Igneous Rocks

The Rock Cycle

The continuous and reversible processes that illustrates how one rock changes to another.

“One rock is the raw material for another”.

The Rock Cycle

Rock Cycle Processes – Crystallization

Rock Cycle Processes - Weathering

Rock Cycle Processes - Lithification

Rock Cycle Processes - Metamorphism

Magma and LavaDifferences• magma is in the

interior of the earth; lava is at the surface.

• magma contains volatiles, dissolved gases, that escape at the surface and so are not present in lava.

• magma cools very slowly; lava cool relatively rapidly, leading to differences in crystal size.

Similarities• Parent material of igneous rocks• Forms from partial melting of rocks at depth•both consist of melt and possibly solid crystals

Rate of cooling and crystallization

A slow cooling rate promotes an interlocking mass of mineral crystals, all visible to the naked eye. This granite crystallized from slow-cooling magma.

Rate of cooling and crystallization

A rapid rate of cooling doesn’t allow the mineral crystals to grow large enough to see with the naked eye. This basalt crystallized from lava.

Rate of cooling and crystallization

If lava cools too rapidly, the orderly repeating crystalline structure, associated with minerals, does not have time to form. This obsidian is not crystalline, instead, it is a considered a glass.

Igneous Rock Classification Criteria #1:Texture

Overall appearance of rock based on size and arrangement of mineral crystals– Texture indicates the environment in which the rock

crystallized since crystal size is determined by the cooling environment

– Extrusive (volcanic) rocks cooled quickly at the surface from lava

– Intrusive (plutonic) rocks cooled slowlyh at depth from magma

– Includes secondary factors such as vesicles (cavities left by escaping gas)

Aphanitic (fine-grained)

• Rapid rate of surface cooling results in microscopic crystals

• The top sample is rhyolite, which has the same compostion as granite

• Aphanitic rocks may exhibit a secondary vesicular texture, like this basalt, as gas escaped from the lava.

Phaneritic (coarse-grained)

• Slow cooling in Earth’s interior results in visible crystals, like this granite

• Crystals approximately the same size.

Pegmatitic (very coarse-grained)

• Composed of crystals >1 cm in diameter

• They form late in the crystallization stage of a magma.

Porphyritic - large crystals embedded in a fine-grained groundmass

• Crystallized in (at least) 2 different environments

• Large crystals, phenocrysts, crystallize first

• Matrix of smaller crystals, the groundmass, crystallize last

• See sample #10.

Glassy

• When molten rock cools too quickly, the resulting rock lacks crystalline structure and is called a glass, like the obsidian (top).

• A glassy-textured rock can also have a vesicular texture, like the pumice shown in the picture.

Pyroclastic– Various-sized

fragments ejected during a violent volcanic eruption

– the most common fragment is ash-sized and the resulting rock is called welded tuff.

– Often appear similar to sedimentary rocks

– Larger fragments form

Pyroclastic

Larger-sized fragments form other types of pyroclastic rocks, such as this vesicular scoria (see sample #7), made from cinder-sized fragments.

Decide if each igneous texture below indicates an extrusive (volcanic), or intrusive (plutonic) origin, based on its texture:

A and C are extrusive and B is intrusiveD probably started out in an intrusive environment and ended up in an extrusive environment

Igneous Rock Classification Criteria #2:Mineral Composition

Igneous rocks are composed primarily of silicate minerals

• Dark (ferromagnesian) silicates– Olivine Group– Pyroxene Group (Augite)– Amphibole Group (Hornblende)– Biotite Mica

• Light (nonferromagnesian) silicates– Quartz– Muscovite mica– Feldspars

Mafic (Basaltic) composition

– Composed of ferromagnesian silicate minerals and calcium-rich feldspar (e.g. labradorite).

– Approximately 50% silica (SiO2) content.

– More dense (heavy) than granitic rocks

– Comprise the ocean floor and many volcanic islands, although also found on continental crust as lava flows, and intrusive bodies.

Ultramafic - Peridotite

• The lowest silica (SiO2) content of the igneous rocks

• Composed entirely of ferromagnesian silicates, with a relatively high iron and magnesium content.

• Rarely found in crust; main constituent of the upper mantle.

Intermediate ( andesitic) composition

• Contains at least 25 percent dark silicate minerals.• Andesite, named after the Andes Mountains in

South America, is associated with explosive volcanic activity.

• None in our collection

Felsic (Granitic) composition

RhyoliteTrachyte porphyry

•Composed of primarily of light silicates •Granite not shown here, is a common example.•Contains up to 70% silica (SiO2).•Major constituents of continental crust.•Sample #10 is a felsic porphyry called trachyte.

Mineralogy of igneous rocks

Origin of Magma

• Earth’s crust and upper mantle primarily composed of solid rock.

• Earth’s outer core is considered molten, but magma has the same composition as mantle and crust, not iron core.

• Geologists conclude that magma originates when solid rock of crust and mantle melts.

• What causes this to happen?

Role of Heat: Geothermal gradient

• Rocks in lower crust and upper mantle are already near melting points.

• Any additional heat (e.g. basaltic magma beneath silica-

rich rocks) may induce melting.

What causes rock to melt• Role of pressure (see animation on

mantle melting)– Melting point increases with depth due to

increased pressure, so rocks that would melt on the surface remain solid at depth.

– Reducing the pressure lowers the melting temperature; decompression melting occurs.

– Occurs at divergent boundaries, where rock is buoyant and ascending so pressure is low.

Decompression melting

Figure 3.14

What causes rock to melt

• Role of volatiles (i.e. water and dissolved gases)

– Volatiles (primarily water) cause rocks to melt at lower temperatures.

– This is particularly important where wet oceanic lithosphere descends into the mantle.

– See animation on mantle melting addition of volatiles is called wet melting

Addition of volatiles (water) lowers the melting point of subducting plate

Bowen’s Reaction Series: Systematic

crystallization of silicate minerals based on their melting points

• High temperature silicates have high melting

points (up to 1200 degrees C):– first minerals to crystallize from molten rock, last

to melt from solid rock. – Includes ferromagnesian silicates and Ca-rich

plagioclase feldspar.

Bowen’s Reaction Series: Systematic crystallization of silicate minerals based on

their melting points

• Low temperature silicates have “low”melting points (as low as 750 degrees C– Last to crystallize from molten rock, and

first to melt from solid rock.– non-ferromagnesian silicates, Na-rich

plagioclase feldspar and K feldspar.

Partial Melting and Magma Formation

Partial melting – incomplete melting of rocks due to differences in mineral melting temperature

• Low temperature minerals melt first and form a magma which migrates upward. Low temperature minerals are the ones with a high silica content (quartz, feldspars etc.)

• Therefore, the product magma or rock of partial melting always is more silica-rich (i.e. granitic) composition than the parent rock.

Partial melting of the solid rock on the left results in a more mafic solid portion and a more felsic magma that is free to rise.

• Mafic (basaltic) magmas originate from partial melting of ultramafic rock in the mantle.

• These magmas form at mid-ocean ridges by decompression melting and at hot spots.

• Much of the oceanic crust is formed by partial melting of ultramafic material.

Formation of Mafic Magmas

These magmas originate from:• partial melting of subducting mafic ocean crust which has

had water added to it. • assimilation of pieces of continental crust, which is usually

felsic.• What is the composition of the Andes of South America?

Formation of Andesitic and Felsic Magmas