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Sedimentary Rocks: Breccia, Conglomerate, Limestone, Sandstone, ShaleMetamorphic Rocks: Gneiss, Marble, Quartzite, Schist, Serpentinite, SlateWhat Are Rocks?Rocks are what the crust of the earth is made of. They are the mountains and the bottom of the ocean. They are everywhere on earth, but often buried under soil. Rocks are made of minerals, like quartz, calcite, feldspars, and micas. Most rocks are made from more than one mineral, but there are quite a few kinds that are made from only one mineral. Minerals are not rocks, rocks are made of minerals. A car is made of steel, glass, and plastic. A rock is like the car, a mineral is like the steel, or glass, or plastic.[ Return to Rock Key Table of Contents ]What Types of Rock Are There?There are three different types of rock:Igneous Rock is formed when a magma cools underground and crystallizes or when it erupts unto the surface of the ground, cools and crystallizes. Magma that erupts onto the surface is called lava. When magma cools slowly underground the crystals are large enough to see. When it cools quickly on the surface, the crystals are very small and you would need a magnifier or a microscope to see them. Sometimes, when the magma cools very quickly, it forms a kind of black glass that you cannot see through.Sedimentary Rock forms from particles, called sediment, that are worn off other rocks. The particles are sand, silt, and clay. Sand has the largest particles while clay has the smallest. If there are a lot of pebbles mixed with the sand, it is called gravel. The sediment gets turned into rock by being buried and compacted by pressure from the weight above it. Another way it becomes rock is from being cemented together by material that has been dissolved in water. Often, both cementing and compaction take place together.Metamorphic Rock is formed by great heat, or pressure, or both. The pressure can come from being buried very deep in the earth's crust, or from the huge plates of the earth's crust pushing against each other. The deeper below the surface of the earth, the higher the temperature, so deep burial also means high temperatures. Another way that high temperatures occur is when magma rises through the earth's upper crust. It is very hot and bakes the rock through which it moves. Hot liquids or gases from the magma also can cause chemical changes in the rock around the magma.[ Return to Rock Key Table of Contents ]What is the Rock Cycle?Rocks, like mountains, do not last forever. The weather, running water, and ice wear them down. All kinds of rocks become sediment. Sediment is sand, silt, or clay. As the sediment is buried it is compressed and material dissolved in water cements it together to make it into sedimentary rock. If a great amount of pressure is exerted on the sedimentary rock, or it is heated, it may turn into a metamorphic rock. If rocks are buried deep enough, they melt. When the rock material is molten, it is called a magma. If the magma moves upward toward the surface it cools and crystallizes to form igneous rocks. This whole process is called the Rock Cycle.[ Return to Rock Key Table of Contents ]What Minerals Form Rocks?The list of minerals that commonly form rocks is short. With a little practice you will recognize most of them when you see them. Descriptions of some of the minerals, as they look in rocks, follow:Quartz: Quartz is the last mineral to crystallize, so in igneous rocks it never has any definite shape. In rocks, it does not show flat faces. It is usually gray in igneous rocks; gray, white, yellow, or red in sedimentary rocks; and gray or white in metamorphic rocks. It has a glassy, or sometimes waxy, look to it.Potassic Feldspars*: (microcline, orthoclase) Potassic feldspars are pink or tan, sometimes white. They show flat, shiny faces in igneous rocks. The crystal grains are usually blocky and nearly rectangular. They look like good china.Plagioclase Feldspars*: (albite, labradorite) Look like the potassic feldspars, except they are white to dark gray, sometimes black. They may show flashes of blue or green.Micas*: (muscovite, biotite, phlogopite) Micas have very thin layers that peel off (or cleave) very easily. In rocks they are usually flakes or layers of flakes. Muscovite is silvery to brown; biotite is black; phlogopite is a reddish brown. Phlogopite may be found in marble.Chlorite*: Like mica, but the flakes are usually not as thin and do not peel apart as easily. The color is medium to dark green, sometimes almost black but with a greenish