3.2: Few Drops of Geology

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Geological Time

Stratigraphy refers to the natural and cultural soil layers that make up an archaeological deposit. The notion is tied up with 19th-century geologist Charles Lyell, who stated that because of natural forces, soils found deeply buried will have been laid down earlier—and therefore be older—than the soils found on top. The age of rock layers are determined by fossils using relative dating and absolute dating. If there are same fossils in more than one layers, these layers have the same age The majority of the time fossils are dated using relative dating techniques. Using relative dating the fossil is compared to something for which an age is already known.

Absolute dating is used to determine a precise age of a rock or fossil through radiometric dating methods. Radioactive minerals occur in rocks and fossils are almost a geological clock. Radioactive isotopes break down at a constant rate over time through radioactive decay. By measuring the ratio of the amount of the stable isotope to the amount of the radioactive isotope, an age can be determined.

Origin of Earth

Pierre-Simone Laplace reached the conclusion that the stability of the Solar system would best be accounted for by a process of evolving from chaos. Laplace suggested that:

The Sun was originally a giant cloud of gas or nebulae that rotated evenly.
The gas contracted due to cooling and gravity.
This forced the gas to rotate faster, just as an ice skater rotates faster when his extended arms are drawn onto his chest.
This faster rotation would throw off a rim of gas, which following cooling, would condense into a planet.
This process would he repeated several times to produce all the planets.
The asteroids between Mars and Jupiter were caused by rings which failed to condense properly.
The remaining gas ball left in the centre became the Sun.

It is frequently accepted nowadays that in addition to the above processes, Earth underwent the heating stage and at some point likely became a “lava ball”, and then cooling stage when water start to condense and make primary ocean. Also, geology and astronomical features of Moon suggest that this body originated from Earth on the some very early stage of Solar system evolution.

Structure of Earth

The Earth consists of concentric layers, core, mantle and crust. Core is in the center and is the hottest part of the Earth. It is solid and possibly made up of metals with temperatures of up to 5,500°C. Mantle is the widest section of the Earth. It has a thickness of approximately 2,900 km. The mantle is made up of semi-liquid magma. Crust is the outer layer of the Earth. It is a thin layer up to 60 km deep. The crust is made up of tectonic plates, which are in constant motion. Earthquakes and volcanoes are most likely to occur at plate boundaries.

Everything in Earth can be placed into one of four major subsystems: lithosphere (land), hydrosphere (water), biosphere (living things), and atmosphere (air). Earth is the only known planet that has a layer of water.

The differentiation of Earth body finally resulted in developing of lighter gas layer on the surface (primary atmosphere), initially very thin and relatively cold. Therefore, water vapour were condensed into primary ocean (primary hydrosphere). According to the principle of actuality, it should be close to today’s volcanic gases 15% of CO\(_2\), plus CH\(_4\) (methane), NH\(_3\) (ammonia), H\(_2\)S, SO\(_2\) and different “acidic smokes” like HCl.

Plate Tectonics

Alfred Wegener is best known as the creator of the theory of continental drift by hypothesizing in 1912 that the continents are slowly gliding around the Earth. According to Wegener, in the beginning of Mesozoic era, there were two large continents, Gondwana and Laurasia which were separated by Tethys Ocean. Gondwana was one of Earth broke up around 180 000 years ago. Moreover, in Permian period, all continents where united in one as Pangaea, which was surrounded by one big ocean.

Mantle is the thickest Earth layer but it slowly moves. Mantle convection breaks the lithosphere into plates and continues to move them around Earth surface. These plates might move alongside each other, move by, and even collide with each other. As a result, oceans basins may open, it may move continents, create mountains, and cause earthquakes. Continents will keep changing their positions due to mantle convection.

Hotspots are the living proofs of mantle convection. In the United States, there are two locations which are considered hotspots: Yellowstone and Hawaii. A hotspot is a place where the intense heat of the outer core radiates through the mantle. The most amazing fact about them is that whereas ocean (Hawaii) or continental (Yellowstone) plates move on, these hotspots stay in place! This is why in the past, Yellowstone was located westward, and Hawaiian volcanoes northward.