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”.

Rock Cycle Processes – Crystallization

Rock Cycle Processes - Weathering

Rock Cycle Processes - Lithification

Rock Cycle Processes - Metamorphism

Magma and Lava

Differences• magma is in the

interior of the earth; lava is at the surface.

• magma contains dissolved gases that escape from 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

Rate of cooling and crystallization process

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 process

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 process

If lava cools too rapidly, the regularly repeating crystalline structure does not form. This obsidian is not a crystalline solid, it is a glass.

Igneous Rock Classification Criteria:Texture and Mineral Composition

• Texture – overall appearance of rock based on size and arrangement of mineral crystals

• Texture indicates the environment in which the rock crystallized

• Crystal size primarily determined by rate of cooling– Extrusive (volcanic) rocks cooled at the surface

from lava– Intrusive (plutonic) rocks cooled at depth from

magma• Includes secondary factors such as vesicles (gas

bubbles) and volcanic inclusions (pyroclasts).

Aphanitic (fine-grained)

• Rapid rate of cooling

• Microscopic crystals

• May contain vesicles (holes from gas bubbles)

Phaneritic (Coarse-grained)

• Slow cooling• Crystals can be

identified without a microscope

• Crystals approximately the same size

Porphyritic – large crystals embedded in a fine-grained matrix

• Minerals form at different temperatures as well as differing rates

• Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass.

Glassy

• Very rapid cooling of molten rock

• Resulting rock lacks crystalline structure

• 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.

– Often appear more similar to sedimentary rocks

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

Igneous CompositionIgneous 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.

Felsic (Granitic) composition

Rhyolite Granite

•Composed of primarily of light silicates.•Contains up to 70% silica (SiO2).•Major constituents of continental crust.

Intermediate ( andesitic) composition

• Contains at least 25 percent dark silicate minerals.

• Andesite associated with explosive volcanic activity.

Igneous compositions• Ultramafic composition

– high in magnesium and iron.– Composed entirely of ferromagnesian

silicates.– Rarely found in crust; main constituent of

the upper mantle.

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

– 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.

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 and Magma Formation

• Formation of basaltic magmas• Most originate from partial melting of ultramafic

rock in the mantle.• Basaltic magmas form at mid-ocean ridges by

decompression melting or at subduction zones.

• Formation of andesitic or granitic magmas• Originate from partial melting of mafic or andesitic

material• Assimilation of surrounding crust as magma rises