Chapter 4Chapter 4Rocks: Mixtures of MineralsRocks: Mixtures of Minerals

Rocks Rocks – A naturally occurring solid mixture of one or – A naturally occurring solid mixture of one or more minerals and organic matter.more minerals and organic matter.

The Rock CycleThe Rock Cycle – The continuous process by which – The continuous process by which new rock forms from pre-existing rock material. Rocks new rock forms from pre-existing rock material. Rocks will actually change their chemical composition.will actually change their chemical composition.WeatheringWeathering – The process in which wind, water, – The process in which wind, water, gravity and heat break down rocks to form fragments gravity and heat break down rocks to form fragments called sediment.called sediment.ErosionErosion – The process in which sediments are – The process in which sediments are washed or blown away from their source.washed or blown away from their source.DepositionDeposition – The process in which eroded material is – The process in which eroded material is dropped and comes to rest.dropped and comes to rest.

TheThe RockRock CycleCycle ActivityActivity

• Write a humorous story in your group about what it would be like to be a rock particle traveling through the rock cycle. Think of this as a story like “The Little Grain That Could”. Then you will present this as a group in class. The winning group gets 20 extra credit points!

• MagmaMagma – partially or completely melted rock that forms igneous rock when it cools.

The Classification of RocksThe Classification of Rocks

• CompositionComposition – The chemical make up of a rock. This describes the mineral composition of the rock.

• TextureTexture – The size, shape and position of the grains that make up the rock and determines it’s quality.

CoarseCoarse – Large grains

MediumMedium – Smaller grains

FineFine – Smallest grains. Almost cannot be seen.

Igneous RocksIgneous Rocks

• IgneousIgneous – Formed when magma cools and solidifies. The type of texture is determined by the composition of the magma and the time it took to cool. Magma contains many different types of minerals in it that will solidify at different rates. Magma forms if either the pressure or temperature increases or the mineral composition changes.

GlassyGlassy – The magma was shock cooled (very rapid cooling) Ex.: Obsidian

Fine GrainedFine Grained – The magma cooled quickly. Ex. : Basalt Coarse GrainedCoarse Grained – The magma took a long time to cool. Ex. : Granite

Igneous Rock FormationsIgneous Rock Formations

• Intrusive Igneous RockIntrusive Igneous Rock – Formed when magma cools under the surface. The grains are large (coarse) because the upper rocks insulate the magma.

• Extrusive Igneous RockExtrusive Igneous Rock – Formed when lava cools and solidifies on the surface. They usually are formed in or around volcanoes and are fine grained or glassy (no crystals)

• ActivityActivity: Draw the intrusive igneous bodies on p. 100 of your book for bonus points.

Sedimentary RockSedimentary Rock

• Formed by the cementing of sediments that was laid down in layers.

• Strata Strata – Layers of sedimentary rock. This makes it easy to identify sedimentary rock.

• The classification of sedimentary rock is based on the way the rock was formed:

A. ClasticClastic – Fragments of rock cemented together. Ex. : Conglomerate (Coarse Grained). B. ChemicalChemical – Formed from solutions of dissolved minerals &

water. Ex. : Halite (salt). C. OrganicOrganic – Formed from the remains of living organisms. Exs. : Coral (tiny sea animals), Limestone (skeletal remains of

sea animals) & Coal (partially decomposed plant matter).

Sedimentary Rock StructuresSedimentary Rock Structures

• StratificationStratification – The layering of sedimentary rocks.

• Ripple MarksRipple Marks – The preserved movement of wind or waves in sedimentary rock.

• Mud CracksMud Cracks – Sediments that were exposed to the air and dried out indicating the location of ancient lake beds.

Metamorphic RockMetamorphic Rock• MetamorphicMetamorphic – means “changed shape”. These are

rocks in which the structure, texture or composition has been changed. Metamorphic change occurs when the temperature or pressure or both have changed.

A. Contact MetamorphismContact Metamorphism – Occurs when the magma is close by the rock formation and heats the surrounding rocks. This usually is limited to a smallsmall area.

B. Regional MetamorphismRegional Metamorphism – Occurs when pressure builds up deep within the Earth or large pieces of crust collide. This occurs over a very largelarge area.

Composition of Metamorphic RockComposition of Metamorphic Rock

• When temperature and pressure change, the minerals within a rock will change to a more stable mineral structure.

Index MineralsIndex Minerals – Certain metamorphic rocks that are used to estimate the temperature, depth or pressure at which the rock went through metamorphism.

Textures of metamorphic rocksTextures of metamorphic rocks – under heat and pressure, the mineral grains are rearranged and a metamorphic rock can be changed into another type of metamorphic rock.

A. FoliatedFoliated – The grains are arranged in bands. Ex. : Shale turns into Slate.

Composition of Metamorphic Rocks Composition of Metamorphic Rocks (cont.)(cont.)

B. Unfoliated Unfoliated – The grains are NOT arranged in bands. These are usually made up of only one or two minerals.

RecrystallizationRecrystallization – Occurs when the minerals change in size or composition and change the type of metamorphic rock.

DeformationDeformation – A change in the shape of a rock due to the force placed on causing it to squeeze or stretch and may create folds or bends in the rock layers.