Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks

Physical Geology, Chapter 3

Quiz 1Monday

Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks

Physical Geology, Chapter 3

Rocks and the Rock Cycle• A rock is a naturally formed, consolidated

material usually composed of grains of one or more minerals.

• There are three types of rocks:– Igneous, metamorphic, and sedimentary

• Each type has different physical appearance (texture)

The Rock Cycle• The rock cycle shows how

one type of rocky material gets transformed into another– Representation of how rocks are

formed, broken down, and processed in response to changing conditions

– Processes may involve interactions of geosphere with hydrosphere, atmosphere and/or biosphere

– Arrows indicate possible process paths within the cycle

•Types of igneous rocks are closely related to the type of magma and the tectonic environment.

•For the most part, volcanic activity is closely linked to interaction between plates (tectonic activity).

The Rock Cycle and Plate Tectonics• Magma is created by melting of rock above a subduction zone

• Less dense magma rises and crystallizes to form igneous rock

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The Rock Cycle and Plate Tectonics• Igneous rock exposed at surface gets weathered into sediment

• Sediments transported to low areas, buried and harden (undergo lithification) into sedimentary rock

The Rock Cycle and Plate Tectonics• Sedimentary rock heated and squeezed at depth to form

metamorphic rock

• Metamorphic rock may heat up and melt at depth to form magma

Igneous Rock Formation • Igneous rocks form when molten rock

crystallizes (cools and solidifies)– Molten rock underground

• magma

– Molten rock on the Earth’s surface • lava

Cooling Basaltic Lava

Diagram of a Magma Chamber

Cooling Basaltic Lava on the surface of the Earth

Diagram of a Magma Chamber

1. Based on this information, what is one way to classify igneous rocks?

Igneous Rocks Classification• Texture or physical appearance is

one way to classify igneous rocks– Texture depends on cooling rate

• Slower cooling rates mean that crystals have more time to form and thus will be larger

More time

Less time

Igneous Rocks Classification• Texture or physical appearance is one way to

classify igneous rocks– Texture depends on cooling rate

• Newton's law of cooling states that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings

One more thing….

A. Fine-grained igneous rock

B. Coarse-grained igneous rock

2. Intrusive

1. Extrusive

2. Match rock texture, cooling rate, and environment

a. “Fast” cooling b. “Slow cooling

Igneous Rocks Formation• Intrusive igneous rocks form when

magma solidifies underground• Granite is a common example

• Extrusive igneous rocks form when lava solidifies at the Earth’s surface

• Basalt is a common example

3. What differences do you see in these two rocks that could have been used for classifying (naming) them?

Granite

Basalt

Igneous Rock Classification• Igneous rock names are based on:

– mineral (chemical)

composition– texture (grain size)

Igneous Rock Chemistry

• Rock chemistry, particularly silica (SiO2) content, determines mineral content and general color of igneous rocks

• Two general end members: Mafic Felsic

Mafic rocks• ~50% silica by weight

• contain dark-colored minerals that are abundant in magnesium (Ma), iron (Fe), and calcium

• Gabbro = coarse-grain, intrusive

• Basalt = fine-grain, extrusive

Igneous Rock Chemistry

Felsic (silicic) rocks • >65% silica (SiO2) by weight

• contain light-colored minerals, such as feldspars and silica, abundant in silica, aluminum, sodium, and potassium

• Granite = coarse-grain, intrusive

• Rhyolite = fine-grain, extrusive

Igneous Rock Chemistry

Intermediate rocks have silica contents between those of mafic and felsic rocks

• Diorite = coarse-grain, intrusive

• Andesite = fine-grain, extrusive

Igneous Rock ChemistryUltramafic rocks

• < 45% silica by weight

• composed almost entirely of dark-colored ferromagnesian minerals

• Most common ultramafic rock is peridotite (intrusive)– Mostly olivine with minor amounts of Ca-rich feldspar and pyroxene

Igneous Rock Classification

Intrusive

Extrusive

SAME chemical

composition

DIFFERENT cooling rates and

environments

DIFFERENTrock names!

