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