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In this Issue:
'Solid as a rock.' Have you heard someone say that before? Rocks have a reputation for being solid, hard, and indestructible. Rocks line river beds and jut above the landscape as mountain peaks; they are fun to collect and sometimes are very beautiful. Each rock is different - some are smooth and round, some are sharp and dangerous. They come in all colors: pink, green, orange, white, red. They are everywhere, and we take their presence for granted and assume that they are unchangeable.
But rocks are not unchangeable! Just like the water cycle, rocks undergo changes of form in a rock cycle. A metamorphic rock can become an igneous rock, or a sedimentary rock can become a metamorphic one. Unlike the water cycle, you can't see the process happening on a day-to-day basis. Rocks change very slowly under normal conditions, but sometimes catastrophic events like a volcanic eruption or a flood can speed up the process. So what are the three types of rocks, and how do they change into each other? Keep reading to find out!
Three types of rock:
Igneous rocks are formed when hot magma (melted rock) is rapidly cooled, either by hitting underground air pockets or by flowing from the mouth of a volcano as lava. Granite, obsidian, and pumice are all common examples of igneous rocks. Pumice is a very porous rock, because when the lava cooled, pockets of air were trapped inside. Because of all those air pockets, pumice can actually float!
Sedimentary rocks are formed by layers of sediment (dirt, rock particles, etc.) being mixed and compressed together for extended periods of time. Common examples of these rocks are limestone, sandstone, and shale. Sedimentary rocks often have lots of fossils in them because plants and animals get buried in the layers of sediment and turned into stone.
Metamorphic rocks are a combination of rock types, compressed together by high pressure and high heat. They usually have a more hard, grainy texture than the other two types. Schist, slate, and gneiss (pronounced like 'nice') are metamorphic rocks.
These rocks change over hundreds of years in the six steps of the rock cycle:
See if you can find sedimentary, igneous and metamorphic rocks where you live. As you study them, think about how they have undergone many slow changes to become what they are. Draw a picture of the rocks you find and then draw a diagram of the whole rock cycle. Next, do some rock experiments.
Rocks are broken down in all sorts of ways — by wind, rain, rivers, and rock slides. But they are also broken apart by freezing and thawing. When water freezes, it expands. Water that has seeped into a rock will expand when frozen, causing cracks in the rock. After it freezes and thaws several times, bits of rock will begin to split off entirely.
Do this experiment to see how it works and find out what kinds of rocks break down the most when frozen.
Look at each rock carefully. Which do you think will break down the most when you freeze and thaw them several times? Why? Write down your prediction, then place the rocks in the plastic bottle and cover them with water. Put the bottle in the freezer. When the water is frozen take it out and let it thaw. After the water melts, put the bottle back in the freezer. Repeat the process 3-5 times.
Take out the rock samples and look at them carefully again. Which one has changed the most? Do you see where small particles have been split off the rock by the freezing water? Was your prediction correct?
Over time whole mountains can be worn down by this freezing and thawing process!
You may be wondering how water gets in rocks in the first place. Aren't rocks solid? Actually, they have tiny pores or pockets that can be filled with water or air (or sometimes oil and natural gas) like a sponge. No rock can soak up as much water as a sponge, but some rocks absorb more water than others - these are called porous rocks.
This experiment will show you how a piece of chalk can absorb a great deal of water in a short time. Chalk is composed of the mineral calcium carbonate. (Different types of rocks are made of different pure minerals; rocks such as limestone have lots of calcium carbonate.) Have older students try this with various types of porous rock, such as pumice - they may need to wait up to a day for the rock to absorb the total amount of water it can hold.
1. Weigh the chalk with the spring scale to determine how heavy it is when dry. To do this, hang a small plastic bag from the hook and place the chalk in the bag. Record how much it weighs.
2. Place the chalk in a cup of water. In five minutes take the chalk out of the water, shake it off and weigh it again. Record the weight. Repeat at five-minute intervals until the chalk no longer increases in weight.
The chalk got heavier because it was absorbing water. After about 10-15 minutes, it absorbed all that it could hold. You can figure out how much water it absorbed, because 1 gram of water = 1 ml of water = 1 cubic centimeter (cc). If the chalk gained 2 grams in water, that means it soaked up 2 ml or 2 cc.