Igneous Rock Textures• Texture refers to the physical appearance of the rock

– size, shape and arrangement of grains or other constituents within a rock

– Fine-grained texture = aphanitic

– Coarse-grained texture = phaneritic

• Texture of igneous rocks is primarily controlled by cooling rate

Aphanitic igneous rock Phaneritic igneous rock

Igneous Rock Textures• Extrusive igneous rocks cooled

quickly at or near Earth’s surface are typically aphanitic or fine-grained (most crystals <1 mm)

• Intrusive igneous rocks cooled slowly deep beneath Earth’s surface are typically phaneritic or coarse-grained (most crystals >1 mm)

…But, what if magma started cooling underground, forming a “mush” of molten rocks, and then erupted?

Coarse-grained (phaneritic), intrusive igneous rock

Fine-grained (aphanitic), extrusive igneous rock

Some Other Igneous Textures

• Igneous rocks with porphyritic texture have two distinct crystal sizes, indicating that the rock underwent a two-stage cooling process.– Larger crystals

(phenocrysts) formed first during slow cooling underground

– Smaller crystals (matrix or ground mass) formed during more rapid cooling on or near the Earth’s surface

phenocryt

Porphyritic Rhyolite

matrix

Some Other Igneous Textures

• A pegmatite is an extremely coarse-grained igneous rock (most crystals >5 cm) formed when magma cools very slowly at depth

• A glassy texture contains no crystals at all, and is formed by extremely rapid cooling (quenching)

Intrusive Rock Bodies• Intrusive rocks exist in bodies or structures that penetrate or

cut through pre-existing country rock

• Intrusive bodies are given names based on their size, shape and relationship to country rock

– Deep intrusions: Plutons

– Shallow intrusions: Dikes, sills, volcanic necks

Intrusive Rock Bodies• Intrusive rocks exist in bodies

or structures that penetrate or cut through pre-existing country rock

• Intrusive bodies are given names based on their size, shape and relationship to country rock

– Deep intrusions: Plutons• Form at depth beneath Earth’s

surface when rising blobs of magma (diapirs) get trapped within the crust

• Crystallize slowly in warm country rock

• Generally coarse-grained rocks

Deep Intrusive Rock Bodies• Pluton

– Large, blob-shaped intrusive body formed of coarse-grained igneous rock, commonly granitic in composition

– Small plutons (exposed over <100 km2) are called stocks, large plutons (exposed over >100 km2) are called batholiths

Sierra Nevada batholith

Intrusive Rock Bodies• Intrusive bodies are given names based on their size, shape and

relationship to country rock– Shallow intrusions: Dikes, sills, volcanic necks

• Form <2 km beneath Earth’s surface

• Chill and solidify fairly quickly in cool country rock

• Generally composed of fine-grained rocks

Intrusive Rock Bodies• Volcanic neck

– Shallow intrusion formed when magma solidifies in throat of volcano

• Dike– Tabular intrusive structure

that cuts across any layering in country rock

• Sill– Tabular intrusive structure

that parallels layering in

country rock

Igneous Rock Identification• Igneous rock names are based on texture (grain size) and

mineralogic composition

• Textural classification– Plutonic rocks (gabbro-diorite-granite) are coarse-grained and cooled

slowly at depth

– Volcanic rocks (basalt-andesite-rhyolite) are typically fine-grained and cooled rapidly at the Earth’s surface

• Compositional classification– Mafic rocks (gabbro-basalt) contain abundant dark-colored

ferromagnesian minerals

– Intermediate rocks (diorite-andesite) contain roughly equal amounts of dark- and light-colored minerals

– Felsic rocks (granite-rhyolite) contain abundant light-colored minerals

•The Earth is not homogeneous (the same throughout). It has a layered structure, and the layers have different chemical compositions (differentiated).

How Magma Forms

• The mantle is solid and is has a basaltic chemical composition high in ferromagnesium silicates

• Basically, magmas form through melting of the mantle

4. How can a solid become molten?

How Magma Forms

How Magma Forms• Addition of heat

– Heat transferred upward (by conduction and convection) from the very hot (>5000°C) core through the mantle and crust

– Rate at which temperature increases with increasing depth beneath the surface is the geothermal gradient

• Gradient is not the same everywhere

How Magma Forms• Decrease pressure

– Melting point of minerals generally increases with increasing pressure

– Decompression melting can occur when hot mantle rock moves upward and pressure is reduced enough to drop melting point to the temperature of the rising rock body

