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Igneous rock: Pumice
Tectonics is the process of movement and deformation of the Earth's crust. This includes the formation of fractures, folds, basins, and mountain ranges. Tectonics is studied within the field of structural geology. The driving force of tectonics lies in the interior of the Earth where much heat is produced. As a result, the Earth's plates move slowly across the surface of the Earth. Plate tectonics is perhaps the most important geological process that the Earth has formed into what it is today.
Causes of tectonics
Because heat in the interior of the Earth, caused by radioactive decay, there are convection currents in the Earth mantle. These convection currents are the engine of plate tectonics because the heat from inside the Earth is finds a way out. The flow in the Earth's mantle occurs in multiple cells and is moving upwards, sideways, and then again into the depths. The tectonic plates move with the flow in the mantle. Because the tectonic plates are in constant motion, and processes in the mantle exert forces on the Earth's crust, there are many places where mechanical stress is present in the Earth's crust. This can be compressive, shear and tension stress. This tension creates deformation of rocks in the Earth's crust.
Tension stress for example, is caused by fracturing, rifts and normal faults. Compression causes initially reverse faults and folds. If this process continues, mountain ranges may form: orogeny. Shear stress causes horizontal movement in faults. If the fracture line is not straight, so-called pull-apart basins may occur along the fault line. Because the fault line is not straight, basins occur at the bends in the fracture line. An example of this is the Dead Sea in Jordan. At low pressure and temperature, the deformation is brittle and results in cracks in the rock. At higher pressure and temperature, plastic deformation occurs, and even hard rocks can be folded without breaking.
Plates and plate boundaries
Plate tectonics is the theory that explains movement of tectonic plates on the Earth. This theory was only accepted by the scientific community in 1960. The Earth's crust is divided into tectonic plates. There are both continental and oceanic tectonic plates. The plates of crust "float" on the underlying plastic mantle. The movement of the Earth's plates is caused by convection currents driven by heat in the Earth's mantle. Most tectonics and volcanism occurs at the plate boundaries.
The continental plates are lighter than the oceanic plates. As a result, the oceanic plate will subduct under the continental plate in case of a collision of these plates. In places where subduction of an oceanic plate occurs is often a deep trench present. In collision zones of tectonic plates mountains will form. Plate tectonics is also the cause of the occurrence of a major proportion of earthquakes and volcanic activity on the Earth. An exception are the hotspots.
On Earth, nine distinct large plates and many smaller and microplates occur. The movement of the plates can be as much of a few millimeters per year to 9 centimeters. In the course of geological time, entire continents can therefore move to a different part of the Earth. In the geological past continents repeatedly merged together and formed the super continents Gondwana and Pangea.
In case of a collision of two continental plates orogeny, mountain building occurs. A good example is the Himalayas that are still uplifted higher by the ongoing collision between the Indian plate and the Eurasian plate. In spreading zones in the oceans, new oceanic crust is created in the so-called Mid Oceanic Ridge. Rift volcanoes provide material for the formation of new oceanic crust.
Uplift and basin formation
By slow changes in the convection currents in the mantle, the temperature can vary in the Earth's mantle. Upon lowering the temperature below the Earth's crust, the crust will sink and a basin is formed. With an increase in the temperature, a uplift area occurs. This process has a major influence on the sedimentation and erosion at the Earth's surface. In some places large amounts of heat are coming up from the mantle. Atthis so-called hot spots, volcanism is common, and because of the uplift also deformation by tectonics.
Because the mantle is plastic, the crust floats (lithosphere) on the mantle (astenophere). Like an iceberg floating in the water searching, the floating plate is continuous seeking equilibrium. If the crust is heavier, it will sink in the course of geological time and float at a deeper level on the astenosphere. If the crust becomes lighter, the crust will rise.
Erosion and sedimentation affect the isostasy, as well as the formation and melting of ice sheets. In northern Sweden, the land is still slowly rising after the melting of a kilometers-thick ice sheet after the last ice age. Also, the presence of a mountain bends down the Earth's crust in the surrounding area due to the large weight. In geological terms, the Earth's crust is not stable, but moving and changing continuously.
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