Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks Physical Geology, Chapter 3

Quiz 1 Monday

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

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

Diagram of a Magma Chamber Cooling Basaltic Lava

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

Cooling Basaltic Lava on the surface of the Earth Diagram of a Magma Chamber

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….

2. Match rock texture, cooling rate, and environment

1. Extrusive A. Fine-grained igneous rock

B. Coarse-grained igneous rock 2. Intrusive

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

(SiO 2 ) 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 (SiO 2 ) 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 Chemistry

Ultramafic 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

SAME chemical composition  Intrusive DIFFERENT cooling rates and environments DIFFERENT rock names!

 Extrusive

• •

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) Fine-grained (aphanitic), extrusive igneous rock …But, what if magma started cooling underground, forming a “mush” of molten rocks, and then erupted?

Coarse-grained (phaneritic), intrusive 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

Some Other Igneous Textures

• A

pegmatite

is an extremely coarse-grained igneous rock (most crystals >5 cm) formed when magma cools

very slowly

depth at • 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 km 2 ) are called

stocks

, large plutons (exposed over >100 km 2 ) 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

mineralogic

composition

(grain size) and – –

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

How Magma Forms

•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

• – –

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

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

•

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

How Magma Forms

• • • 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

•

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.

Bowen’s Reaction Series • 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

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 diorite

or magmas will similarly crystallize into

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

than their source rocks\ produces magmas less mafic 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

produce one of involves the mixing of more and less mafic magmas to 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

Magma, Rock Types, and Plate Tectonic Setting

Rock Starting Magma Ending Magma Basalt Gabbro Mafic Mafic Andesite Diorite Rhyolite Granite Mafic (usually) Felsic (silicic) Intermediate Felsic Melting Processes Mantle melting due to decreased pressure or increased heat Partial melting of mantle through addition of water (assimilation, differentiation, or magma mixing) Partial melting of lower crust Tectonic Setting Divergent Hot spot (intraplate) Subduction zones Convergent boundaries Hot spot

End of Chapter 3… …Wednesday, Chapter 4

Volcanism and Extrusive Igneous rocks