Types of Rocks

• All rocks are composed of minerals.

• There are about 3500 known minerals in Earth’s crust.
• Each combination of rock-forming minerals results in a rock with a unique set of properties.

• Mineral a natural, usually inorganic solid that has a characteristic chemical composition, an orderly internal structure, and a characteristic set of physical properties
• Molten rock cools to form igneous rock.

• Nearly all igneous rocks are made of crystals of various minerals.

• Igneous rock rock that forms with magma cools and solidifies
• Extrusive igneous rock cools on Earths surface
• Intrusive igneous rock cools while trapped beneath Earth’s surface
• Remains of older rocks and organisms form sedimentary rocks.

• All rock breaks down over thousands of years.

• Weathering the natural process by which atmospheric and environmental agents, such as wind, rain, and temperature changes, disintegrate and decompose rocks
• As pieces of rock accumulate, they can form another type of rock.

• Sedimentary rock a rock formed from compressed or cemented layers of sediment

• Sediment accumulated pieces of rock and other particles
• Loose sediment forms rock in two ways.

1. Layers of sediment get compressed from weight above, forming rock.
2. Minerals dissolved in water seep between bits of sediment and “glue” them together.

• Sedimentary rocks are named according to the size of the fragments they contain.
• Rocks that undergo pressure and heating without melting form metamorphic rock.

• Heat and pressure within Earth cause changes in the texture and mineral content of rocks.

• Metamorphic rock a rock that forms from other rocks as a result of intense heat, pressure, or chemical processes
• Old rocks in the rock cycle form new rocks.

• The sequence of events in which rocks can be weathered, melted, altered, and formed is described by the rock cycle.

• Rock formation occurs very slowly, often over tens of thousands to millions of years.
• The relative age of rocks can be determined using the principle of superposition.

• The principle of superposition states the following:

• Assuming no disturbance in the position of the rock layers, the oldes will be on the bottom, and the youngest will be on top.
• Radioactive dating can determine a more exact, or absolute, age of rocks.

• The radioactive elements that make up minerals in rocks decay over billions of years.

• Physicists have determined the rate at which these elements decay.

• Geologists can use this data to determine the age of rocks.
• Distinguish between chemical and physical weathering.

• Explain how chemical weathering can form underground caves in limestone.

• Describe the importance of water to chemical weathering.

• Identify three different physical elements that can cause erosion.