How Magma Forms• Add water or other contaminant

– Hot water under pressure• Water becomes increasingly

reactive at higher temperatures

• At sufficient pressures and temperatures, highly reactive water vapor can reduce the melting point of rocks by over 200°C

– Mineral mixtures• Mixtures of minerals, such as

quartz and potassium feldspar, can result in the melting of both at temperatures hundreds of degrees lower than either mineral would melt on its own

• The mantle is solid and is has a basaltic chemical composition

• Basically, magmas form from melting of the mantle

• Mantle melts due to:

– Increased heat

– Decreased pressure

– Addition of water

How Magma Forms

Magma Crystallization and Melting Sequence

• Minerals crystallize in a predictable order (and melt in the reverse order), over a large temperature range, as described by Bowen’s Reaction Series

Magma Crystallization and Melting Sequence8

• Bowen’s Reaction Series has two branches.

– Discontinuous branch• Ferromagnesian minerals (olivine,

pyroxene, amphibole, biotite) crystallize in sequence with decreasing temperature

• As one mineral becomes chemically unstable in the remaining magma, another begins to form

– Continuous branch• Plagioclase feldspar forms with a

chemical composition that evolves (from Ca-rich to Na-rich) with decreasing temperature– Often forms zoned crystals

Bowen’s Reaction Series

Lessons from Bowen’s Reaction Series• Large variety of igneous rocks is produced by large

variety of magma compositions

• Mafic magmas will crystallize into basalt or gabbro if early-formed minerals are not removed from the magma

• Intermediate magmas will similarly crystallize into diorite or andesite if minerals are not removed

• Separation of early-formed ferromagnesian minerals from a magma body increases the silica content of the remaining magma

– Differentiation or fractional crystallization

– Can produce granites from basaltic magma

• Minerals melt in the reverse order of that in which they crystallize from a magma

Magma Evolution• A change in the composition of a

magma body is known as magma evolution

• Magma evolution can occur by:– differentiation,

– partial melting,

– assimilation,

– or magma mixing

Magma Evolution• Differentiation or fractional crystallization

involves the changing of magma composition by the settling of denser early-formed ferromagnesian minerals

• The removal of some components means that the relative % of the other components remaining in

the magma increases (enriched)

Magma Evolution• Incomplete or partial melting produces magmas less mafic

than their source rocks\– Minerals melt in the reverse order of that in which they crystallize

from a magma

– Lower melting point minerals are more felsic in composition

Magma Evolution

• Assimilation occurs when a hot magma melts and incorporates more felsic surrounding country rock

Magma Evolution• Magma mixing involves

the mixing of more and less mafic magmas to produce one of intermediate composition

Putting it all together: Igneous Activity and

Plate Tectonics

• Igneous activity occurs primarily at or near tectonic plate boundaries

• Mafic igneous rocks are commonly formed at divergent boundaries

– Increased heat flow and decreased overburden pressure (decompression melting) produce mafic (basaltic) magmas from melting of the mantle

Igneous Activity and Plate Tectonics

• Intermediate igneous rocks are commonly formed at convergent boundaries

– Partial melting of basaltic oceanic crust due to the addition of water produces intermediate magmas

Igneous Activity and Plate Tectonics

• Felsic igneous rocks are commonly formed adjacent to subduction zones (convergent boundary)

– Water from subducting crust lowers melting point

– Resulting hot rising magma causes partial melting and assimilation of the granitic continental crust

Igneous Activity and Plate Tectonics

• Intraplate volcanism– Rising mantle plumes can

produce localized hotspots and volcanoes when they produce magmas that rise through oceanic or continental crust

• Hawaii is an example of intraplate volcanism in oceanic crust

• Yellowstone is an example of intraplate volcanism in continental crust

Rock Starting Magma

Ending Magma

Melting Processes Tectonic Setting

Basalt Gabbro

Mafic Mafic Mantle melting due to decreased pressure

or

increased heat

Divergent

Hot spot (intraplate)

Andesite

Diorite

Mafic (usually)

Intermediate Partial melting of mantle through addition of water

(assimilation, differentiation,

or magma mixing)

Subduction zones

Rhyolite

Granite

Felsic (silicic)

Felsic Partial melting of lower crust Convergent boundaries

Hot spot

Magma, Rock Types, and Plate Tectonic Setting

End of Chapter 3……Wednesday, Chapter 4

Volcanism and Extrusive Igneous rocks