tint.Hornblende: Hornblende is dark green to black. It shows nearly flat, shiny faces in almost rectangular or long thin needle like crystals in rock. Hornblende is usually found in dark colored metamorphic rocks; sometimes in igneous rocks.Actinolite and Tremolite: Actinolite and tremolite are usually in long thin blades or needle like crystals. Actinolite is dark green; tremolite is white to gray. The crystals may be parallel to each other, or spread from a point. Actinolite is usually found in schists or gneisses. Tremolite may be found in marble.Olivine*: Olivine in rocks is an olive green to greenish yellow. In rocks it is in rounded grains. If there is much of it, it is almost sugary. It is found mostly in dark colored igneous rocks.Calcite and Dolomite: The color is usually white, but can be other colors when impure. Crystal grains show flat shiny faces, often shaped like parallelograms. Calcite and dolomite are both soft. They are easily scratched with a steel point. Powdered calcite will fizz in white vinegar; dolomite will not. The minerals are found in limestone or dolostone ( the rock is dolostone, the mineral is dolomite) and marble.note: Names marked with an asterisk (*) are groups of related minerals.[ Return to Rock Key Table of Contents ]Collecting RocksRocks are easier than minerals to collect. That is because they are found nearly everywhere. If you want to start a rock collection, try to find pieces of rock that are freshly broken off a ledge. A ledge is a bed of rock that is sticking out of the ground, or the side of a mountain. It is not loose, but is still part of the bedrock below the soil. Pieces of rock that have been buried in the soil, or rolled in a stream or river are not good to collect. It is difficult to see what they are or what they are made of and you really don't know where they came from..Collect clean fresh specimens.Make a label that has the name of the rock and the location where it was collected. Assign a number to each rock. Record in a notebook the name, location where you found it, and number of the rock. Paint a small white rectangle on each rock, and write the rock's number on it. [ Return to Rock Key Table of Contents ]Safety While Collecting RocksAlways wear safety glasses or goggles when breaking rocks. Use only hammers that are intended for breaking rocks. Do not use a carpenter's claw hammer (a hard rock can break sharp steel splinters off a claw hammer). Do not climb on dangerous ledges or on quarry walls. Stay away from quarry walls, they might collapse. Never enter mine tunnels. They are very dangerous! If possible, always collect with an adult. [ Return to Rock Key Table of Contents ]Using The Rock KeyAs you use The Rock Key, you will find a lot of links. They are blue and are underlined. Links let you jump from one place to another in the key. The Yes and no choices are links. "Clicking" on them jumps to the next question that you need to answer. If it is the last question to finding the name of the rock, the link jumps to the description of the rock. The numbers for each step in the key are not important if you are using the key in your web browser. They are included so, if you wish, paper copies may be used.You can back up to earlier questions you were asked, by clicking Go Back. Of course, you can go back one step to, where you were, at any time by clicking the Back button at the top of your Browser.In the Mineral Descriptions there is always a part called, Compare To:, with the names of rocks that look a lot like the one you are reading about. The rock names are links. If you click on one, you jump to the description of that rock.In order to use The Rock Key there are a few things you need to know:Crystals: Crystals are what minerals form when they are free to grow in nature; like the quartz crystal in the first drawing. In rocks, crystals grow up against each other. They cannot grow as the quartz crystal did in open space. Crystals in rocks have straight edges and they very often show flat shiny faces that reflect light like tiny mirrors. They look more like the second drawing.Grains: Grains that are not crystals in rock do not have flat shiny faces. They are rounded, like grain of sand, or jagged, like a piece of broken rock.Grain Size: Grain size in rocks can mean the size of crystal grains or of fragments:Coarse Grained: most of the rock is made of grains as large as rice, or larger.Medium Grained: the individual grains can be seen without a magnifier, but most of the rock is made of grains smaller than rice.Fine Grained: the individual grains can not