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Rock Cycle & Igneous Rocks

Chapter 5

Minerals vs. Rocks

• 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.

Three Types of Rocks1. 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 Rock CycleThe 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

Stop here for 1st Lecture

Show Rock Cycle Video

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Igneous Rocks• 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.

Magma• 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 Rock

Extrusive igneous 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 Rock

Intrusive, or plutonic, igneous 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

How to tell if you have an igneous rock…

• 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

Bowen’s Reaction Series

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Steps in IdentificationUse Bowen’s Scale to determine identity of

igneous rocks.

1.Determine the color (indicates mineral composition)

2.Determine the texture (indicates cooling history) – Phaneritic=large grains – Aphanitic=small 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

Textures

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)

Igneous Composition• 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.

6.1 Formation of Sedimentary Rocks

Weathering and Erosion

• Sediments: small pieces of rock that are moved and deposited by water, wind and

gravity

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Weathering

• Weathering produces sediment from rock and mineral fragments

• Weathering can be either chemical or physical in nature

• Sediments produced by weathering can range in size from a microscopic particle to a huge boulder

• During physical weathering the minerals remain chemically unchanged

Erosion: removal and transport of sediment

Agents of erosion:• Wind• Moving water• Gravity• Glaciers

Deposition

• Occurs when transported sediments are deposited on the ground or sink to the bottom of a body of water

• Settles with the largest grains at the bottom of the layering and the smallest grains at the top

Energy of transporting agents

• Fast moving water moves larger particles better

• As the water slows down, the largest particles settle out first followed by the next largest particles

• Wind can only move small particles (this is why sand dunes only contain fine sand)

Lithification:

• Sediment is deposited in low areas such as valleys and ocean basins– Bottom layers are subjected to intense

pressure and temperatures– This process is called lithification: THE

PHYSICAL AND CHEMICAL PROCESSES THAT TRANSFORM SEDIMENT INTO SEDIMENTARY ROCK

Lithification: COMPACTION• The weight forces the sediment grains to get

closer and closer together causing physical changes to occur

• Mud can contain up to 60% water that gets squeezed out

• Sand does not compact as much as mud does because sand is mostly quartz

• Grain to grain contact in sand forms a supporting framework that maintains open space between the grains

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Lithification: CEMENTATION

• Minerals like calcite CaCO3 and iron oxide Fe2O3 flow through the open spaces left by compaction

• The minerals linger and cement the grains together forming sedimentary rock

Sedimentary Features:

• Sedimentary rock tells geologists the history of the area

• BEDDING: predominant feature of sedimentary rock is the horizontal layering– Feature results from the

way water or wind causes the sediment to settle out

– Two types of bedding

Bedding Type #1: Graded Bedding• Bedding in which heavier and coarser particles

are located near the bottom of the sedimentary rock

Bedding Type #2: Cross-Bedding• Inclined layers of sediment deposited along a

horizontal surface

Other “Bedding” Types

• Mud Cracks– Polygonal cracks

formed in drying mud

– Form in sediments that are sometimes exposed to air

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Other “Bedding” Types• Ripple Marks

– Small ridges formed on the surface of sediment layer by moving wind or water

– May be symmetrical or asymmetrical

• symmetrical ripples – wave ripples

• asymmetrical ripples –water or wind current ripples

– Often found in siltstone and mudstone

Fossils

• The best known feature of sedimentary rock is the propensity to find fossils in it

• During lithification the animal’s parts (like the shell) can be replaced by minerals and turned to rock which make up a fossil

Types of Sedimentary Rock

• Clastic / Detrital – made up of solid particles (gravel, sand, silt and clay) derived from preexisting rocks through weathering

• Chemical – sedimentary rocks that result from inorganic chemical processes or from the chemical activities of organisms

Clastic / Detrital Sedimentary Rocks

• Coarse-grained– Gravel sized fragments– Types of rock formed: conglomerate (rounded gravel), breccia

(angular gravel)– Formed by high-energy flows of water

• Medium-grained– Sand fragments– Type of rock formed: sandstone– Formed by stream & river channels, beaches, deserts– Importance: due to pore spacing, fluids can move through and thus

hold reservoirs of oil, natural gas, and groundwater• Fine-grained

– Silt and mud fragments– Type of rock formed: siltstone, shale, mudstone– Importance: low porosity resulting in a barrier to movement of

groundwater & oil.

Chemical Sedimentary Rock

• Inorganic Sedimentary Rock – Evaporite – form as a

result of crystal grains precipitating out of a supersaturated body of water

– Type of rock formed: rock gypsum (made up of the mineral gypsum) & rock salt(made up of the mineral halite)

Chemical Sedimentary Rock

• Biochemical – form as a result of the chemical processes of organisms, many times the remains of once-living plants and/or animals.– Types of rock formed:

limestone, chert, and coal.– Form in shallow-water,

swamp, and coastal environments

chert

coal

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Importance of Sedimentary Rock

• Provide historical timeline for area. Leaves a ‘footprint’ of all that’s come before us.– Past plants and animals– Bedrock– Ancient rivers, lakes and shorelines

• Provides resources– Oil, natural gas, and coal– Building materials– Uranium– Phosphate and iron

Metamorphic Rocks

Metamorphic Rock

• Meta means change• Morphe means form• A metamorphic rock forms when high

temperature and pressure combine and change the texture, mineral composition or chemical composition of a rock without melting it

Metamorphic Textures• Metamorphic Rocks are classified into two

textural groups1) Foliated- layers and bands (left)2) Nonfoliated- blocky crystal shapes (right)

Grades of Metamorphism

• Low-grade metamorphism is associated with low temperatures and pressures

• High-grade metamorphism is associated with high temperatures and pressures

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Types of Metamorphic Rock

1) Regional Metamorphism2) Contact Metamorphism3) Hydrothermal Metamorphism

Regional Metamorphism

• When a large belt regionally has metamorphed

• Results include changes in mineral and rock types, plus folding and deformation of rock layers

• Mountains can be an example of regional metamorphism

Contact Metamorphism

• When molten material (like an igneous intrusion) comes into contact with solid rock contact metamorphism has occurred

• Lava cools so contact metamorphism from lava occurs most near the lava

• As the distance from an intrusion increases it is harder for contact metamorphism to occur since the temperature has dropped

Hydrothermal Metamorphism• When hot water reacts

with rock and alters its chemical and mineral composition hydrothermal metamorphism has occurred

• Ore deposits of gold, copper, zinc, tungsten and lead are formed in this manner

Economic Importance of Metamorphic Rocks and Minerals

• Metamorphism of ultrabasic igneous rocks produce the minerals talc and asbestos

• Graphite can be formed by the metamorphism of coal