Structural geology is the systematic study of the deformed rocks which constitute the planet Earth’s upper layers of geological materials. Thus, it is the study of the deformation of the Earth from a planetary perspective as the major structural features are described. It also describes the situation and processes by which the upper layers of the planet undergo horizontal and vertical displacement.
In the universe, each planet or moon exhibits its own distinctive process and style of deformation. This reflects the degree of movement of the planetary interior as well as its ratio of volume. As a planet radius increases, the rate of cooling of the planet drops, since there is less surface area to cool across. Thus, larger planets are hotter and more mobile, due to the dependence of rock strength on temperature. This causes a planet interior to be able to flow plastically.
While the bed of rock lying on the surface of a planet is brittle and relatively weak, the rocks that lie deeper underneath are stronger, due to pressure. However, when we reach some depth, the effect of increasing temperature causes the rock strength to drop as it become weaker the deeper we go. Hence, the a layer of high rock strength is found near the surface of a planet. This geological layer is called lithosphere, with ‘litho’ meaning rock. The most fundamental structural property of the Earth is the distinction between ocean basins and continents, which are the most obvious features of its surface. The boundary that divides continent from ocean is found at 2000 m below sea level.
The continents of the planet consists largely of 100/3000 million years old rocks, which have been repeatedly deformed. They are rich in quartz and feldspar.
Lower Mantle
The lower mantle is one of the five geological layers of Earth. It is different from the other geological layers of the Earth, being the site of molten rocks that are ejected to the surface through volcanoes. Although it is also molten rocks, it is more solid than the upper mantle as it flows very slowly, like a very dense liquid.
The main process that helps shape the planet take place in the lower mantle, playing an important role in creating volcanoes. Hot convection forces in this layer cause molten rocks and mineral to boil so intensely that they explode through the Earth’s crust, giving birth to volcanoes and the formation of rock islands in the oceans (Pacific).
Characteristics
The lower mantle is the thickest continuous layer of the Earth’s as it constitutes more than 50% of its size and the 70% of the planet’s mass. It measures about 1,550 miles (2,500 km) thick, having an estimated temperature of 4,000° F (2,204° C). It is believed that this layer is made mostly of magnesium, silicon, and oxygen, with smaller amount of iron, calcium, and aluminum. Combined together, these elements form a rare mineral called perovskite.
The age of the lower mantle is hard to figure out, simply because its molten rocks cannot be reached to take a sample and study it. However, since this layer was formed very early in the geological history of the Earth, it is probably billions of years old. At the same time, the lower mantle’s rocks change their form slowly and regularly as the layer convects and boils, with older rocks changing into new, younger ones.
You must remember that convection is the propagation of heat in a liquid. Hot melted rock is less dense and lighter than cooler molten rock, which is more dense and heavier. As the hotter lighter rock rises up to the top, the cooler and dense one sinks down.
Below, diagram of the Earth geological layers, showing the upper and lower mantle.
Upper Mantle
The upper mantle of the planet Earth is the geological layer which is located right below the crust. It is not made up of solid materials but it is rather a very thick, slow-moving liquid of molten rocks. The crust has been formed by these molten rocks that have been ejected from deep below over hundreds of millions of years. Layers upon layers of sediment that derive from once upon-a-time living creatures and vegetation lie on top of the crust. Consisting of the lithosphere and the asthenophere, it is estimated that the upper mantle measures about 217 miles (350 km) thick. The lithosphere in turn is also a part of the deep part of the crust, overlapping both.
As the upper mantle drifts slowly around the Earth surface, so does the crust lying on top of it. This geological process is called plate tectonics. According to the theory, plate tectonics have shaped the continents, islands, mountains, causing the strongest earthquake on the planet. The upper mantle and the crust move so slowly that it cannot be observed on a day to day, or a week to week, basis, for it takes millions upon millions of years to shape huge mountain ranges, such as the Andes or the Himalaya.
Composition
The top section of the upper mantle (the lithosphere) is rigid, almost solid, and it is 40-mile deep. It is composed of peridotites. A peridotite is a plutonic igneous rock, which consists of olivine. Olivine is in turn a yellowish green mineral made up of silicate containing iron and magnesium, mainly (Mg, Fe)2SiO4. Since peridotites are heavier than most of the rocks found in the Earth crust, they have a tendency to sink down deep into the bottom of the crust and the upper mantle.
The asthenosphere, on the other hand, consists of about 150 miles of molten rocks, which are made up mostly of silicon and magnesium, but it also contains smaller amount of magnesium, iron, aluminum, and calcium. Diamonds and other rocks also erupt onto the surface of the Earth from deep in the upper mantle in some places of the planet.
Below, Diagram/Drawing of Earth Geological Layers, Showing the Upper Mantle.
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