Volcanoes

Volcanoes are associated with the boundaries of the plates that make up Earth’s top layer (the lithosphere) as well as with the so-called hot spots in the middle of plates. Most of Earth’s volcanoes are found along the “ring of fire” that circles the Pacific plate. Plates meet along this ring which causes tremendous tension or friction, as one plate moves beneath another. The rock melts and some of it moves upward as magma. As pressure mounts a volcanic eruption is the result. These eruptions can be explosions or quiet outpourings of lava. The lava solidifies as ash, volcanic glass and igneous rocks. Glasses occur when the lava cools so quickly that the atoms in the lava have no time to order themselves. Obsidian, for example, is one of these silicate mixtures called natural glass. If the lava had been allowed to cool more slowly, then granite may have been the result.

What are sedimentary rocks?

Sedimentary rocks form when loose materials are compacted and cemented by Earth’s processes. These materials include bits and pieces of minerals and other rocks and minerals from oceans and lakes. This process sounds simple but it can take millions of years for those collections of small particles or sediments to become rocks.

When rocks are changed by wind, water, ice, and gravity, particles – some small, some large – form sediment.

Sedimentary rock can be formed in bodies of water as well as in dry, desert-like conditions. In other case, sediment is formed from wind, water, the buildup of natural remains, or some kind of chemical reaction.

One way sedimentary rock is formed is from the buildup of shell fragments, corals and animal skeletons in sedimentary beds in the bottom of oceans, lakes and other waterways. Sometimes beds of sedimentary rock form from chemical solutions such as calcium carbonate or sodium chloride, trickling into lakes without outlets or into cutoff arms of the sea. The calcium carbonate or sodium chloride then hardens into sedimentary rocks. Regardless of the way the layers of sediment are deposited they all form into rock through the same process of nature – the layers of sediment are cemented or bonded together over long periods of time. The bonding agent is usually one of three common materials – calcite, silica or iron oxide. Calcite is found in water that percolates between the sediment beds.

Clastic sedimentary rocks are formed from fragments of minerals plus other rocks, organic matter, or both. Most sedimentary rocks that contain fragments larger than two milimetres, as well as sand stones and shales are in the clastic category.

Some sedimentary rocks are formed from solutions. These rocks which include some limestones, rock gypsum and rock salt are nonclastic. Clastic sedimentary rocks are classified according to grain size. Thus, a sandstone can be composed of quartz, calcite, rock particles, feldspar or a combination of all of these. Shale can also be composed of these same things but the “things” are smaller in size.

Дата добавления: 2015-07-19