Sunday, September 30, 2007
Loess is a geologically recent deposit of silt or material which is usually yellowish or brown in color and consisting of tiny mineral particles brought by wind to the places where they now lie. It is a product of past glacial activity in an area. It is a sedimentary deposit of mineral particles which are finer than sand but coarser than dust or clay, deposited by the wind. Loess is a type of silt which forms fertile topsoil in some parts of the world. Loess deposits are usually a few meters thick. One of the key characteristics of these deposits is the ‘cat steps'. The soil has few clay particles to hold it together. It is composed mainly of quartz crystals which slide easily against each other, and is therefore very subject to erosion. Because of this, there are mini-earth slides, which form the steps.
Loess was formed during the time after the Ice Age when glaciers covered a great portion of the earth. When the climate warmed up, the warm temperatures melted the glaciers creating tremendous flows of water down into a valley or river, and exposing vast plains of mud. When these plains dried, strong winds blew the exposed sediments and swept the finer materials from the flood plains into huge clouds of dust, which were deposited into the bluffs, that is, bold steep banks. As silt accumulated, higher bluffs were formed. Often several loess deposits are stacked on top of each other, because each individual glacier produced new loess deposits. Topsoils made up of loess are found in the central and northwestern parts of United States, in central and eastern Europe, and in eastern China.
Posted by Princessa at 9/30/2007 10:29:00 PM
Hills are elevations of the earth's surface that have distinct summits, but are lower in elevation than mountains. Hills may be formed by a buildup of rock debris or sand deposited by glaciers and wind. Hills may be created by faults. Faults are a slight crack in the earth which can cause earthquakes. Hills are formed when these faults go slightly upward. The most famous hills in the world are the Loess hills. The Black Hills are also famous. You can find hills in low mountain valleys, valleys, plains, and even in your own backyard.
Hills are also formed by deep erosion of areas that were raised by disturbances in the earth's crust. Erosion forms hills by carrying away all of the soil on a mountain, causing a hill to be left behind. Humans also make hills by digging soil up and dumping it in a giant pile. Volcanoes are also another way that hills are formed. Volcanoes form hills when they erupt. During the eruption, volcanic ash is spewed through the air; after the eruption, the lava or molten rock hardens and builds up a thick layer of lava rock. The ash falls on the hardened lava causing a layer of ash to form on the hill. When rain falls, this layer of ash mixes with the rainwater to form black colored water. This black water will freeze causing the lava rock to crack and crumble and eventually erode to form a hill.
Posted by Princessa at 9/30/2007 08:52:00 PM
Over long periods of time, mountains are created by tremendous forces in the earth with a steep top usually shaped up to a peak or ridge. Mountains occur more often in oceans than on land; some islands are the peaks of mountains coming out of the water. Mountains are formed by volcanism, erosion, and disturbances or uplift in the earth's crust. Most geologists believe that the majority of mountains are formed by geological forces heat and pressure producing changes under the earth's crust and movements in the earth's crust. They call this movement plate tectonics. This theory sees the crust of the earth divided into a number of vast rigid plates that move about at the rate of a few centimeters a year. The uplift is caused by the collision of plates below the earth's surface that triggers various geologic processes that produce this crustal uplift. Other processes are caused by horizontal compression that is the deformation of crustal strata which produces folds or wrinkles. The Himalayas, for example, were raised by the compression that accompanied collision of the Indian plate with the Eurasian plate. Another example is Europe's Alps and Jura mountains which were also formed by horizontal compression, generated in their case by collision with the African plate and the Eurasian plate.
Some ranges of low mountains are raised by nontectonic processes, and are caused by sculpturing effects of differential erosion. Erosion occurs when wind, rain and ice are present. Mountains are impacted by erosion through the combined action of wind, rain and ice changing the shapes of the mountains.
Volcanism causes mountains to form. Examples of mountains formed by periodically dangerous volcanic action are Mount Ranier and Mount Saint Helens in the United States, Mount Erebus in Antarctica, Mount Vesuvius in Italy, and Mount Fuji in Japan. Many of these volcanic mountains have summit craters that still emit steam and debris; others that no longer show signs of volcanic activity may only be dormant, not extinct. Shield volcanoes found in Mauna Loa and Mauna Kea in Hawaii are less spectacular even when quite high.
Posted by Princessa at 9/30/2007 08:29:00 PM