Rocks

1. Igneous - Crystallized from hot, molten rock. Examples: granite, basalt

2. Sedimentary - Fragments of sediment laid down by water or wind become compressed or cemented over time Examples: sandstone, shale, limestone

3. Metamorphic - Rocks changed by heat and/or pressure or chemical activityExamples: gneiss, schist, slate, marble

The Rock Cycle

Through the rock cycle, one type of rock can be converted into another.

Igneous Rocks

• The word igneous means "fire-formed."• Igneous rocks crystallized from hot, molten magma or

lava, as it cooled. – Magma is hot, molten rock beneath the surface of the Earth.– Lava is hot, molten rock which has flowed out on the surface of

the Earth.

• Igneous rocks make up more than 90% of Earth's crust, by volume.

Extrusive Igneous Rocks

Extrusive or volcanic rocks form from lava, which cooled on the Earth's surface.

Examples: Basalt, rhyolite, andesite, obsidian

Intrusive Igneous Rocks

Intrusive or plutonic igneous rocks form from magma which cooled beneath the surface of the Earth.

• Examples: Granite, gabbro, diorite

Cooling History and Grain Size

• The texture of a rock is a description of its grain size. • Cooling rates influence the texture of the igneous rock. • Lava cools much more quickly than magma because

lava is on the surface of the Earth, where temperatures are much lower than they are at depth.

Cooling History and Grain Size

Extrusive rocks = quick cooling = fine grained Intrusive rocks = slow cooling = coarse grained

Extrusive vs. Intrusive

Rhyolite - fine-grained, extrusive igneous rock.

Granite - coarse-grained, intrusive igneous rock.

Igneous Rock Classification

Igneous rocks are classified on the basis of:

1. Texture (or grain size) 2. Composition

Igneous Rock Composition Groups

1. Silica-rich2. Intermediate3. Silica-poor

Silica-rich Rocks1. High percent silica. 2. Light-colored. 3. Has light-colored minerals such as quartz and

potassium feldspar.

Examples: granite, rhyolite.

Intermediate Rocks1. Intermediate in composition between silica-rich and

silica-poor. 2. Mixture of light and dark minerals.

Examples: diorite, andesite.

Silica-poor Rocks1. Iron and magnesium rich. 2. Dark-colored. 3. Has dark minerals such as olivine, pyroxene, and

amphibole.

Examples: gabbro, basalt.

Very silica-poor Rocks1. Very iron and magnesium rich. 2. Typically green in color due to abundant olivine.

Example: Peridotite.

Igneous Rock Classification Silica-rich (silicic)

Intermediate Silica-poor (mafic)

Very silicapoor(ultramafic)

Fine

Rhyolite Andesite Basalt

Coarse

Granite Diorite Gabbro Peridotite

Basalt

• The most common igneous rock.• Ocean crust is dominated by basalt. Covers about 70%

of Earth's surface. • Islands like Hawaii and Iceland are made of basalt. • Fine-grained texture • Dark color because it contains ferromagnesian (Fe and

Mg) minerals, along with feldspar.

Granite

• Earth's continental crust is dominated by granite. • Coarse-grained texture.• Light color because it is dominated by light-colored

minerals like quartz and feldspar.

Bowen's Reaction Series

Minerals in igneous rocks crystallize in a particular order, at particular temperatures.

Sedimentary Rocks

• Cover about 75% of the world's land area.

• Form when loose sediment (gravel, sand, silt or clay) becomes compacted and/or cemented to form rock.

• The process of converting sediment to sedimentary rock is called lithification.

Sediment is deposited in horizontal layers. A major characteristic of sedimentary rock is layering, also called bedding or strata.

Sedimentary Rocks

Sedimentary rocks contain the fossil record, which preserves the evolving story of life on Earth.

Sedimentary Rocks

What can sedimentary rocks tell us?

• Locations of ancient sedimentary environments (seas, reefs, deltas, beaches, rivers, lakes deserts, glaciers, and mountains).

