Chapter 21.3

1. Chapter 21.3 Rocks and Minerals

2. Objectives… • Identify what materials make up rocks? • How are scientists able to tell how old a rock is http://www.history.com/video.do?name=How_the_Earth_Was_Made&bcpid=9787674001&bclid=15202215001&bctid=15346371001 http://www.history.com/video.do?name=How_the_Earth_Was_Made&bcpid=9787674001&bclid=15202215001&bctid=15346371001 • Explain the difference between different rock types Key Terms: • Mineral, igneous rock, weathering, sedimentary rock, metamorphic rock

3. All rocks are composed of minerals • Minerals have a definite crystal structure, a regular repeating internal alignment of atoms and molecules • The Earth’s crust contains over 4,000 known minerals • Only about 20 of these are found regularly • Some of these are feldspar, pyroxene, mica, olivine, dolomite, quartz, amphibole, and calcite

4. Rocks can be porous, granular, or smooth • Rocks can have essentially any physical characteristic • These characteristics give scientists insight as to how they were formed

5. Molten rock cools it hardens to form igneous rock • Nearly all rocks are made of crystals of various minerals • In general, the longer the rock takes to cool the longer the crystals have to form

6. Igneous rock can be extrusive, if it cools on the Earth’s surface • Basalt is an example of an extrusive igneous rock • Basalt makes up much of the Hawaiian Islands

7. Igneous rock can be intrusive, because they cool inside the Earths surface • Granite cools underground and large crystals can be observed

8. Remains of older rocks form sedimentary rocks • Hard rock (not the café) and soft rock all break down during a process known as weathering • Pieces of this rock can be moved by gravity, water, and wind • These pieces accumulate to form sedimentary rocks

9. Loose sediments form rock in two ways, compression or cementation • The weight of rock layers can be enough to squeeze layers of sediment together • Dissolved minerals can form a type of “glue” between sediment particles to hold them together

10. Sedimentary rocks are named according to the size of the fragments they contain • Conglomerates are made of small pebbles cemented together • Rocks made of sand are oddly enough called sandstones • Rocks made of fine mud or silt are called mudstones or siltstones • Limestone is a rock commonly made of shells of fossilized aquatic life

11. Rocks that change because of pressure and heating without melting form metamorphic rock • As one goes deeper in the Earth, we know that pressure and temperature increase • The word metamorphic literally means to change form • Limestone, though sedimentary rock, can turn into marble at high temperatures

12. Metamorphic rock can be changed by two methods; heat only or a combination of heat and pressure • Regardless of how it is changed, metamorphic rock experiences a chemical change • Therefore its physical and chemical properties change • Any fossils which were once inside are now destroyed and lost forever

13. The Rock Cycle (click here)

14. The principle of cross-cutting relations states that a geologic feature that cuts across existing rock must be younger than the rock that surrounds it • The principle of lateral continuity states that layers on opposing sides of a valley are the same age because they were once one continual layer

15. How Old are Rocks? • Since the Earth changes, it can be difficult to know exactly how old rocks are • The positions of layers can show relative ages • The principle of superposition states that the oldest layers are on the bottom and youngest layers will be on top, provided that there has been no disturbance (Lets use some play-dough again!)

16. Radiometric dating can give the absolute age of rocks • Isotopes, or atoms of elements with different numbers of neutrons, decay at different rates • By knowing these rates of decay, and knowing amounts left in rock, scientists can determine the exact age of rocks • Contrast this with the principles of superposition and cross-cutting relations.