Rocks and the Rock Cycle Ch. 4 - pg. 88 – Earth Science

What is a rock? a mixture of minerals, rock fragments, volcanic glass, organic matter or other natural materials How are rocks different from minerals? Rock Cycle (p. 91) process that creates and changes rocks. shows the three main types of rocks and how they are created. shows how rocks can change from one type to another. The Rock Cycle shows how each type of rock can change from processes such as: Melting Weathering and Erosion Compaction and Cementation Heat and Pressure Cooling

Rock Cycle: Sedimentary Rock… with heat and pressure can form a Metamorphic Rock. Metamorphic Rock… melts, and later cools, to from an Igneous Rock. Igneous Rock… after weathering and erosion into sediments, which can then solidify into a Sedimentary Rock. Igneous Rock… with heat and pressure can also become a Metamorphic Rock.

Igneous Rock: formed from cooling magma that hardens Igneous Rock: formed from cooling magma that hardens. These are the most common rocks on Earth. Magma is formed from heat deep below Earth’s surface. As it becomes hotter, it rises towards the surface. As magma reaches the Earth’s surface, it is called lava. Intrusive igneous rocks – rocks that form from magma BELOW the Earth's surface. These are found at the surface only after the layers of rock and soil that once covered them have been removed by erosion. These take a long time to cool which produces mineral grains that are large enough to see with an unaided eye. Extrusive igneous rocks – rocks that are formed as lava cools ON the surface of the Earth. Lava cools quickly once it reaches the surface. The quick cooling rate keeps mineral grains from growing large. Therefore, these rocks are fine grained. Pg. 96

Classifying Igneous Rocks: Igneous rocks are formed by different types of magma (or lava) Basaltic Rocks – Igneous rocks which are dense, dark colored rocks. They form from magma that is rich in iron and magnesium. This lava flows from volcanoes in Hawaii How does this explain the black beach sand common in Hawaii? Granitic Rocks – Igneous rocks are light-colored rocks of lower density than basaltic rocks. This magma is thick and stiff and contains a lot of silica but lesser amounts of iron and magnesium. Andesitic Rocks – igneous rocks that have mineral compositions between those of basaltic and granitic rocks. Many volcanoes around the rim of the Pacific Ocean form andesitic magma.

Metamorphic Rock: rocks that have changed because of changes in temperature and pressure or the presence of hot, watery fluids. Below the Earth’s surface there is a high amount of pressure from the layers above them. This pressure causes the formation of metamorphic rock. (Read Pg. 100) Can be changed in the form of the rock, the composition of the rock, or both. Can form from igneous, sedimentary or other metamorphic rocks.

Classifying Metamorphic Rocks: Foliated Rocks – when mineral grains line up in parallel layers. Slate and gneiss Read pg. 101 Nonfoliated Rocks – when mineral grains grow and rearrange, but do not form layers. Sandstone and marble Read pg. 102

Sedimentary Rock: formed when sediments are pressed and cemented together, or when minerals form from solution. 75% of the rocks exposed at the surface are sedimentary rocks. These often form in layers. This can be very valuable when trying to find the age of rocks...the older layers are on the bottom because they were deposited first.

Classifying Sedimentary Rocks Detrital Sedimentary Rocks – made from broken fragments of rocks. Caused by the following processes... Weathering and Erosion – the breakdown of rocks into smaller pieces when they are exposed to air, water, or ice. The rock is unstable and breaks down chemically and mechanically. Compaction – the process that forms sedimentary rocks when layers of sediments are compressed by the weight of layers above them. Cementation – the process in which sediment grains are held together by natural cements that are produced when water moves through rock and soil. Detrital rocks have granular textures, much like granulated sugar. They are named according to the shapes and sizes of the sediments that form them. Often contain the minerals quartz and feldspar.

Classifying sedimentary rocks continued... Chemical Sedimentary Rocks – these form when dissolved minerals come out of solution. This forms sediments which eventually turn into rocks. They are not made from pieces of preexisting rocks. Limestone Made from calcium carbonate which is carried in ocean water. Rock Salt Forms from water that is rich in salt – halite. (pg. 107)

Classifying Sedimentary Rocks continued.... Organic Sedimentary Rocks – rocks that are made from once-living things. Fossil-rich limestone – animals such as mussels, clams, corals and snails make their shells from calcium carbonate. When they die, shells accumulate on the ocean floor. When these shells are cemented together, this type of limestone forms. Chalk – when you write with “naturally occurring” chalk, you're crushing and smearing the calcite-shell remains of once-living ocean organisms. Coal – forms when pieces of dead plants are buried under other sediments in swamps. These plant materials are chemically changed by microorganisms. These sediments are compacted over millions of years to form coal, an important source of energy. (pg. 108)