Older students can figure out what percentage of its volume the chalk absorbed. If the piece of chalk is a cylinder, they can find out its volume using the formula Volume = πr 2(h) where r is the radius and h is the height. (For example, if you have a piece of chalk 2 cm in diameter and 5 cm long, the formula would be 3.14 x 1 2(5) = 15.7 cc.) If it is a different shape, or they're trying this with other rocks, they can use the displacement method: pour 100 ml of water into a graduated cylinder and add the rock. The water level will rise, and the amount it rises is the rock's volume. If the water level rises to 125 ml, for example, then the volume of the rock is 25 ml, or 25 cc. Once you know the volume of the water absorbed and the volume of the rock, you can find what percentage of its volume the rock absorbed. Just divided the volume of water by the volume of the rock (e.g. 2cc/25cc = .08 = 8%).
Try this experiment with several different kinds of rock. Calculate the percentage of its volume that each rock can absorb. Which absorbs the most? Based on what you learned in the previous experiment, how do you think this affects the erosion of that type of rock?
Certain rocks with carbonate compounds erode or dissolve when they come in contact with acidic chemicals. Carbon dioxide in the atmosphere sometimes can produce rain that is slightly acidic. Over time, this acid rain erodes rocks. You can see it happen quickly if you use vinegar, which is a much stronger acid than acid rain.
Use a piece of limestone, which is made of calcium carbonate. Put the limestone in a bowl, pour a little vinegar on top of it and watch what happens. It will fizz and form bubbles, because the vinegar reacts with the carbonate ions. As it reacts, it dissolves the limestone. When you're done watching it fizz, take the limestone out and look in the bottom of the bowl. You should see a layer of sediment made of small particles of calcium acetate, a chemical made when the acid and carbonates react.
Try it again, but this time use chalk, which is also made of calcium carbonate. Set one piece of chalk in a cup of vinegar and one in water. The chalk will immediately start reacting with the vinegar, making quite a show! Pour off the liquid after about an hour and compare the chalk that was in the water to the chalk in the vinegar. Is there sediment in the bottom of the vinegar cup? Is there any sediment in the water cup?
You can try this experiment again with different strengths of acid. Try lemon juice, diluted lemon juice, coke, or anything else you can think of!
Science Spotlight: Gemstones
Wind and water and blowing sand can, over time, rub away the rough edges of rocks, leaving smooth stones. In nature this is called 'weathering.' But weathering can happen at home, too! Many people use a rock tumbler to shape and polish the rocks they collect. A rock tumbler mixes the stones with several types of grit to rub away the roughness, just like rocks 'tumbled' with sand in an ocean or river. The results are smooth, shiny rocks. When people tumble gemstones (usually semi-precious ones they find), they can end up with beautiful colors and can even use the polished stones to make their own jewelry.
A gem or a gemstone is any mineral that can be cut and polished for jewelry or other decoration. The most precious gems are chosen for their beauty, rarity, and durability. Semi-precious gems usually have one or two of these characteristics, but fall short in some area. Fluorite, for instance, is very beautiful but it is too soft and will scratch easily. Agate, quartz, and amethyst are other examples of semi-precious gems.
Diamonds are gemstones that are considered very precious, and for good reason. Though they are made of carbon, one of the most common elements, diamonds are generally regarded as some of the most beautiful gemstones. They are relatively rare, because much diamond is not of jewelry quality. Diamond mines usually have 1 part diamond to 40 million parts other rock, but a diamond high-quality enough to be in an engagement ring is the product of the removal and processing of 200 to 400 million times its volume of rock! The diamond's strongest point, however, is its durability. It is the hardest substance found in nature, four times harder than the next hardest natural substance, corundum (sapphire and ruby). The grit in a regular rock tumbler wouldn't have much effect on a diamond! It also has the highest melting point, and conducts heat five times better than the second best element, silver.
Erosion Speed. Erosion of rocks usually happens very slowly, but once in awhile a catastrophic event speeds things up. On May 18, 1980, the Mount St. Helens volcano in Washington State erupted. By the end of the nine-hour eruption, the mountain was 1,314 feet shorter than it had been on May 17th! The landscape was changed beyond recognition in a very short time, though it would have taken centuries to do so by normal erosion.
Diamond Fame. Most diamonds are very small, so when someone finds a large one, it's a big deal. The largest diamonds in the world are famous - they even have their own names! Sometimes diamonds are named after their owners, like the blue Hope Diamond or the Burton-Taylor diamond (Richard Burton bought it for Elizabeth Taylor). The Excelsior is one of the biggest diamonds ever found. Before it was cut into 10 pieces it weighed 995 carats (most diamonds in engagement rings are one carat or less). Read about some of the world's largest diamonds on this site.
See the rock cycle 'in action' with this interactive rock cycle animation.