Structure and Origin of Rocks

1. Structure and Origin of Rocks • What materials make up rocks? • All rocks are composed of minerals. • mineral: a natural, usually inorganic solid that has a characteristic chemical composition, an orderly internal structure, and a characteristic set of physical properties • There are more than 4,000 minerals in Earth’s crust.

2. Structure and Origin of Rocks, continued • Molten rock cools to form igneous rock. • Nearly all igneous rocks are made of crystals of various minerals. • igneous rock: rock that forms when magma cools and solidifies • Igneous rock can be extrusive or intrusive. • Extrusive igneous rock cools on Earth’s surface. • Intrusive igneous rock cools while trapped beneath Earth’s surface.

3. Structure and Origin of Rocks, continued • The remains of older rocks form sedimentary rocks. • All rock breaks down over thousands of years by a process known as weathering. • As pieces of broken down rock accumulate, they can form another type of rock—sedimentary rock. • weathering: the natural process by which atmospheric and environmental agents, such as wind, rain, and temperature changes, disintegrate and decompose rocks • sedimentary rock: a rock formed from compressed or cemented layers of sediment

4. Structure and Origin of Rocks, continued • Loose sediment forms rock in two ways. • Layers of sediment can be compressed from weight above them and form rock. • Minerals dissolved in water seep between bits of sediment and “glue” them together. • Sedimentary rocks are named according to the size of fragments that they contain.

5. Structure and Origin of Rocks, continued • 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. • The word metamorphic comes from the Greek word metamorphosis, which means “to change form.” • metamorphic rock: a rock that forms from other rocks as a result of intense heat, pressure, or chemical processes

6. Structure and Origin of Rocks, continued • Metamorphic rocks may be changed in two ways. • by heat alone • by heat and pressure (more common) • The most common types of metamorphic rocks are formed by heat and pressure deep in the crust.

7. Structure and Origin of Rocks, continued • 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. • Depending on the circumstances, any type of rock can become any other type of rock.

8. Rock Cycle

9. How Old Are Rocks? • How are scientists able to tell how old a rock is? • Scientists have developed several methods to determine the age of rocks on a geologic time scale.

10. How Old Are Rocks? • The position of a rock can show its relative age. • The relative age of a rock can be determined by using the principle of superposition. • The principle of superposition states the following: • Assuming no disturbance in the position of the rock layers, the oldest will be on the bottom, and the youngest will be on top.

11. Law of Superposition

12. How Old Are Rocks? continued • Radioactive dating can determine a more exact, or absolute, age of rocks. • The radioactive elements that make up minerals in rocks decay at different rates, often over millions and billions of years. • Physicists have determined the rate at which these different elements decay. • Geologists can use this data to determine the absolute age of rocks.