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| Sunday, 17 July 2005 |
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Junior Observer |  |
| News Business Features Editorial
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Solid as a rock
Thousands of different types of rocks and minerals have been found on Earth. Most rocks on the Earth's surface comprise only eight elements (oxygen, silicon, aluminium, iron, magnesium, calcium, potassium, and sodium), but these elements are combined in a number of ways to make rocks that are very different from each other. All rocks are made of two or more minerals. When rocks break down into smaller and smaller pieces, they turn into sand. If you look at sand under a microscope, you will see that the sand is made up of the same minerals as the rocks that it came from. When plants start to sprout up in sand, it is turning from being just small bits of rock to soil. Rocks are constantly being formed, worn down and then formed again. This is known as the rock cycle. It takes millions of years for rocks to change. Erosion is a key part of the rock cycle. Water is a main cause of erosion. When it falls as acid rain, it can dissolve rocks that are sensitive to acid. Marble and limestone wither when exposed to the rain. When the rain falls very heavily, as in monsoons, flooding can happen. Rivers with a lot of rushing water can cause mud slides and erode river banks. The action of waves on a beach causes erosion. The waves pound on the rocks and over time, cliffs crumble. That is why you will often find sand and little pebbles on beaches. Rushing water, like what you find in rivers that move quickly in the mountains, or strong waves on the shores of oceans, roll rocks around. This wears off the sharp edges of the rocks, and that is why river rocks are smooth and beach pebbles look polished.
The freeze and thaw cycle causes mountains to crumble over time and large rocks to break down into little rocks. Water which gets into cracks in the rocks expands during the freeze cycle, making the cracks bigger. When the cracks fill up with water during the thaw period, it allows more water to go deeper into the rock, making the rocks split apart when they freeze again. Wind, when it carries bits of sand and grit, can blast away layers of rocks. The wind can easily pick up little bits of sand and sandblast the rocks that are in its way. Sometimes only the soft layers of the rock are eroded, leaving interesting shapes. This kind of erosion usually happens only in very dry, desert-like areas. Geologists classify rocks into three groups, according to how they were formed. The three rock groups are igneous, sedimentary, and metamorphic rocks. Igneous rocks Igneous means made from fire or heat. When volcanoes erupt and the liquid rock comes up to the earth's surface, new igneous rock is formed. When the rock is liquid and inside the earth, it is called magma. When the magma hardens inside the crust, it turns into granite. Most mountains are made of granite. It cools very slowly and is very hard. When the magma gets to the surface and flows out during a volcano eruption, the liquid is called lava. Lava flows down the sides of the volcano. When it cools and hardens, it is called obsidian, lava rock or pumice - depending on what it looks like. Igneous rocks form when molten lava (magma) cools and turns to solid rock. The magma comes from the Earth's core which is molten rock. The core makes up about 30 per cent of the Total Earth Mass. There are five kinds of igneous rocks, depending on the mix of minerals in the rocks. Granite contains quartz, feldspar and mica. Diorite contains feldspar and one or more dark mineral with feldspar being dominant. Gabbro contains feldspar and one or more dark mineral with the dark minerals being dominant. Periodotite contains iron and is black or dark. Pegmatite is a coarse-grained granite with large crystals of quartz, feldspar and mica. Obsidian is nature's glass. It forms when lava cools quickly on the surface. It is glassy and smooth. Pumice is full of air pockets that were trapped when the lava cooled when it flowed out onto the surface. It is the only rock that floats. Metamorphic rocks
Metamorphic rocks are rocks that have changed. The word comes from the Greek meta and morph which means to change form. Metamorphic rocks were originally igneous or sedimentary, but changed due to the movement of the Earth's crust. When the Earth's crust moves, it causes rocks to get squeezed so hard that the resultant heat and pressure cause the rock to change form. Marble is an example of a sedimentary rock that has changed into a metamorphic rock. Metamorphic rocks are the least common of the three kinds of rocks. Foliated metamorphic rocks have layers, or banding. Slate is transformed into shale and can be split into smooth slabs. Schist is the most common metamorphic rock. Gneiss has a streaky look because of alternating layers of minerals. Non-foliated metamorphic rocks are not layered. Marble is transformed limestone. More on sedimentary rocks later |
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The whole earth is made of rocks and minerals. Inside the earth, there
is a liquid core of molten rock and outside, there is a hard crust made up
of rocks and minerals. Much of the crust is covered by water, sand, soil
and ice. If you dig deep enough, you will always hit rocks. Below the
loose layer of soil, sand and crumbled rocks is bedrock, which is a solid
rock. 
