish-brown Similar chemical composition of granite Dark color is the result of small amounts of metallic ions in an otherwise clear, glassy substance

Glassy textured rock that forms when large amounts of gas escape from lava Voids are quite noticeable Resembles fine shards of intertwined glass Typically found in deposits with obsidian Will float when placed in water

Medium-gray, fine-grained rock Volcanic origin Commonly exhibits a porphyritic texture

Intrusive equivalent of andesite Coarse-grained rock Looks like gray granite, but lacks visible quartz crystals Can have a salt-and-pepper appearance

Very dark green to black, fined-grained rock Composed mostly of pyroxene and calcium-rich plagioclase feldspar When porphyritic, contains small, light-colored feldspar phenocrysts Most common extrusive igneous rock Upper layers of oceanic crust are composed of basalt

Intrusive equivalent of basalt Very dark green to black, coarse grained rock Composed mostly of pyroxene and calcium-rich plagioclase feldspar Uncommon on the continental crust, but makes up a significant portion of the oceanic crust

Temperatures in the upper crust increase about 25 degree C per kilometer

Melting occurring at higher temperatures with increasing depth Reducing pressure lowers the melting temperature Solid, hot mantle rocks will ascend to regions of lower pressure, inducing melting

Minerals crystallize in a systematic fashion based on their melting points As minerals crystallize, the composition of the liquid portion of the magma continually changes

Earlier-formed minerals and denser than the liquid portion of the magma and sink to the base of the magma chamber

The formation of one or more secondary magmas from a single parent magma

As magma migrates through the crust, it may incorporate some of the surrounding rock

During the ascent of two chemically different magma bodies, the more buoyant mass may overtake the slower-rising body

Cooled, emplaced magma into preexisting rocks Classified by their orientation to the surrounding rock

Table-like

Cut across existing structures

Parallel to features like sedimentary strata

Irregularly shaped

a tabular, discordant pluton Serves as tabular conduits to transport magma Parallel groups are called dike swarms

A tabular, concordant pluton Tend to accumulate magma and increase in thickness Closely resembles buried lava flows

Largest intrusive body Surface exposure of 100+ square kilometers (smaller bodies are termed stocks) While expansive, most are less than 10km thick

Suspended blocks of country rocks found in plutons

Forcibly injected between sedimentary strata Causes the overlying strata to arch upward

Igneous rocks are formed by the cooling and crystallization of molten rock.

How quickly the molten rock cools and crystallizes, and where this occurs with respect to Earth's surface.

A fine-grained texture is the product of rapid cooling and crystallization of lava.

Intrusive Below the surface

The igneous rock exhibits mineral crystals too small to see with the naked eye.

Liquid Portion Solid Portion Gaseous Portion

Muscovite and Pyroxene

Greater silica Lower melting temperature

Mafic igneous rocks are rich in Fe, Mg, and Ca.

Gabbro

Quartz, K-feldspar, and plagioclase feldspars, with relatively fewer biotite and amphiboles.

Silicates

Upper Mantle

Quartz and Potassium Feldspar

Lava cools rapidly.

Vesicular

Pegmatitic

The rock was likely formed by a violent volcanic eruption.

Porphyritic

Obsidian

Batholith

The surface of a lava flow

Fine-grained

The dark crystals formed first, undergoing slow cooling, and the remainder cooled quickly.

Olivine Mantle

A mineral that makes up a relatively small portion of the total rock composition.

Granite is phaneritic and rhyolite is aphanitic.

Andesite Porphyry

The both exhibit a vesicular texture.

The introduction of water

Higher melting points are determined by higher pressures

Geothermal Gradient

Assimilation Magma mixing Crystal settling Magmatic differentiation

Bowen's Reaction Series

Olivine

Quartz

Felsic magma and mafic residue

Quartz

Basaltic magma partially melts the continental crust, which is more felsic in composition.

Decomposition melting of the mantle

The melting temperature of mantle rocks to decrease.

Heat from basaltic magma partially melting overlying crust

Dike

Sill

Batholith

Because batholiths form deep within Earth, the occurrence of one at the surface implies that if has since been uplifted and/or the rocks above have been removed.

Fine-grained