Italy's Mount Etna provides a vivid image of one of the birthplaces of igneous rock during a night eruption. Igneous rock forms when magma cools and solidifies. Lava is magma that has reached the Earth's surface. Etna also shows off the etymology of igneous, from the Latin word for fire. Rocks are so common that most of us take them for granted—cursing when we hit them with the garden hoe or taking advantage of them to drive in tent pegs on summer camping trips.
To geologists , a rock is a natural substance composed of solid crystals of different minerals that have been fused together into a solid lump. The minerals may or may not have been formed at the same time. What matters is that natural processes glued them all together. Extremely common in the Earth's crust, igneous rocks are volcanic and form from molten material.
They include not only lava spewed from volcanoes, but also rocks like granite, which are formed by magma that solidifies far underground. Typically, granite makes up large parts of all the continents. The seafloor is formed of a dark lava called basalt, the most common volcanic rock. Basalt is also found in volcanic lava flows, such as those in Hawaii, Iceland, and large parts of the U.
Granite rocks can be very old. Weathering therefore occurs more slowly in granite than in layered sedimentary rocks. Rainfall and temperature can affect the rate in which rocks weather. High temperatures and greater rainfall increase the rate of chemical weathering. Rocks in tropical regions exposed to abundant rainfall and hot temperatures weather much faster than similar rocks residing in cold, dry regions. Soils affect the rate in which a rock weathers. Soils retain rainwater so that rocks covered by soil are subjected to chemical reactions with water much longer than rocks not covered by soil.
Soils are also host to a variety of vegetation, bacteria and organisms that produce an acidic environment which also promotes chemical weathering. Minerals in a rock buried in soil will therefore break down more rapidly than minerals in a rock that is exposed to air. The longer a rock is exposed to the agents of weathering, the greater the degree of alteration, dissolution and physical breakup. Lava flows that are quickly buried by subsequent lava flows are less likely to be weathered than a flow which remains exposed to the elements for long periods of time.
Chemical weathering is a process where minerals in a rock may be converted into clays, oxidized or simply dissolved.
Silicates comprise almost all minerals in igneous rocks and are also important components in metamorphic rocks. Not all silicates, however, survive weathering processes to become incorporated into sedimentary rocks. Figure 6. For example, interlocking silicate grains in fresh granite gradually decay along crystal boundaries due to conversion to clays.
Eventually cracks open around the boundaries, the rock weakens and easily disintegrates. Water dissolves some of the solid, leaving behind an altered material and producing a solution containing substances drawn from the original solid coffee grounds. The acid rainwater than reacts with minerals on the exposed rock face. Together with advancing ice, the rocks carve out a path as the glacier slumps down the mountain.
When the glacier begins to melt, it deposits its cargo of soil and rock, transporting the rocky debris toward the sea. Every year, rivers deposit millions of tons of sediment into the oceans. Without the erosive forces of water, wind, and ice, rock debris would simply pile up where it forms and obscure from view nature's weathered sculptures.
Although erosion is a natural process, abusive land-use practices such as deforestation and overgrazing can expedite erosion and strip the land of soils needed for food to grow. All rights reserved. Share Tweet Email. Why it's so hard to treat pain in infants. This wild African cat has adapted to life in a big city.
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A conservative estimate is that each of these steps would take approximately 20 million years some may be less, others would be more, and some could be much more. How long might it take for this entire process to be completed? Unless they are re-eroded and moved along, sediments will eventually be buried by more sediments. At depths of hundreds of metres or more, they become compressed and cemented into sedimentary rock.
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