The changing Earth

THE EARTH FORMED FROM A CLOUD OF DUST and gas drifting through space about 4,600 million years ago. Dense minerals sank to the centre while lighter ones formed a thin rocky crust. However, the first known life forms- bacteria and blue green algae- did not appear until about 3,400 million years ago, and it was only about 700 million years ago that more complex plants and animal began to develop. Since then, thousands of animal and plants species have evolved; some, such as the dinosaurs, survived for many million years, while others died not quickly. The Earth itself is continually changing. Although continents neared their present locations about 50 million years ago, they are still drifting slowly over the planet’s surface, and mountain ranges such as Himalayas-which began to form 40 million years ago are continually being built up and worn away. Climate is also subject to the change: the Earth has undergone a series of ice ages interspersed with warmer period (the most recent glacial period was at its height about 20,000 years ago).






       THE EARTH’S CRUST

THE EARTH IS THE SOLID outer shell of the Earth. Its include the continental crust (about 40 kilometres thick) and oceanic crust (about six kilometres thick). The crust and the top most layer of the mantle form the lithosphere. The lithosphere consist of semi- rigid plate that move relative to each other on the underlying asthenosphere (a partly molten layer of mantle). This process is known as plate tectonics and helps explain continental drift. Where two plates move apart, there are rifts in the crust. In mid ocean, this movement results in the sea floor spreading can form rift valleys. When plates move towards each other, one may be sub ducted beneath (forced under) the other. In mid ocean, this causes oceanic trenches, seismic activity, and an arc of volcanic islands. Where oceanic crust is sub ducted beneath continental crust or where continents collide, lands may be uplifted and mountains formed plates may also slide past each other- along the San Andreas fault, for example. Crustal movements on continents may result in the earthquakes, while the movements under the seabed can lead to tidal waves.




        FAULTS AND FOLDS

THE CONTINUOUS MOVEMENTS of the Earth’s crust plates can squeeze, stretch or break rock strata, deforming them and producing faults and folds. A fault is a fracture in a rock along which there is a movement of one side relative to the other. The movement can be vertical, horizontal, or oblique (vertical or horizontal). Faults develop when rocks are subjected to compression or tension. They tend to occur in the hard, rigid rocks, which are more likely to break then bend. The smallest faults occur in single mineral crystals and are microscopically small, whereas the largest – the Great Rift valley in Africa, which formed between 5 million and 100,000 years ago – is more than 9,000 kilometres long. A fold is a bend in a rock layer caused by the compression. Folds occur in the elastic rocks which tend to bend rather than break. The two main types of folds are anticlines (upholds) and synclines (down folds). Folds vary in size in few millimetres  long to folded the mountain ranges hundreds of kilometres long, such as the Himalayas and the Alps, which are repeatedly folding. In addition to folds and faults, other features associated to the rock deformation include the boudins, mullions, and en echelon features.




 Mountain building

THE PROCESSES INVOLVED in mountain building- termed orogenesis – occur as a result of movement of the Earth’s crustal plates. There are three main types of mountains: volcanic mountains, Fold Mountains, block mountains. Most volcanic mountains have been formed along the plate’s boundaries where the plates have come together or move apart and lava and other debris have been ejected on to the Earth’s surface. The lava and debris may have built up to form a dome around the vent of a volcano. Fold Mountains are formed where the plates push together and cause the rock to buckle upwards. Where oceanic crust means less dense the continental crust, the oceanic crust is forced under the continental crust. The continental crust is buckled by the impact. This how the folded mountain ranges, such as the Appalachian Mountains in the North America, were formed. Fold Mountains are also formed where two areas of continental crust meet. The Himalayas, for example, began to form when India collide with the Asia, buckling the sediments and part of the oceanic crust between them. Block Mountains are formed when the block of land is uplifted between two faults as a result of compression or tension in the Earth’s crust. Often, the movement along the faults has taken place gradually over the millions of years. However, the two plates may cause n earthquake by suddenly sliding the past each other along a fault line.