Rocks Chapter 6
Three Major Types of Rock • Igneous: forms when magma, or molten rock, cools and hardens. • Sedimentary: forms when sediment deposits are compressed and cemented together. • Metamorphic: forms when existing rock is altered by changes in temperature, pressure, or chemical processes
The Rock Cycle The series of processes in which rock forms, changes from one type to another, is destroyed, and forms again by geological processes
Properties of Rocks • Bowen’s Reaction Series The simplified pattern that shows the order in which minerals crystallize from cooling magma according to their chemical composition & melting point
As magma cools and solidifies, minerals crystallize out of the magma in a specific order that depends on their melting points.
Olivine Pyroxene Amphibole Biotite mica Potassium feldspar Muscovite mica Quartz
More Properties of Rocks • Chemical Stability of Minerals • Physical Stability of Minerals • zones of weakness called “joints”
Igneous Rocks • Three factors that affect whether rock melts • Temperature, pressure, & presence of fluids • Magma- molten material beneath the surface of the earth (parent material) • Lava- molten material on the surface of the earth • Crystallization- cooling and solidification of rocks into an orderly pattern. All material does not solidify at the same rate
Igneous rocks form from two processes- melting and cooling +hardening (crystallization)
Types and Textures of Igneous Rocks Two main types • Intrusive igneous rock • Coarse-grained Texture form underground from cooled magma. Reach the surface through weathering and erosion • Extrusive igneous rocks • Fine-grained Texture form when lava or melted rock cools rapidly on the earth’s surface
Texture of Rocks • Texture describes the size, shape, and arrangement of crystalline grains. • Crystal size is the most important factor in determining texture • Crystal size is determined by the speed of hardening (rate of cooling) • Slower cooling larger crystals • Faster cooling smaller crystals • Intermediate cooling glass-like texture • The longer the magma stays liquid, the larger the crystals.
Families of Igneous Rocks • Three families with different mineral compositions • Felsic Rock = Granite family • High is silica, light in color, main minerals are orthoclase feldspar and quartz • Formed during mountain building • Intrusive, fine or course grained, formed from thick slow moving magma Granite
Families of Igneous Rocks • Mafic Rock = Basalt family • Low in silica, rich in iron, dark colored, main minerals are biotite, hornblende, augite, and olivine • Extrusive, may be fine or course grained, formed from thin, hot magmas Basalt
Families of Igneous Rocks • Intermediate Rocks = Diorite family • Little or no quartz • Medium colored rocks, main minerals are feldspar, hornblende, and micas • May be fine or course grained Diorite
Intrusive Igneous Rock Structures • Batholiths—large masses of magma • Stocks—smaller masses than batholiths • Laccoliths—sheet-like masses of magma spreading upward • Sills—magma flowing between layers of rock • Dikes—magma flowing through vertical cracks next
Extrusive Igneous Rock Structures • Volcano—vent through which magma, gases or ash is expelled • Volcanic neck—solidified central vent • Lava flows—flat extrusions of magma • Lava plateau—flat-topped mountain formed by volcanic action • Tuff—volcanic ash next
Sedimentary Rocks • Sedimentary rocks form when sediments harden into rocks. They are classified according to the kind and size of sediments that form them. • Sedimentary rocks form under two main processes: • Weathering and Erosion • Compaction and cementation
Weathering—breaking rock down • Erosion—moving material away • Compaction—the weight of overlying sediments causes pressure, pushing the fragments closer together and squeezing out air and water. • Cementation- dissolved material carried by water is left between fragments and provide a cement to hold the fragments together. Cement may be silica (SiO2,) calcite (CaCO3) or iron oxide ( FeO).
3 Classes of Sedimentary Rock • Clastic, chemical, and organic • Clastic Sedimentary Rocks—formed from weathering of rocks that already exist • Range in grain size from gravel to sand to mud • Conglomerate = gravel size sediments • Sandstone = sand size sediments • Shale = mud size sediments • Running water collects particles of all sizes and causes them to be smoothed down and more rounded. • The farther a particle travels in water, the more rounded and smaller it becomes.
Sandstone Conglomerate Shale
Chemical sedimentary rocks form from materials that were once dissolved in water • Some form when material settles out due to temperature change • Some form when water evaporates and leaves the minerals behind Halite, rock salt
Organic Sedimentary rocks form from the remains of living things • Coal and limestone are examples • Fossils are often present Coal Limestone with fossils
Sedimentary Rock Features • Stratification—arrangement of visible layers, oldest layer on the bottom, younger layers on top. • Ripple marks—small waves on surfaces of rocks caused by wind or water. • Mud cracks—caused by periodic drying out of mud that formed into rock. • Fossils—remains of ancient living things preserved or imprinted in rock. • Concretions—lumps of precipitated material • Geode—hollow concretion with inward facing crystals Features indicate conditions at time of formation next
Mudcracks Ripple marks Geode Fossilized paw back
Metamorphic Rock • Metamorphism- changing one type of rock to another by heat, pressure, and chemical processes. • All metamorphic rocks form from existing igneous, sedimentary or metamorphic rocks. Most form deep within the crust of the earth.
Two types of metamorphism • Regional metamorphism • Large areas of Earth’s crust are affected by heat and pressure caused by movements in the Earth’s giant tectonic plates • Most metamorphic rock forms this way • Contact metamorphism • Hot magma pushes through existing rocks • Rocks near or touching the magma are metamorphosed
Classification of Metamorphic Rocks • Foliated Texture—visible parallel bands • Extreme pressure flattens the crystals in the original rock and push them into parallel bands • Slate, schist and gneiss are common foliated metamorphic rocks
Foliated metamorphic rocks Schist = minerals have a stretched appearance (parent rock is phyllite) Slate= foliated but no visible minerals present (parent rock is shale) Gneiss has clear bands of dark and light material (parent rock is granite)
Nonfoliated Metamorphic Rocks • Rocks without bands or aligned minerals are nonfoliated • Original rock has only one type of mineral grain • Mineral grains of original rocks are round or square.
Nonfoliated Metamorphic Rocks Hornfels Quartzite
Alabama’s Official State Rock • Used in the Washington Monument and the Lincoln Memorial. Marble