• Ancient climates– humid tropical coal swamps, – dry windswept deserts, – glacial ice sheets, – high temperatures and high sea levels.

Sedimentary rocks also hold the fossil fuels and energy resources on which our culture depends - coal, oil, natural gas. Careful reading of the rock record allows exploration geologists to find these critical resources.

Sedimentary Rocks

How is sediment formed?

Sediment forms from the weathering and erosion of rocks, as part of the rock cycle.

Weathering of granite in a humid climate

1. Feldspars undergo hydrolysis to form clay. 2. Biotite and amphibole undergo hydrolysis to form clay, and oxidation

to form iron oxides.3. Na, Ca, and K ions are lost in solution and washed away. 4. Small amounts of Si from feldspars, biotite, and amphibole are lost

in solution. 5. Quartz remains as sand grains due to its resistance to weathering.

Fate of the Weathering Products

• Clay minerals form shale• Iron oxides form cement, ochre, or iron ore• Dissolved Na, Ca, and K ions form limestone,

evaporites, or become included in shale• Dissolved Si ions form chert, silica cement, or

diatomite• Unaltered quartz grains form sandstone

Types of Sedimentary Rocks

• Clastic Sedimentary Rocks (also called terrigenous or detrital)

• Chemical / biochemical Sedimentary Rocks

• Organic Sedimentary Rocks (Coal)

Types of Sedimentary Rocks1. Clastic sedimentary rocks (also called terrigenous or

detrital) – Conglomerate or Breccia – Sandstone – Siltstone – Shale or Claystone

2. Chemical/biochemical sedimentary rocks 3. Organic sedimentary rocks (coal)

Types of Sedimentary Rocks1. Clastic sedimentary rocks (also called terrigenous or

detrital) 2. Chemical/biochemical sedimentary rocks

– Evaporites – Carbonate sedimentary rocks (limestone and dolostone or

dolomite) – Siliceous sedimentary rocks (chert, diatomite)

3. Organic sedimentary rocks

Types of Sedimentary Rocks1. Clastic sedimentary rocks (also called terrigenous or

detrital) 2. Chemical/biochemical sedimentary rocks3. Organic sedimentary rocks

– Peat – Lignite– Bituminous coal – Anthracite coal

Clastic Sedimentary Rocks

Clastic sedimentary rocks are derived from the weathering of pre-existing rocks, which have been transported to the depositional basin.

Clastic Texture

• Clasts (larger pieces, such as sand or gravel) • Matrix (mud or fine-grained sediment surrounding the clasts)

• Cement (the chemical "glue" that holds it all together) Types of cement:

• Calcite • Iron oxide • Silica

Clastic Sedimentary Rocks are Classified by Grain Size

• Gravel - Grain size greater than 2 mm • Sand - Grain size 1/16 to 2 mm • Silt - Grain size 1/256 to 1/16 mm • Clay - Grain size less than 1/256 mm

Clastic Sedimentary Rocks are classified by grain size

Grain size Rock name

Gravel Conglomerate = rounded clasts Breccia = angular clasts

Sand Sandstone

Silt Siltstone

Clay Shale = fissile Claystone = massive

Chemical/Biochemical Sedimentary Rocks

Form within the depositional basin from chemical components dissolved in the seawater.

Chemicals are removed from seawater and made into rocks by chemical processes, or biological processes (such as shell growth).

Chemical/Biochemical Sedimentary Rocks

1. Evaporites - form from the evaporation of water 2. Carbonate rocks - form by chemical processes and

biochemical processes 3. Siliceous rocks - form from chemical processes (silica

replacing limestone) or biochemical processes (silica-secreting organisms)

Evaporites

1. Rock salt - composed of halite (NaCl). 2. Rock gypsum - composed of gypsum (CaSO4 .

2H2O)

3. Travertine - composed of calcium carbonate (CaCO3) – a carbonate rock; forms in caves and around hot springs.

Carbonate Rocks 1. Limestones

– Micrite (microcrystalline limestone) – Oolitic limestone – Fossiliferous limestone – Coquina – Chalk – Crystalline limestone – Others

2. Dolostones or dolomites

Siliceous rocks • Diatomite - made of microscopic planktonic organisms

called diatoms. Resembles chalk, but does not fizz in acid.

