Chapter 4 magma, Igneous, rocks and intrusive Activity

Rocks in the lower crust and upper mantle

are near their melting points

Crustal rocks exhibit a considerable range

(40 percent to 70 percent)

Light (or nonferromagnesian) silicates

Contain more potassium, sodium, or calcium than iron and magnesium

Example andesitic or intermediate composition

Andesite

Types of Igneous Textures

Aphanitic(fine-grained) texture phaneritic(coarse grained) texture

Assimilation

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

Pyroclastic Rocks Welded tuff

Ash particles are hot enough to fuse together Can contain walnut-sized pieces of pumice and other rock fragments Covers vast portion of previous volcanically active areas of the western United States

example of Basaltic or mafic composition

Basalt Gabbro Scoria

Igneous Rocks Classification

Based on texture and composition

Pyroclastic Rocks Tuff

Common pyroclastic rock Composed of ash-sized fragments cemented together

Pyroclastic Rocks

Composed of fragments ejected during a volcanic eruption

Pyroclastic Rocks Volcanic breccia

Composed of particles larger than ash

Andesitic or intermediate composition

Contain 25 percent or more dark silicate minerals (amphibole, pyroxene, and biotite mica) Associated with volcanic island arcs

Granitic (Felsic) Igneous Rocks Granite

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

Igneous rocks are composed primarily of silicate minerals

Dark (or ferromagnesian) silicates Light (or nonferromagnesian) silicates

Basaltic or mafic composition

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

Granitic (Felsic) Igneous Rocks Obsidian

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

there are three ways to create magma

Decrease in pressure Introduction of water Heating crustal rocks above their melting temperature

Crystal settling

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

example Phaneritic (coarse-grained) texture

Diorite

Classification of plutons Tabular—table-like

Discordant Concordan

Magma mixing

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

Example of Light (or nonferromagnesian) silicates

Examples include quartz, muscovite mica, and feldspars, quartz

Pegmatitic texture

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

Granitic (Felsic) Igneous Rocks Rhyolite

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

Laccoliths

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

Pyroclastic (fragmental) texture

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

Granitic (Felsic) Igneous Rocks Pumice

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

Example of Granitic or felsic composition

Gray granite rhyolite pumice

Magma is generated in the uppermost mantle

Greatest amounts are produced at divergent plate boundaries Lesser amounts are produced at subduction zones Can also be generated when crustal rocks are heated

Generating Magma from Solid Rock Temperature increase: melting crustal rocks

Heat from nearby magma sources can melt the surrounding crustal rocks Can also form melt from heat generated during continental collisions

extrusive

Igneous rocks which form by the crystallization of magma at the surface of the Earth. They are characterized by fine-grained textures because their rapid cooling at or near the surface did not provide enough time for large crystals to grow

Andesitic (Intermediate) Igneous Rocks Diorite

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

Basaltic (Mafic) Igneous Rocks Gabbro

Intrusive equivalent of basalt Very dark green to black, fine-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

Classification of plutons Massive

Irregularly shaped

phenocrysts

Large crystals

Batholith

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

Granitic or felsic composition

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

Magma consists of three componets

Liquid portion = melt Solids, if any, are crystals of silicate minerals Volatiles are dissolved gases in the melt that vaporize at surface pressure

Emplacement of batholiths

Magma at depth is much less dense than the surrounding rock In the mantle, the more buoyant magma pushes aside the host rock and rises in Earth through a process called shouldering

Andesitic magma

Magmatic differentiation of mantle-derived basaltic magma Can also form when basaltic magmas assimilate crustal rocks

Andesitic (Intermediate) Igneous Rocks Andesite

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

Decompression melting

Melting occurs 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

Bowen's reaction series

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

magma

Molten rock usually located deep within the mantle of the Earth that occasionally comes to the surface through cracks in the mantle or through the eruption of volcanoes

Granitic magmas

Most form when basaltic magma ponds beneath the continental crust Melted crustal rocks alter the magma composition Can form from magmatic differentiation of andesitic magma

primary or primitive magmas

Most originate from partial melting of mantle rocks at oceanic ridges

Emplacement of batholiths 2

Nearer to Earth's surface, the rocks are cooler and brittle Upward movement is accomplished by stoping, where the overlying blocks of country rock sink through the magma

Generating Magma from Solid Rock Addition of water

Occurs mainly at subduction zones As an oceanic plate sinks, heat and pressure drive water from the crust and overlying sediments Fluids migrate into the overlying wedge of mantle The addition of water lowers the melting temperature of the mantle rocks to trigger partial melting

columnar jointing

Occurs when igneous rocks cool and develop shrinkage fractures that produce elongated, pillar-like columns with 6 sides

Ultramafic composition

Rare composition of mostly olivine and pyroxene Composed almost entirely of ferromagnesium minerals

General characteristics of magma

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

Aphanitic (fine-grained) texture

Rapid rate of cooling Microscopic crystals

Factors affecting crystal size:

