Physical Geology: Chapter 4- Igneous Rocks

Sills

- Igneous layers

Bowen's Reaction Series

- Predicts the sequence of minerals that appear from magma - Two branches 1. Silicate tetrahedra link to form different minerals 2. Plagioclase changes composition - Assumes one homogeneous magma cools

Batholiths, Plutons, Stocks

- Regions of granite classified by size

Veins

- Smaller dikes

Dikes

- Cut across layers - Can be vertical, in sheets or ring

Dike

-A sheetlike discordant igneous intrusion that cuts across layers of bedded country rock -the major route of magma transport in the crust.

ultramafic rocks

An igneous rock consisting primarily of mafic minerals and containing less than 10 percent feldspar

Igneous rocks occur at

1. Divergent Plate Boundaries • Mid-ocean ridges • Continent-continent divergent boundaries 2. Convergent Plate Boundaries • Subduction zones 3. Hot Spots • Mantle plumes

porphyry

An igneous rock of mixed texture in which large crystals (phenocrysts) "float" in a predominantly fine-grained matrix

Differentiation

: processes that occur in the magma chamber • Rocks of different composition arise from magma • First minerals to crystalize are last to melt

andesite

An intermediate igneous rock with a composition between that of dacite and that of basalt; the extrusive equivalent of diorite

diorite

A coarse-grained intermediate igneous rock with a composition between that of granodiorite and that of gabbro; the intrusive equivalent of andesite

peridotite

A coarse-grained, dark greenish gray, ultramafic intrusive igneous rock composed primarily of olivine with smaller amounts of pyroxene and other minerals such as spinel or garnet; the dominant rock in Earth's mantle and the source rock of basaltic magmas

gabbro

A dark gray, coarse-grained igneous rock containing an abundance of mafic minerals, particularly pyroxene; the intrusive equivalent of basalt

basalt

A dark, fine-grained, mafic igneous rock composed largely of plagioclase feldspar and pyroxene; the extrusive equivalent of gabbro

Obsidian

A dense, glassy volcanic rock, usually of felsic composition

granite

A felsic, coarse-grained igneous rock composed of quartz, orthoclase feldspar, sodium-rich plagioclase feldspar, and micas; the intrusive equivalent of rhyolite

batholith

A great irregular mass of intrusive igneous rock that covers at least 100 km2; the largest type of pluton

pluton

A large igneous intrusion, ranging in size from a cubic kilometer to hundreds of cubic kilometers, formed deep in the crust

magma chamber

A large pool of magma that forms in the lithosphere as rising magmas melt and push aside surrounding solid rock

rhyolite

A light brown to gray, fine-grained felsic igneous rock; the extrusive equivalent of granite

granodiorite

A light-colored, coarse-grained intermediate igneous rock that is similar to granite in containing abundant quartz, but whose predominant feldspar is plagioclase, not orthoclase; the intrusive equivalent of dacite

dacite

A light-colored, fine-grained intermediate igneous rock with a composition between that of rhyolite and that of andesite; the extrusive equivalent of granodiorite

stock

A pluton less than 100 km2 in area

magmatic differentiation

A process by which rocks of varying composition arise from a uniform parent magma as various minerals are withdrawn from it by fractional crystallization as it cools, changing its composition

Pyroclast

A rock fragment ejected into the air by a volcanic eruption

sill

A sheetlike concordant igneous intrusion formed by the injection of magma between parallel layers of bedded country rock

vein

A sheetlike deposit of minerals precipitated in fractures or joints in country rock, often by a hydrothermal solution

pegmatite

A vein of extremely coarse-grained granite, crystallized from a water-rich magma in the late stages of solidification, that cuts across much finer grained country rock and may contain rich concentrations of rare minerals

ophiolite suite

An assemblage of rocks, characteristic of the sea-floor but found on land, consisting of deep-sea sediments, submarine basaltic lavas, and mafic igneous intrusions

discordant intrusion

An igneous intrusion that cuts across the layers of the country rock it intrudes

concordant intrusion

An igneous intrusion whose boundaries lie parallel to layers of bedded country rock.

How does magmatic differentiation account for the variety of igneous rocks?

Because different minerals crystallize at different temperatures, the composition of magma changes as it cools and various minerals are withdrawn by crystallization.

Fractional crystallization

Crystals form in the magma are segregated from the remaining liquid

Mafic rocks

Dark-colored igneous rock containing minerals such as pyroxenes and olivines that are rich in iron and magnesium and relatively poor in silica

How are igneous rocks classified?

Igneous rocks can be divided into two broad textural classes: coarse-grained rocks, which are intrusive and therefore cooled slowly; and fine-grained rocks, which are extrusive and cooled rapidly. Igneous rocks can also be classified on the basis of their silica content using a scale that runs from felsic (rich in silica) to ultramafic (poor in silica).

partial melting

Incomplete melting of a rock that occurs because the minerals that compose it melt at different temperatures

What are the forms of igneous intrusions?

Large intrusive igneous bodies are called plutons. The largest plutons are batholiths, which are thick horizontal masses extending from a funnel-shaped central region. Stocks are smaller plutons. Less massive than plutons are sills, which lie parallel to the layers of bedded country rock, and dikes, which cut across those layers. Veins form where water is abundant, either in the magma or in the surrounding country rock.

Lava

Magma that flows out onto Earth's surface.

How do plate tectonic processes affect magma production?

Magmas are produced at two types of plate boundaries. At spreading centers, peridotite rises from the mantle and undergoes decompression melting to form basaltic magma. At subduction zones, subducting oceanic lithosphere undergoes fluid-induced melting to generate magmas of varying composition. Mantle plumes within lithospheric plates are also sites of decompression melting that produce basaltic magmas.

How and where do magmas form?

Magmas form at places in the lower crust and mantle where temperatures are high enough for partial melting of rock. Because the minerals within a rock melt at different temperatures, the composition of magmas varies with temperature. Pressure raises the melting temperature of rock, and the presence of water lowers it. Because melted rock is less dense than solid rock, magma rises through the surrounding rock, and drops of magma come together to form magma chambers.

fluid-induced melting

Melting of rock induced by the presence of water, which lowers its melting point

volcanic ash

Pyroclasts less than 2 mm in diameter, usually glass, that form when escaping gases force a fine spray of magma from a volcano.

Magma Chamber

Rising mass of magma pushes aside crustal rocks as it rises through the crust

fractional crystallization

The process by which the crystals formed in cooling magma are segregated from the remaining liquid rock, usually by settling to the floor of the magma chamber

Country rock

The rock surrounding an igneous intrusion

decompression melting

The spontaneous melting of rising mantle material as it reaches a level where pressure decreases below a critical point, without the introduction of any additional heat

Ultramafic

Very dark or green

Kimberlite

Volcanic pipes, contain diamonds

Mafic

darker colored • biotite, amphibole, pyroxene, olivine • Silica frameworks, double and single chains, isolated tetrahedra

Vesicles

holes from escaping gases

Felsic

lighter colored • quartz, feldspar, muscovite • silica frameworks, sheets

Pumice

volcanic rock made of glass and ash

Tuff

volcanic rock with glass, ash, and rock fragments

Intermediate

• Can't say light OR dark • Roughly equal amounts of both colors