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The Earth

Earth is the third planet from the sun that is part of our solar system with eight planets and smaller dwarfs. The composition of the Earth is stony with an atmosphere around it. Atmospheric weather phenomena take place. Like the other planets, the Earth rotates around the sun.

The surface of the Earth 71% consist of oceans and seas and the rest is land. The presence of water is necessary for the existence of life on Earth. Another characteristic is that the tectonic plates an move slowly over the mantle under the influence of plate tectonics. This process has given shape to the Earth for a large extent. The Earth is the only known planet that contains life.

Earth orbit

The Earth revolves around the sun in a year. At the same time the Earth rotates around its own axis, almost once a day. This causes day and night, because only one side of the Earth can be face the sun. Because the Earth's axis is at an angle of 23 degrees with the plane state of the Earth's orbit, there are seasons. In every season a different latitude of the Earth is facing the sun. The Moon revolves around the Earth is a natural satellite. The gravitation of the moon causes the tides in the seas and oceans.

The Milankovi? cycles play a major role in the periodic change of climate on Earth. Multiple periodic variations in the shape of the Earth's orbit, and the position of the Earth's axis have large effects on the amount of sunlight that falls on Earth and the geographical distribution. It involves variations in the eccentricity of Earth's orbit, obliquity of the Earth's axis and the precession of the Earth's axis. These variations all have a different period when the change occurs. This changes the climate periodically and may cause ice ages for example, which in turn have a great influence on the sea level. Variations in sea level are generally well reflected in the stratigraphy of sedimentary rocks. See also the article on Milankovi? cycles.

The stratification (limestone from the Jurassic period) in the picture shows a clear cyclicity. This regularly thinning and thickening of the layers may be caused by periodic changes in the position of the Earth's axis, such as, for example, the Milankovich cycles. The harder limestone represent a period of shallower sea level than the softer marly layers that reflect a deeper depositional environment.

Origin of the Earth

Like the other planets, Earth was probably formed by the clumping of particles by the mutual attraction of a large cloud of particles. The particles gathered first in a kind of disk due to the rotation. Then
the planets formed from the disk, and the sun with the largest mass in the center. Then the planets grew by impacts of meteorites and comets. As a result, also water and gases came to Earth which later formed its atmosphere. The moon was probably formed by a very large impact of another large object on Earth, where a large portion of the material came in orbit around the Earth and formed the Moon. In the beginning the Earth was for the most part a molten mass which rapidly cooled due to the absence of an atmosphere.

The age of the Earth is investigated using radiometric dating of the oldest rocks and crystals. The Earth turns out to be 4.57 billion years old. In the first period of the Precambrian era on Earth intense volcanism took place and soon the basic structure of the Earth with the core, mantle, and crust was present. Pretty soon rocks formed and the process of plate tectonics started. Research on ancient zircon crystals shows that
liquid water must have been already present 4.3 billion years ago.

The first known life was in any case already present 3.5 billion years ago, but possibly life on Earth originated even earlier. How life originated exactly is still unknown. Life on Earth has ultimately changed the composition of the atmosphere by the production of oxygen by photosynthesis. Around 2.3 billion years ago, oxygen was therefore already present in large quantities in the atmosphere. About the first period of life on Earth little is known. First, many rocks from that time are not preserved. Second, the first lifeforms existed only in soft tissues that remain poorly preserved. Only since the Cambrian era and the Cambrian explosion of life, a lot more is known about the development of life on Earth.

The major trend in the evolution of life on Earth became more complex and consisted of more and more species. However, there have also been cases where a significant portion of life was eradicated during several mass extinctions. For example, the extinction of the dinosaurs at the K-Pg boundary.

Structure of the Earth

The Earth consists of a core in the center, the mantle and the Earth's crust at the surface. The core is present from depths of up to 2,900 kilometers to the centre at 6370 km depth. The core consists of a solid inner core
and a liquid outer core. The mantle is located between the crust and the core of the Earth. The top of the mantle is the Mohorovi?i? discontinuity. This limit can only be determined with seismic survey , and it is believed that this limit is caused by differences in the composition of the rock. The mantle comprises a major proportion of the volume of the Earth, because the core is relatively small, and the crust is relatively very thin. The mantle is about 2,900 kilometers thick.

A schematic representation of the inside of the Earth. 1. Continental crust, 2. Oceanic crust, 3 and 4 mantle 5 outer core (liquid), 6 inner core (solid), A Mohorovi?i? discontinuity, B Gutenberg discontinuity, C Lehmann discontinuity. Creative Commons License.

The crust is the outer hard layer of the Earth. The earth's crust consists of igneous rocks such as Basalt , metamorphic rocks, and sedimentary rocks. The top of the crust forms the land surface or seabed. There are two types of crust. Continental crust and oceanic crust. Under the oceans oceanic crust is relatively thin with a thickness up to 10 kilometers. The oceanic crust consists of Basalt, Diorite and Gabbro with a layer of deep-sea sediments on top. Continental crust has a lower density than the oceanic crust and is composed of igneous rocks, metamorphic rocks, and many kinds of sediments. The continental crust has a thickness of 35 to 40 kilometers, but  can reach up to 80 kilometers thickness near mountains.

The crust 'floats' on the mantle, which behaves plastically. Due to plate tectonics the plates float on the Earth's mantle. Due to the higher density of oceanic crust, these will be subducted during a collision of tectonic plates.

The Earth has a magnetic field with a north pole and a south pole. In the course of geological history, north and south poles have been reversed many times. The direction of the magnetic field during forming is captured in the rock, this is an important factor in research into the movements of the Earth plates.

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