be seen without a magnifier (or microscope).Layers: Layers in rocks show in different ways.In some rocks different colored minerals are lined up in ribbons.. Usually there are two colors, often black and white, or green and white, of black and tan or pink. Ribbon like layers are found in the rock, gneiss. In schists, the layers are most often thin layers of mica or chlorite around lens shaped masses of feldspar or quartz. The top and bottom is almost always mica or chlorite. In sandstones, different sized sand grains sometimes show as different colors. When the grains are sorted by running water or wind, they show different shades of the same color. The layers in slate are very thin and straight. The top and bottom layers are usually flat and quite smooth.Ribbon like LayersMica like LayersParticle LayersThin Cleavage Layersin Gneissin Schistin Sandstonein SlateGas Bubbles: Gas bubbles in rock are sort of round or elongated holes. In pumice, the bubbles may be very tiny to the size of a match head. They are a glass froth that may look something like a sponge or gray, glassy soap bubbles. In scoria or vesicular basalt, the bubbles are larger, often as large as peas. They look like small pockets in the rock.[ Return to Rock Key Table of Contents ]The Rock Key1. Is the rock made of crystal grains? (Does it have a lot of flat, shiny faces - maybe tiny to small - that reflect light like little mirrors? You may need to use a magnifier.) The rock is made of crystal grains with flat shiny surfaces. . . Go to 2 There are no (or not many) shiny, flat, crystal grains. . . Go to 3[ Return to Rock Key Table of Contents ] 2. Does the rock have both layers and crystal grains? (Look carefully for layers , especially along the edges of the rock. You may need a magnifier.) The rock has both layers and crystals. . . Go to 4 The rock has crystals, but it has no layers. . . Go to 5[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ]3. Does the rock have layers but not crystal grains? (Look carefully for layers, especially along the edges of the rock. You may need a magnifier.) The rock has layers, and crystal grains are not visible. . . Go to 11 The rock has no layers, and crystal grains are not visible. . . Go to 12[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ]4. Do the layers look like ribbons or bands of minerals running through the rock; and is the rock kind of blocky? (The bands of minerals may be straight or wavy. The rock breaks into blocky chunks, not along its layers.) The rock has crystals, layers that look like ribbons or bands of minerals running through it, and is kind of blocky . It is. . . Go to 23 The rock has crystals and layers that are thin and do not look like ribbons of minerals. It breaks along the layers. It is. . . Go to 24[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] 5. Is the entire rock mostly light colored, compared to other rocks? (Look at the whole rock, not just mineral grains in the rock.) The rock is mostly light colored or light gray minerals grains. . . Go to 6 The rock is mostly medium gray to very dark colored minerals. . . Go to 7[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] 6. Can you scratch glass with the rock? (If it does, the rock is hard. If it doesn't, the rock is soft.) (Safety note: keep the glass flat on your desk, not in your hand. Carefully press a point of the rock against the glass and pull it about 2cm. Look to see if it scratched the glass. Do not hit the glass with the rock.) The rock scratches glass. It has crystals, but has no layers. . . Go to 9 The rock does not scratch glass. It has crystals, but has no layers. It is. . . Go to 25[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] 7. Is the rock mostly light or medium gray, not very dark gray or black? The rock is mostly light to medium gray, has crystal grains, and is not layered. It is. . . Go to 31 The rock is mostly very dark gray or black. . . Go to 8[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ]8. Can you see crystal grains in most or all of the rock without using a magnifier? The rock is coarse or medium grained, has crystals, and no layers. . . Go to 10 The rock is fine grained, has crystals, and no layers. It is. . . Go to 32[ Go Back ] ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ]9. Can you see crystal grains in most or all of the rock without using a magnifier? The rock is mostly crystal grains. It is medium or coarse grained, has no layers, and is light colored. It is. . . Go to 30 The rock is mostly fine grained, it has crystal grains, has no layers, and