• Chert - massive and hard, microcrystalline quartz. May be dark or light in color. Often replaces limestone. Does not fizz in acid.

Organic Sedimentary Rocks - Coal

Composed of organic matter (plant fragments). With increasing depth of burial (temperature and

pressure): • Peat • Lignite • Bituminous coal • Anthracite coal

Organic Sedimentary Rocks - Coal

• Coal is a fossil fuel. Electric utility companies use more than 90% of the coal mined in the U.S.

• Chemicals derived from coal are used in making plastics, tar, synthetic fibers, fertilizers, and medicines.

Metamorphic Rocks

• Metamorphic means "changed form."• Metamorphism causes changes in the texture and

mineralogy of rocks.• Rocks are changed or metamorphosed by:

1. High temperatures 2. High pressures 3. Chemical reactions caused by solutions and hot gases

Types of Metamorphism

1. Contact metamorphism Alteration of rock by heat adjacent to hot molten lava or

magma.2. Regional metamorphism Alteration of rock over a large area by heat and

pressure due to deep burial or tectonic processes.

Types of Metamorphic Rocks

Metamorphic rocks are separated into two groups on the basis of texture. • Foliated• Non-foliated (or granular)

Foliation = Laminated structure in a metamorphic rock resulting from the parallel alignment of sheet-like minerals (usually micas).

Foliated Metamorphic Rocks

In order of increasing grade of metamorphism:• Slate• Phyllite • Schist • Gneiss

Foliated Metamorphic Rocks

Slate - Mica flakes are microscopic in size. Derived from the regional metamorphism of shale. Note the relict sedimentary bedding (vertical).

Phyllite - Mica flakes are very fine-grained; other minerals such as garnet or staurolite may also be present. Derived from the regional metamorphism of shale.

Foliated Metamorphic Rocks

Schist - Mica flakes are visible to the unaided eye. Derived from the regional metamorphism of shales or fine-grained volcanic rocks.

Foliated Metamorphic Rocks

Gneiss - Coarse-grained rock with minerals segregated into light and dark layers or bands. Derived from the regional metamorphism of high-silica igneous rocks, and muddy sandstones.

Non-foliated Metamorphic Rocks

Non-foliated or granular metamorphic rocks are composed of equidimensional grains such as quartz or calcite. There is no preferred orientation. The grains form a mosaic.

Non-foliated Metamorphic Rocks

Marble - Composed of finely- to coarsely-crystalline calcite or dolomite. Derived from the metamorphism of limestone or dolostone. Commonly white or gray. May be pink.

Non-foliated Metamorphic Rocks

Quartzite - Composed of finely- to coarsely-crystalline quartz. Derived from the metamorphism of quartz sandstone.

Non-foliated Metamorphic Rocks

Greenstone - Contains iron- and magnesium-rich green minerals such as chlorite and epidote. Fine-grained texture. Derived from the low-grade metamorphism of basalt.

Non-foliated Metamorphic Rocks

Hornfels - Very hard, fine-grained rock. Derived from the contact metamorphism of shale and other fine-grained rocks.

Metamorphic Index Minerals

Certain minerals form during metamorphism, under specific pressure and temperature conditions. These minerals can be used as a guide to metamorphic pressures and temperatures. They are called metamorphic index minerals.

Metamorphic Index Minerals

• Chlorite and muscovite form at relatively low temperatures.

• Biotite and garnet form at somewhat higher temperatures and pressures.

• Staurolite and kyanite form at intermediate to high temperatures and pressures.

• Sillimanite forms at the highest temperatures and pressures.

Metamorphic Index Minerals

From studies of minerals in metamorphic rocks it is possible to determine the conditions under which the rocks formed.

Metamorphic Index Minerals

Recap: The Rock Cycle