Rate of cooling Amount of silica Amount of dissolved gases

Example of Porphyritic texture

Rhyolite porphyry andesite porpyry

Dark (or ferromagnesian) silicates

Rich in iron and/or magnesium

Vesicular texture

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

Phaneritic (coarse-grained) texture

Slow cooling Large, visible crystals

Porphyritic texture

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)

Magmatic differentiation

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

volcanic or extrusive igneous rocks

The solidification of lava or volcanic debris forms

Concordan

are parallel to features like sedimentary

Partial melting

This process produces most magmas

Basaltic (Mafic) Igneous Rocks Basalt

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

Glassy texture

Very rapid cooling Ions are frozen in place before they can unite in an orderly crystalline structure

Most common volatiles in magma

Water vapor (H2O) Carbon dioxide (CO2) Sulfur dioxide (SO2)

Greatest amounts magma

are produced at divergent plate boundaries

Tabular Intrusive Bodies Dike

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

Batholith

a very large igneous intrusion extending deep in the earth's crust

intrusive

also called plutonic rock, igneous rock formed from magma forced into older rocks at depths within the Earth's crust, which then slowly solidifies below the Earth's surface, though it may later be exposed by erosion

Xenoliths

are suspended blocks of country rocks found in plutons

Granitic magmas have high silica content

are viscous (thick), and erupt at a lower temperature

Plutonic rocks are observed

at the surface following periods of uplifting and erosion of overlying rocks

Tectonic processes trigger melting

by reducing the melting point

Earth's crust and mantle are primarily

composed of solid rock

Discordant

cut across existing structures

Most magma that erupts

erupts is basaltic (mafic) magma

Partial melting of intermediate rocks yields

felsic magmas

slow Rate of cooling

fewer but larger crystal

Igneous rocks

form as molten rock (magma) cools and solidifies

The chemical makeup of an igneous rock can be inferred

from the silica content

granitic

is an igneous rock composed of mostly two minerals: quartz and feldspar. It is an intrusive rock, meaning that it crystallized from magma that cooled far below the earth's surface.

A pluton

is cooled, emplaced magma into preexisting rocks

Silica content as an

indicator of composition

laccolith

is a sheet intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.

mafic

is an adjective describing a silicate mineral or rock that is rich in magnesium and iron, and hence is a portmanteau of "magnesium" and "ferric". Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite.

Texture

is influenced by cooling history

Mineralogy

is influenced by the chemical composition of the parent magma

Crystallization

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

texture

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

geothermal gradient

is the rate of increasing temperature with respect to increasing depth in the Earth's interior. Away from tectonic plate boundaries, it is about 25 °C per km of depth (1 °F per 70 feet of depth) near the surface in most of the world.

Incomplete melting of rocks

known as partial melting

Silicon and oxygen atoms

link together first to form a silicon−oxygen tetrahedron (the basic building block of silicate minerals)

Silica content influences

magma behavior

Most originate from partial melting

mantle rocks at oceanic ridges

fast rate of cooling

many small crystal

Basaltic magmas have much lower silica content

more fluidlike behavior, and erupt at a higher temperature

example of glassy texture

obsidian

Example of Dark or ferromagnesian silicates

olivine, pyroxene, amphibole, and biotite mica

Pegmatitic texture

pegmatite

example Ultramafic composition

peridotite( main constituent of the upper mantle)

Magma that crystallizes at depth forms

plutonic or intrusive igneous rocks

igneous

produced under conditions involving intense heat, as rocks of volcanic origin or rocks crystallized from molten magma.

example of vesicular texture

pumice, vesicular basalt

felsic

refers to igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron (ferric

example Aphanitic (fine-grained) texture

rhyolite

Pyroclastic Rocks Names do not imply mineral composition and are identified with a modifier for example

rhyolitic tuff

Basaltic

s a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill.

sill

s a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock

lava

s the molten rock expelled by a volcano during an eruption

groundmass

smaller crystals

Tabular Intrusive Bodies Sill

tabular, concordant pluton Tend to accumulate magma and increase in thickness Closely resembles buried lava flows May exhibit columnar jointing

Geothermal gradient

temperatures in the upper crust increase about 25o C per kilometer

Country Rock

term meaning the rock native to an area. It is similar and in many cases interchangeable with the terms basement and wall rocks. The term is used to denote the usual strata of a region in relation to the rock which is being discussed or observed.

During partial melting

the melt is enriched in ions from minerals with the lowest melting temperature

When the remaining magma solidifies

the mineralogy will be different from the parent magma

Plutons are classified

their orientation to the surrounding rock

dike

type of later vertical rock between older layers of rock. Technically, it is any geologic body which cuts across: flat wall rock structures, such as bedding. massive rock formations, usually igneous in origin.

example Pyroclastic (fragmental) texture

welded tuff

Partial melting of mafic rocks

yields intermediate magmas

Partial melting of ultramafic rocks

yields mafic magmas