Chemical composition
The mass of the Earth is approximately 5.98×1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminum (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.
Internal structure
The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents.
| Geologic layers of the Earth | ||||
Earth cutaway from core to exosphere. Not to scale.
| Depth km | Component Layer | Density g/cm³ | |
| 0–60 | Lithosphere | — | ||
| 0–35 | ... Crust | 2.2–2.9 | ||
| 35–60 | ... Upper mantle | 3.4–4.4 | ||
| 35–2890 | Mantle | 3.4–5.6 | ||
| 100–700 | ... Asthenosphere | — | ||
| 2890–5100 | Outer core | 9.9–12.2 | ||
| 5100–6378 | Inner core | 12.8–13.1 |
Tectonic plates
| Earth's main plates | |||
A map illustrating the Earth’s major plates.
| Plate name | Area 106 km² | |
| African Plate | 61.3 | ||
| Antarctic Plate | 60.9 | ||
| Australian Plate | 47.2 | ||
| Eurasian Plate | 67.8 | ||
| North American Plate | 75.9 | ||
| South American Plate | 43.6 | ||
| Pacific Plate | 103.3 |
According to plate tectonics theory, the outermost part of the Earth’s interior is made up of two layers: the lithosphere, comprising the crust, and the solidified uppermost part of the mantle. Below the lithosphere lies the asthenosphere, which forms the inner part of the upper mantle. The asthenosphere behaves like a superheated material that is in a semi-fluidic, plastic-like state.
The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: convergent, divergent and transform. The last occurs where two plates move laterally relative to each other, creating a strike-slip fault. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.
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Surface
The Earth’s terrain varies greatly from place to place. About 70.8% of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes, oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.
The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts also act to reshape the landscape.
The continental crust consists of lower density material such as the igneous rocks granite and andesine. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors. Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust. The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth’s surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine. Common carbonate minerals include calcite (found in limestone), aragonite and dolomite.
The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops. Close to 40% of the Earth’s land surface is presently used for cropland and pasture, or an estimated 1.3×107 km² of cropland and 3.4×107 km² of pastureland.
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III. Answer the questions:
1. What is the difference between a terrestrial planet and a gas giant?
2. What is the Earth’s shape?
3. Why is equatorial diameter larger than pole to pole diameter?
4. What are the highest and lowest places in the world?
5. Are iron, oxygen, silicon, magnesium, sulfur, nickel, calcium, and aluminum the main elements?
6. What are the main layers of the Earth’s interior?
7. The crust is separated from the mantle by the Mohorovičić discontinuity, is not it?
8. How many layers do the outermost part of the Earth’s interior have? Describe them.
9. How many types of plate boundaries do you know?
10. What can you tell about underwater and surface landscape?
11. What are the reasons of the planetary surface reshaping?
12. How many rock types do you know? Describe them.
13. What is the pedosphere?
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