The earth’s globe is made up of three layers. The surface layer is called the crust. It’s thickness varies from about 8 km under the oceans to about 30 to 50 km under the continents. The next layer is the mantle, made up of harder, denser, rocks than the crust. It is about 3000 km thick. Next is the core, believed to be liquid outside, then as far as we know, solid right at the centre.
Igneous rocks
The rocks in the crust are classified into three main groups, according to how they were formed: igneous rocks, metamorphic rocks and sedimentary rocks.
‘Igneous’ comes from the Latin word for fire, and igneous rocks are formed from molten material called magma that has cooled and hardened. Magma is molten rock. It probably come from the mantle where solid rock is turned into liquid through a combination of great heat and pressure. When this happens, some of the magma is forced up through cracks in the earth’s crust. An active, or erupting, volcano forces out streams of magma in the form lava. Lava cools very quickly when it reaches the air and becomes one form of igneous rock.
The type of igneous rock formed by cooling lava is called extrusive, because it is forced out or extruded from the interior of the earth. The speed at which the lava cools causes small crystals to form and the rocks are therefore smooth-surface of fine-textured like basalt and obsidian, although pumice is porous becaise of the gas bubbles.
Another type of igneous rock is called intrusive and here the magma cools slowly beneath the earth’s surface. This sometimes forms large masses called batholiths. These rocks are coarse-textured with large crystals resulting from slow cooling. Typical intrusive igneous rocks are granite, gabbro and syenite.
Minerals found in both extrusive and intrusive igneous rocks include felspar, hornblende, mica, quartz and pyroxene. Some of the oldest rocks on earth are igneous, but so are some of the newest. Every time you hear of a volcano erupting and pouring out lava, you know that when the lava cools, new igneous rocks will be formed.
Sedimentary rocks
Sedimentary rocks are formed by the slow but never-ending process of erosion or weathering which breaks down the hardest rocks to small particles. Over millions of years these particles of rock are carried from their original site by the wind, rivers, glaciers and ice sheets to be deposited somewhere else. Eventually, layers of rock sediment are built up and become compacted and cemented together to form new rocks. Sedimentary rocks form a small part of the earth’s crust, but they form a large part of the surface of the earth, because it is only on the surface that weathering and erosion takes place. Sedimentary rocks are important because they sometimes contain fossils (the remains of plants and animals) which give clues to what life was like on earth hundreds of thousands of years ago.
Metamorphic rocks
Metamorphism is a word that means change from one thing to another. Metamorphic rocks are rocks that were originally either igneous or sedimentary and have been changed by heat or pressure or a combination of both. Metamorphism is caused by the upwelling of molten magma, or by great movements of the earth that squeeze and crumple rocks into mountain ranges.
When magma forces itself to between cracks int he earth’s crust, it is extremely hot and its heat bakes and hardens rocks around it. Sometimes the heat releases substances from the rocks and this also changes their composition. When mountains are formed, pressure crushes and grinds the rocks. When heat and pressure are combined, other changes takes place. This is how we get marble, which was originally sandstone. Slate is another example, formed from compressed clay and shale. As slate is in layers which can be split easily, it can be used for school blackboards and roofing tiles. Today it is mainly used for flooring tiles.
Igneous rocks
The rocks in the crust are classified into three main groups, according to how they were formed: igneous rocks, metamorphic rocks and sedimentary rocks.
‘Igneous’ comes from the Latin word for fire, and igneous rocks are formed from molten material called magma that has cooled and hardened. Magma is molten rock. It probably come from the mantle where solid rock is turned into liquid through a combination of great heat and pressure. When this happens, some of the magma is forced up through cracks in the earth’s crust. An active, or erupting, volcano forces out streams of magma in the form lava. Lava cools very quickly when it reaches the air and becomes one form of igneous rock.
The type of igneous rock formed by cooling lava is called extrusive, because it is forced out or extruded from the interior of the earth. The speed at which the lava cools causes small crystals to form and the rocks are therefore smooth-surface of fine-textured like basalt and obsidian, although pumice is porous becaise of the gas bubbles.
Another type of igneous rock is called intrusive and here the magma cools slowly beneath the earth’s surface. This sometimes forms large masses called batholiths. These rocks are coarse-textured with large crystals resulting from slow cooling. Typical intrusive igneous rocks are granite, gabbro and syenite.
Minerals found in both extrusive and intrusive igneous rocks include felspar, hornblende, mica, quartz and pyroxene. Some of the oldest rocks on earth are igneous, but so are some of the newest. Every time you hear of a volcano erupting and pouring out lava, you know that when the lava cools, new igneous rocks will be formed.
Sedimentary rocks
Sedimentary rocks are formed by the slow but never-ending process of erosion or weathering which breaks down the hardest rocks to small particles. Over millions of years these particles of rock are carried from their original site by the wind, rivers, glaciers and ice sheets to be deposited somewhere else. Eventually, layers of rock sediment are built up and become compacted and cemented together to form new rocks. Sedimentary rocks form a small part of the earth’s crust, but they form a large part of the surface of the earth, because it is only on the surface that weathering and erosion takes place. Sedimentary rocks are important because they sometimes contain fossils (the remains of plants and animals) which give clues to what life was like on earth hundreds of thousands of years ago.
Metamorphic rocks
Metamorphism is a word that means change from one thing to another. Metamorphic rocks are rocks that were originally either igneous or sedimentary and have been changed by heat or pressure or a combination of both. Metamorphism is caused by the upwelling of molten magma, or by great movements of the earth that squeeze and crumple rocks into mountain ranges.
When magma forces itself to between cracks int he earth’s crust, it is extremely hot and its heat bakes and hardens rocks around it. Sometimes the heat releases substances from the rocks and this also changes their composition. When mountains are formed, pressure crushes and grinds the rocks. When heat and pressure are combined, other changes takes place. This is how we get marble, which was originally sandstone. Slate is another example, formed from compressed clay and shale. As slate is in layers which can be split easily, it can be used for school blackboards and roofing tiles. Today it is mainly used for flooring tiles.
No comments:
Post a Comment