Hard Rock Café Background Information Created by Linda Sebastian Clermont Northeastern School District

Related Topics: earth science, fossils, dinosaurs, minerals, physical change, measuring volume

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Background Information:

• The Three Types of Rocks. Although there seem to be many different kinds of rocks, they all can be divided into three main groups, according to the way in which they were formed.
  • Igneous (meaning formed from fire) rocks are formed from molten material (magma) in or below the Earth's crust. The hot magma breaks through cracks in the Earth's surface and forms rock called extrusive rock. Liquid magma that reaches the Earth's surface is also called lava. Extrusive rock is either glassy or made up of very fine crystals; because it cools so quickly large crystals do not have an opportunity to form (examples: basalt, pumice, obsidian). Rocks formed from magma that cooled below the Earth's surface are called intrusive rocks and are coarser and have much larger crystals because the magma cooled slowly (examples: granite, quartz, feldspar, mica).
  • Sedimentary rocks were formed from different sediments that accumulated for several thousand years and then were cemented tightly together. One kind of sediment that forms sedimentary rock includes such materials as sand, clay, silt, pebbles, and gravel. Streams and rivers carry these sedimentary materials to lakes or oceans, where they settle to the bottom. As the sediment accumulates, layers are formed, which then slowly change into rock. This change happens because the weight of the top layers presses the sediment in the bottom layers tightly together (examples: conglomerate, sandstone, shale, soft coal). A second kind of sediment that forms sedimentary rock includes the remains of many plants and animals that live in the ocean and take calcium carbonate from the water to form shells or skeletons (examples: limestone, chalk). A third kind of material that forms sedimentary rock includes such chemicals as salt and calcium carbonate that are dissolved in the seawater. Because of changes in ocean conditions, the water can no longer hold these chemicals in solution and they deposit out, forming accumulations that later harden into rock (examples: rock salt, very pure limestone). Sedimentary rocks often contain the remains of early animals and plants, called fossils, embedded in them.
  • Metamorphic rocks are igneous and sedimentary rocks that were buried deeply under other rocks and then changed by heat and pressure. Metamorphic means "changed in form." Some changes are physical in nature, where the original materials in the rock were only rearranged. Other changes are chemical in nature, where new minerals were formed (examples: gneiss from granite, quartzite from sandstone, slate from shale, marble from limestone, hard coal from soft coal, and graphite from hard coal).
  • Looking for Clues. By observing rocks individually, you can deduce much about them. Smooth, round rocks have most likely been acted upon by running water. Flat rocks are usually pieces of sedimentary rock. Pebbles and smaller rocks embedded or cemented in the rock probably mean that the rock is a conglomerate. Crystals in the rock usually mean that the rock is igneous.
• The Rock Cycle. See information contained in the A Rock Is Born game (See Ready to Print.)
• Properties of Rocks and Minerals. Note: The properties listed below are a sampling of terms commonly used by geologists.
  • Color is determined by examining the powdery trail left on a streak plate (piece of dull white tile). Note: Many minerals exist in more than one color.
  • Hardness can be measured formally using the Mohs' scale of hardness. The higher the number on the scale, the harder the mineral. Hardness can also be measured informally by rubbing two rocks or minerals against one another. Whichever gets scratched is the softer rock or mineral.
  • Luster describes the ability of a rock or mineral to reflect light. The following words can be used to describe luster and are ordered from most to least luster:
    • glassy
    • shiny
    • sparkly (having embedded bits of crystals)
    • pearly
    • greasy
    • earthy (no luster)
  • Texture describes the surface of a rock or mineral.
    • coarse: large grains or chunks visible with a hand lens
    • fine: no large grains or chunks visible with a hand lens
  • Transparency describes whether light can pass through a rock or mineral.
    • transparent (light passes through easily)
    • translucent (light barely passes through)
    • opaque (light does not pass through)
• Archimedes and the Bathtub. Archimedes was a Greek philosopher, mathematician, physicist, and inventor. The best-known story about Archimedes concerns his discovery of the principles of buoyancy and the law of density or specific gravity. In ancient Greece, Hieron II (a prince) had been given a gold crown, which he had sworn to dedicate to the gods when he became king. He asked Archimedes to determine if the crown was made of pure gold. The problem was great since Archimedes was not to damage the crown in any way. (Some versions say the king was having a new crown made, but he did not trust his goldsmith, so he ordered Archimedes to devise a way to see if the goldsmith used the exact amount of gold. This version also says Archimedes would be put to death if he failed.) One day, the answer came to Archimedes as he climbed into his bathtub and noticed the overflow of water. He jumped out of the tub and ran naked through the streets, shouting, "Eureka! Eureka!," which in Greek means, "I have found it! I have found it!" It had occurred to him that, if the crown were made of pure gold, it should take up the same amount of space as an equal weight of pure gold. If the crown and the equal weight of pure gold were placed in turn into a tub full of water, equal volumes of water should overflow. When he found the overflow to be different, he knew that the king had been cheated.