Igneous Rock

Magma – Molten rock under the surface Temperature = 600o – 1400oC

Magma Chamber – Large pool of magma in the lithosphere

Magma chamber

- most all magma consists of silicon and oxygen (silicate) and the other 6 common elements.

Aluminum = Al Iron = Fe Calcium = Ca Sodium = Na Potassium = K Magnesium = Mg

- magma rises to the surface because it is less dense than the surrounding material. Why is it less dense?

- when magma solidifies it is than called igneous rock - extrusive igneous rock: magma that cools at or near

the surface - intrusive igneous rocks: magma that cools and

crystallizes below the surface - 95% of the crust is made of igneous rock

- magma at the surface is called lava

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- lava flows - pyroclastic materials ejected particles

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Composition of Magma - defined by silica content

Felsic - Silica rich magma; - more than 65% silica; abundant sodium, potassium, aluminum

Mafic - Silica poor magma

- 45% to 52% silica; abundant calcium, iron, magnesium

Intermediate - Compositions between felsic and mafic

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- Viscosity: Resistance to Flow

- temperature is a factor - composition is a more important control

- formation of silica tetrahedra networks controls viscosity - silica rich (felsic) magma/lavas are thick, viscous and resist flow - silica poor (mafic) magma/lavas are thinner, have a lower viscosity and don’t resist flow

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Origin of Magma

- Bowen’s Reaction Series - shows how mafic, intermediate, and felsic magmas could derive from an original parent mafic magma

- Discontinuous branch - Continuous branch

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- The Origin of Magma at Spreading Ridges

- (a) Melting temperature rises with increasing pressure - (b) Melting temperature decreases when water is present

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- The Origin of Magma at Spreading Ridges

- Melting is initiated by a pressure decrease at spreading ridges - Presence of water also decreases melting temperature - Partial melting explains how mafic magmas are derived from an ultramafic source

- The Origin of Magma at Subduction Zones

- Partial melting of a mafic crust results in intermediate and felsic magmas - Melting of sediments and contamination with silica rich continental crust rocks also change the magma composition

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Figure 4.6 The Origin of Magma: Magma forms beneath spreading ridges, because as plates separate, pressure is reduced on the hot rocks and partial melting of the upper

mantle begins. Invariably, the magma formed is mafic. Magma also forms at subduction zones where water from the subducted plate aids partial melting of the upper mantle. This

magma is also mafic, but as it rises, melting of the lower crust makes it more felsic.

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- The Origin of Magma at Hot Spots

- Hot mantle rock rises - Decrease in pressure melts mantle rock, creating

magma

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A mantle plume beneath oceanic crust with a hot spot. Rising magma forms a series of

volcanoes that become younger in the direction of plate movement.

A mantle plume with an overlying hot spot yields flood basalts, and some of the continental crust melts to form felsic magma.

- Chemical Changes in Magma

- Crystal settling: the physical separation of minerals by crystallization and gravity. Mafic minerals form first, leaving the melt richer in silica.

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Early formed ferromagnesian silicates such as olivine crystallize and because of their density settle to the bottom of the

Ferromagnesian silicates continue to form and settle.

The remaining melt becomes richer in silicon, sodium, and potassium because much of the iron and magnesium originally present is now in the

- Assimilation: when magma reacts with preexisting country rock, changing the composition of the magma

- Mixing: when two magma chambers come into contact and mix with each other

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! Dark inclusions in granitic rocks in the Sierra Nevada in California.

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Fragments of rock dislodged by rising magma may melt and become incorporated into the magma, a process called assimilation, or they may remain

as inclusions. Magma mixing is shown where mafic magma is injected into felsic

Igneous Rocks

- Igneous Rocks form from crystallizing from a melt, or by explosive volcanic activity

- Igneous Rock Textures

- Refers to the size, shape, and arrangement of mineral grains - Size relates to cooling rate, and indicates an intrusive or extrusive origin

- Aphanitic: fine-grained, to small to see, rapid cooling - Phaneritic: coarse grained, slow cooling - Porphyritic: phenocrysts (big crystals) and groundmass, two-stage cooling history - Glassy: no crystal structure/no grains visible - Vesicular: gas cavities - Pyroclastic: fragments generated by explosive volcanism. Fine grain ash that becomes consolidated and solid

- Texture depends on:

