Rocks and the Rock Cycle

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

Characteristics of magma• Magma is molten

rock that originates in the mantle, at depths of 100-350 km.

• Igneous rocks form from molten rock when it cools and crystallizes into the rock-forming minerals.

• Magma that reaches the surface is called lava.

Igneous RocksVolcanic (extrusive) igneous rocks form from lava on the surface.

Plutonic (intrusive) igneous rocks from magma beneath the surface and are exposed later

Volcanic Rock – Rapid rate of cooling • Volcanic rocks

crystallize from lava on the surface.

• Lava contacts air and water and cools rapidly, inhibiting large crystal formation.

• The most common volcanic rock is basalt.

Plutonic Rock – Slow rate of cooling

• Plutonic rocks crystallize from magma in the interior of the crust, which are later exposed.

• Magma is insulated and cools slowly, allowing large crystals to form.

• Most common plutonic rocks are from the granite family.

Igneous Rock Classification

• Texture– (overall appearance of rock, but

especially pertains to mineral crystal size)

– linked to how rock formed since crystal size depends on rate of cooling

• Mineral composition – chemical makeup of magma or lava

Texture

– Size and arrangement of crystals• Slow cooling rate promotes the growth

of fewer but larger crystals• Fast rate forms many small crystals• Very fast rate forms glass

– Overall appearance (vesicles, glassy etc.)

Phaneritic - Coarse-texture

Granite

Rhyolite

Aphanitic - Fine texture

Porphyritic – Bimodal crystal size

• Large crystals, phenocrysts, form slowly from magma.

• Magma erupts; crystals are suspended in an aphanitic matrix, the groundmass.

Obsidian

Glassy texture – indicates lack of crystalline structure due to rapid cooling

Granitic (Felsic) Composition• Composed of

primarily of light silicates

• Contains high amounts of silica (SiO2)

• Major constituents of continental crust.

Basaltic (Mafic) Composition

• Composed of dark silicates and calcium-rich feldspar

• More dense than granitic rocks

• Comprise the ocean floor as well as many volcanic islands.

Sediment• solid rock or mineral

fragments transported and deposited by water, wind, ice or gravity

• dissolved minerals which evaporate or precipitate from water, or are secreted by organisms

• accumulate over time as loose unconsolidated layer.

Sedimentary rocks form when sediment from weathered and eroded rock is compacted or

cemented into solid rock

Fig. 3-2, p.46

Sedimentary rocks• Formed by the lithification of

sediment • compaction by burial• cementation by minerals from ground water

• Sedimentary rocks contain evidence of past environments

• Provide information about climate• Often contain fossils

Clastic Sedimentary Rockscomposed of solid sediment from weathered rocks

conglomeratesandstone

organic shale

Chemical Sedimentary Rocks

• Composed of minerals precipitated from surface or ground water (chemical sediment).

• The upper picture shows rock salt on the Bonneville Salt Flats.

• The lower photo shows veins of rock gypsum embedded in mudstone.

• Both of these chemical sedimentary rocks are examples of evaporites.

Biogenic Sedimentary Rocks

• composed of sediment of biological origin (e.g. shell fragments, plant material).

• The upper photo is limestone, composed of an ancient coral reef in the Guadaloupe Mountains of Texas.

• The lower photo is of bituminous coal. • COAL

Coal Formation

Metamorphic rocks form when heat, pressure or chemical reaction alters existing rocks

Fig. 3-CO, p.44

MetamorphismThe transition of one rock into another by

temperatures and/or pressures unlike those in which it formed.

Metamorphic rocks are produced from• Igneous rocks• Sedimentary rocks• Other metamorphic rocks

Metamorphism• Progresses incrementally (low to high-

grade)

• Rock remains solid throughout process

• Agents of metamorphism– heat (between 150° and 800° C)– pressure (5 km below the surface the

pressure is about 1500 x greater than atmospheric pressure

– chemically-active pore fluids.

Heat – from the Geothermal Gradient or rising magma

Pressure from overlying rocks

Low grade to high grade Metamorphism (left-right)

Types of Metamorphism

• Contact or thermal metamorphism – usually caused by contact of a rising magma body with host rock

• Regional metamorphism (mountain building) – caused by tectonic plate interactions

• Hydrothermal metamorphism – chemical alterations from hot, ion-rich water

Changes due to metamorphism

• recrystallization of mineral grains and chemical precipitates

• Formation of new, stable minerals

• Foliation – parallel alignment of minerals due to pressure.

Foliation

Granite is a common parent rocks of Gneiss (“Nice”)

Igneous Granite Metamorphic Gneiss

Sandstone is the parent rock for quartzite

Sandstone - sedimentary Quartzite - metamorphic

Limestone is the parent rock for marble

Limestone - sedimentary Marble - metamorphic