Chapter 4 Rocks

The Rock Cycle Rock--- mixture of minerals, volcanic glass, organic matter, or other material Rock Cycle-- model showing processes that create and change rock 1. Sedimentary rock can be changed by heat and pressure into metamorphic rock 2. Metamorphic rock can melt and form igneous rock 3. Igneous rock can be broken into fragments that may later form sedimentary rock

The Rock Cycle Conservation of matter- rock cycle never destroys elements of rocks but merely redistributes them James Hutton recognized the rock cycle in 1788 by observing Siccar Point, Scotland Rock cycle—a continuous and dynamic process

Rock Cycle Discussion Question: What primary processes are involved in the rock cycle?

Igneous Rock Igneous rocks form from magma found deep under Earth’s surface. Magma reaching the surface flows from a volcano as lava. Magma trapped below the surface forms large- grained intrusive igneous rock when it cools. Magma cooling at or near Earth’s surface forms small-grained extrusive igneous rock.

Igneous Rock Contain iron and magnesium but very little silica. 4. Basaltic igneous rocks are dark-colored and dense. Contain iron and magnesium but very little silica. Basaltic lava flows freely from a volcano. Basalt: A fine-grained igneous rock that is usually black in color. The specimen shown is about two inches (five centimeters) across.

Igneous Rock 5. Granitic igneous rocks are lower density and lighter color. Contain more silica and less iron and magnesium. Granitic magma is thick and stiff. Granite: The specimen above is a typical granite. It is about two inches across. The grain size is coarse enough to allow recognition of the major minerals. The pink grains are orthoclase feldspar, and the clear to smoky grains are quartz or muscovite. The black grains can be biotite or hornblende. Numerous other minerals can be present in granite.

Igneous Rock Andesitic rocks have a more balanced composition of minerals and density than basaltic or granitic rocks. Andesite: The specimen shown is about two inches (five centimeters) across and has a porphyritic texture.

Igneous Rock 7. Crystal size, large or small, can help identify an igneous rock as intrusive or extrusive. 8. Volcanic glass rocks cool so quickly that few crystals form. 9. Some rocks have holes formed around once- trapped air and other gases.

Igneous Rock B. Igneous rocks are classified in two ways. Where they formed—intrusive (under the Earth’s surface) or extrusive (at or near the Earth’s surface) Magma type– basaltic, granitic, or andesitic.

Igneous Rocks Discussion Questions: What does mineral crystal size tell about how its formed?

Sedimentary Rocks Sedimentary rocks-mostly found on the exposed surface of Earth Sediments—loose materials such as mineral grains, rock fragments, bits of plant and animal remains They come from rocks that have been weathered and eroded Form sedimentary rocks when pressed or cemented Most common rocks on Earth

Sedimentary Rocks B. Detrital sedimentary rocks- made from broken fragments of other rocks Detrital rocks often have a granular texture. Rocks are named according to size and shape of sediments. Sediment size can be large like gravel or small like clay. Sediments can be well-rounded or have sharp angles. Conglomerates—made of rounded sediments Breccias—made of sharp sediments

Sedimentary Rocks 3. Compaction Erosion brings sediments to new location They pile up The sediments on the bottom get crushed by the weight of the sediments on top Enough to make rock from very small sediments (clay)

Sedimentary Rocks 4. Cementation Large sediments (sand and pebbles) won’t stick together just by pressure Water dissolves minerals like calcite (natural cements) and soaks through When sediments dry, they are stuck together

Sedimentary Rocks C. Chemical sedimentary rocks—formed from minerals left by evaporating liquids Limestone forms from calcite, which was calcium carbonate in solution. Rock salt forms from halite, which was salt in solution.

Sedimentary Rock D. Organic sedimentary rocks—made from remains of once-living plants or animals. Chalk—made of microscopic calcite-shell remains of animals. Coal—made of plant remains, chemically changed by microorganisms and compacted over millions of years.

Sedimentary Rock Discussion Question: How do detrital, chemical, and organic sedimentary tocks differ?

Metamorphic Rocks Metamorphic rocks- Changed by temperature, pressure, and hot fluids. Heat and Pressure result from one layer of rock on top of another. Sometimes temperature and pressure are great enough to melt rock, forming magma. Sometimes pressure flattens mineral grains in rocks without melting them. As pressure and temperature continue to increase over time, one type of rock can change into several different metamorphic rocks.

Metamorphic Rocks 2. Hot, water-rich fluids can move through rock, chemically changing it.

Metamorphic Rocks B. Classification of metamorphic rocks-by composition and texture 1.Foliated texture-mineral grains flatten and line up in parallel layers or bands 2.Nonfoliated texture-mineral grains grow and rearrange but do not form layers

Metamorphic Rocks Foliated Rocks Mineral grains flatten and line up Break along smooth, flat sheets Shale (sedimentary) becomes Slate Granite (igneous) becomes gneiss

Metamorphic Rocks Nonfoliated rocks Minerals are just rearranged, not flattened Sandstone (sedimentary) becomes quartzite Limestone (sedimentary) becomes marble

Metamorphic Rocks DISCUSSION QUESTION: What are three factors that can change rocks?