1) rate of cooling 2) rate of loss of volatiles (water, sulfur, gases..) 3) amount of silica content(SiO2) – the more silica

the more viscous (thicker) the melt

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- Effects of Cooling

1) Slow cooling = coarse grained ex. granite 2) Rapid cooling = fine grained ex. Rhyolite 3) Very rapid cooling (quenching) = Glass 4) Slow to fast cooling = slow cooling followed

by fast cooling. Porphyritic

Matrix (groundmass)

Phenocrysts

- Cooling rate controlled by:

1) size of mass of magma 2) shape or surface area 3) intrusive (slow cooling) vs. extrusive (fast

cooling)

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aphanitic

! aphanitic porphyritic phaneritic porphyritic

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phaneritic

phenocryts

- Classifying Igneous Rocks - Based on texture and composition - Chart shows relative proportions of chief mineral components and the textures of some common igneous rocks

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Classification of Igneous Rocks: This diagram shows the percentages of minerals, as well as the textures of common igneous rocks. For example, an

aphanitic (fine-grained) rock of mostly calcium-rich plagioclase and pyroxene is basalt.

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- Common Intrusive and Extrusive Igneous rocks

Extrusive (Aphanitic texture)

Intrusive (Phaneritic texture)

Ultramafic

Peridotite: makes up the upper mantle. High in Mg & Fe; low in silica

Mafic

Basalt: makes up the upper part of the oceanic crust, lava flows, volcanoes

Gabbro

Intermediate

Andesite Diorite

Felsic

Rhyolite Granite: basement rock for most continents-rk below sedimentary rock

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- Common Igneous Rocks for which texture is the main consideration

Intrusive Igneous Bodies

- Country rock: rock that is cut into by other rock

- Concordant: pluton that has boundaries parallel to the layering in the country rock

- Discordant: pluton that has boundaries that cut across the layering in the country rock

- Plutons: usually Granite - forms when magma cools and crystallizes within the

crust

Composition Felsic Mafic

Texture Vesicular Pumice Scoria

Glassy Obsidian

Pyroclastic or

Fragmental

Volcanic Breccia

Tuff/Welded tuff

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- different types: defined by their shape and relationship to the country rock

- Dikes and Sills - Laccoliths - Volcanic pipes/necks - Batholiths/stocks

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- Batholiths - largest of all plutons - covers more than 100km2 (60 miles2)

- Stock - pluton covering less than 100 km2 - maybe a segment of a larger pluton

- Dike - Discordant Igneous rock - Dike swarm: a group of dikes

- Sill - Concordant Igneous rock

- How to tell the difference between a lava flow and a sill?

- vesicles indicate flow - intrusion of sill: both top and bottom rock affected - chill margin: flows only on bottom sills on top and bottom

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- Laccoliths - concordant, but not tabular like a sill; have a

mushroom-like shape

- Volcanic pipe - connects crater to magma chamber

- Volcanic neck - is what remains after all the material

surrounding the volcanic pipe erodes

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Devils Tower, Wyoming Devils Tower National Monument was the first area in the United States so designated. The tower rises more than 260 m above its base and it can be seen from 48 km away. The vertical lines result from intersections of fractures called columnar joints (see Chapter 5), but according to Cheyenne legend, a gigantic grizzly bear made the scratches. Traditionally, Devils Tower was thought to be a volcanic neck, but it now seems more likely that it is an eroded laccolith.

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Volcanic Neck Shiprock, a volcanic neck in northwestern New Mexico, rises nearly 550 m above the surrounding plain. One of the dikes radiating from Shiprock is in the foreground.

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- Origin of Batholiths - assimilation: large igneous bodies melt their way into the crust/country rock - limited: too little heat in magma to assimilate large amounts of country rock - emplacement: magma forces its way up and pushes the country rock aside. Country rock falls underneath the magma.

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Emplacement of a Hypothetical Batholith As the magma rises, it shoulders aside and deforms the country rock.

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- stoping: when magma rises into the crust/country rock by breaking off and engulfing pieces of country rock.

! Stoping takes place when magma rises into the crust by

detaching and engulfing pieces of country rock.

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Some of the detached blocks may be assimilated, and some may remain as inclusions

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Igneous Rock (texture)

Intrusive Extrusive

Coarse Grain Fine grain Phaneritic Aphanitic Slow Cooling Fast Cooling Really Slow Cooling = Pegmatite Really Fast Cooling = glass Ex. Gabbro, Diorite, Granite Ex. Basalt, Andesite, Rhyolite

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Mafic

45-52% Low Silica Rich in Fe & Mg High Melting Point

Low Viscosity (thin) Low Water

Felsic

> 65% High Silica Low in Fe & Mg Low Melting Point High Viscosity (thick)