is light colored. It is. . . Go to 29[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ]10. Is the rock coarse grained? (If the rock is coarse grained, most of the rock mostly is made of crystals that are as large, or larger, than rice. If you can see the crystals without a magnifier, but they are smaller than rice, the rock is medium grained.) The rock is made of coarse crystal grains. It has no layers, and is dark colored. It is. . . Go to 34 The rock is made of medium crystal grains. It has no layers, and is dark colored. It is. . . Go to 33[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ]11. Using the point of a steel nail, can you scrape grains of sand off the rock? (Hold the rock over a clean sheet of paper and scrape it hard with the point of the nail. Rub your finger over the paper. Can you feel grains of sand?) The rock has layers. It is made of grains of sand. The rock is. . . Go to 38 The rock has layers and is not made of grains of sand. . . Go to 13[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ]12. Does the rock have gas bubbles in it? (It may look something like a sponge. Look for rounded holes, or glassy bubbles in the rock. They may be tiny like a pinhead, small, or large like a pea ) The rock has gas bubbles. . .Go to 15 The rock has no gas bubbles. . .Go to 17[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ]13. Does the rock look like it is composed of mostly only one mineral and has many thin flat layers? (The layers are less than 2mm thick, mostly thinner. not thick layers) The rock has many thin flat layers, seems to have only one mineral, and usually no visible crystals. The rock is. . . Go to 27 The rock is mostly one mineral, but the layers are thicker (usually more than 4mm). . .Go to 14[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] 14. Is the rock definitely green in color, and does it feel slippery? The rock is mostly green and slippery. The rock is. . . Go to 28 The rock is not green and slippery. The rock is. . . Go to 39[ Go Back [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] 15. Is the rock light in weight and mostly light colored (probably gray)? The rock is full of gas bubbles, is light in weight, and is light colored. The rock is. . . Go to 35 The rock is heavy, dark colored, and has some gas bubbles, but the bubbles are mostly larger. . . Go to 16[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] 16. Is the rock dark colored, glassy, with gas bubbles in it? (Does it have some jagged or sharp points? ) The rock is dark colored, glassy, with gas bubbles in it. The rock is. . . Go to 36 The rock is gray or black, has a few gas pockets in it, and has no layers. It is not glassy. The rock is. . . Go to 32[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] 17. Does the rock look like black glass with no bubbles in it? (It may have some white "snowflakes" in it ,or some reddish bands in it) The rock looks like black glass. The rock is . . Go to 37 The rock does not look like a black glass. . . Go to 18[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] 18. Using the point of a steel nail, sand can be scraped off the rock. (Use the point of a steel nail to scrape the rock over a sheet of clean paper. Can you feel sand on the paper?) Sand can be scraped off the rock. . . Go to 19 Sand cannot be scraped off the rock. . . Go to 20[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] 19. Does the rock contain sand and larger pieces of rock or pebbles? The rock is composed of sand and pebbles or other larger pieces of rock. . . Go to 22 The rock is made of sand, but not pebbles or other larger pieces of rock. The rock is. . . Go to 38[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ]20. Can the rock scratch glass? (Safety note: keep the glass flat on your desk, not in your hand. Carefully press a point of the rock against the glass and pull it about 2cm. Look to see if it scratched the glass) The rock scratches glass, but it is not made of sand. . . Go to 21 The rock does not scratch glass. It is not composed of visible crystals. It is. . . Go to 40[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] 21. Is the rock white, yellowish, tan, or reddish? The rock is.. . . Go to 26 The rock is either black or gray. The rock is. . . Go to 32[ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ]22. Are the larger pieces of rock (that are mixed with the sand) rounded pebbles, not blocky or jagged? The larger pieces are rounded pebbles. The rock is. . . Go to 41 The larger pieces are jagged and blocky. The rock is. . . Go to 42</b