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