Rock Cycle & Igneous Rocks

Chapter 5

Minerals are homogeneous, naturally occurring, inorganic solids. Each mineral has a definite chemical

composition and a characteristic crystalline structure.

A mineral may be a single element such as copper (Cu) or gold (Au), or it may be a compound made up of a number of elements.

About 3,500 different minerals have been described, but only about 24 are common.

Rocks are made up of one or more minerals.

1. Igneous Rocks: Formed from the cooling of molten rock.

2. Sedimentary Rocks: Formed in layers as the result of moderate pressure on accumulated sediments.

3. Metamorphic Rocks: Formed from older "parent" rock (either igneous or sedimentary) under intense heat and/or pressure at considerable depths beneath the earth's surface.

The inter-relationship among the rock types is called …

THE ROCK CYCLE.

Basically a “recycling machine” of rocks.

http://science.cc.uwf.edu/sh/curr/rockcyc/rockcyc.htm

Igneous rocks are called fire rocks and are formed either underground or above ground.

Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks.

Also formed when volcanoes erupt, causing the magma to rise above the earth's surface. When magma appears above the earth, it is called lava. Igneous rocks are formed as the lava cools above ground.

Show Rock Cycle Video

http://www.youtube.com/watch?v=aCnAF1Opt8M

Magma is liquid and usually less dense than the surrounding solid rock As it moves upward to cooler regions of the Earth, it crystallizes into an igneous rock.

Magma can cool on the Earth's surface, where it has erupted from a volcano called extrusive rock Or under the Earth's surface, where it has intruded older rocks called intrusive rock.

The composition of magma is limited to the eight common elements of the earth's crust and combine within a melt to form silicate minerals. These silicate minerals include

feldspars (plagioclase feldspar, potassium feldspar) quartz micas (muscovite, biotite) pyroxenes (augite) amphiboles (hornblende) olivine

These minerals make up over 95% of the volume of the common igneous rocks, making igneous rocks easy to identify.

Extrusive igneous rocks, or volcanic rocks, form when magma reaches the Earth's surface a volcano and cools quickly.

Most extrusive (volcanic) rocks have small crystals.

Examples include: basalt, rhyolite, and andesite.

Intrusive igneous rocks, or plutonic rocks, form when magma cools slowly below the Earth's surface. Most intrusive rocks have large, well-formed crystals.

Examples include: granite, gabbro, and diorite

Igneous rocks are recognized by: the interlocking texture of the grains the presence of vesicules (holes) in

extrusive igneous rocks may be dark-colored and heavy may display two grain sizes, one

much larger than the other

Use Bowen’s Scale to determine identity of igneous rocks.

1. Determine the color (indicates mineral composition)

2. Determine the texture (indicates cooling history) Pegmatitic = very large/very course grains Phaneritic = large/course grains (visible to the eye) Aphanitic = small/fine grains

(too small to identify with the naked eye) Porphyritic = fine grains mixed with larger grains Glassy = glass-like Vesicular = holes

Example: A light-colored, fine-grained rock = rhyolite

Coarse grained (phaneritic): Slow cooling

Porphyritic:Two phases of cooling:one very slow, one slow

Pegmatitic: Slow cooling plus high water content

Fine grained (aphanitic): Fast cooling

Porphyritic:Two phases of cooling:one slow,one fast

Glassy: Fast cooling plus high silica content

INTRUSIVE EXTRUSIVE

3rd Type – Volcanic – vesicular (bubbles/holes)

Ultramafic rocks are dominated by olivine and/or pyroxene.

Mafic rocks are dominated by plagioclase and pyroxene (even if you can't see them with the naked eye) and smaller amounts of olivine.

Intermediate rocks are roughly even mixtures of felsic minerals (mainly plagioclase) and mafic minerals (mainly hornblende, pyroxene, and/or biotite). There is little or no quartz.

Felsic rocks are mostly feldspar (especially K-feldspar), at least 10% quartz, and less than 15% mafic minerals (biotite, hornblende).

INTRUSIVE COMPOSITIONS EXTRUSIVE COMPOSITIONS

Peridotite =

Ultramafic

Gabbro =

Mafic

Diorite =

Intermediate

Granite =

Felsic

Basalt =

Mafic

Andesite =

Intermediate

Rhyolite =

Felsic

Rare occasions only. We will not learn these.

Igneous Texture and Composition Video:http://www.youtube.com/watch?v=laVDypLGdbs

Intro. to Sedimentary Rockshttp://www.youtube.com/watch?v=Etu9BWbuDlY

Intro. to Metamorphic Rockshttp://www.youtube.com/watch?v=1oQ1J0w3x0o