# General Astronomy/Earth
## Earth's Interior

Based on models of the earth (well supported by the scientific evidence available), the theory of plate tectonics presupposes that the Earth's outermost surface (crust) is made up of two sub-layers, the rigid lithosphere and the semi-molten asthenosphere. Beneath the crust is the mantle, a layer made up of molten rock (magma).
Heat is generated in the Earth's core by uranium, potassium, and thorium.

## Earth's Surface
### Plate Tectonics

This map shows all of the Tectonic Plates that make up the Earth's crust Plate tectonics is a theory of geology underlying the observed phenomenon of continental drift.
The rigid stone making up the lithosphere "floats" on the fluid-like asthenosphere, and, in areas where magma wells up from the mantle below the crust, may develop fractures. This results in the lithosphere being separated into contiguous masses of solid earth and rock known as tectonic plates. Due to slow currents in the asthenosphere's material, the plates floating atop this material undergo slow motions in different directions. These floating plates regularly jostle one another, with interactions along these boundaries being responsible for earthquakes and volcanos.
Tectonic plates are roughly divided into two types: continental and oceanic plates. The distinction is based on the thickness of the plates; oceanic plates are thinner than continental plates; as a result, the oceanic plates generally lie below sea level, while the continental plates project above sea level.
The same currents that move tectonic plates with respect to one another also tend to bring molten material closer to the surface at points where a circulation cell is drawing the material upward. In minor cases, this can result in a hot spot, where the material of the plate is eaten away from below, leaving openings for volcanos such as those producing the islands of Hawaii. As plates move over mantle fissures, chains of volcanos can form. In major cases, the rising magma continuously pushes itself into the midst of the plate, eventually cooling to form new plate material. Thus, some tectonic plates are being widened by the addition of new material, via upwelling of magma from below. Currently the Atlantic oceanic plate is in the process of expanding, fuelled by a continuous input of magma along the North Atlantic Ridge. The region of Iceland, which straddles this ridge, is thus gaining territory at a rate of a few centimeters per century.
The boundaries between plates are known as fault lines. Individual faults are regions where two plate sections are moving with respect to one another. Pressure builds up along these regions as the plates are drawn in different directions by the underlying currents; when the pressure is released by the abrupt motion of the plates, an earthquake results. One of the most famous systems of fault lines is the boundary of the Pacific plate, known as the Ring of Fire due to the amount of seismic activity along this boundary. On the easternmost edge of the ring is the San Andreas Fault, on the boundary of the North American and Pacific plates. This fault line is responsible for the earthquake activity in the western United States, most notably the 1906 San Francisco Earthquake.
In some areas one plate will begin to overrun another. The plate being thus overrun will have its edge pressed downward below the leading edge of the other plate. This process, called subduction, results in the edge of this plate being exposed to the greater pressures and temperatures within the asthenosphere, and the rock making it up may begin to liquify. The resulting build-up of magma under pressure may find explosive release through the lithosphere above it, resulting in a volcanic eruption. In addition, the pressure exerted on both plates during a collision may cause the plates to buckle upward some distance from the plate boundaries. The resulting geographical features are known as upthrust mountain ranges. One example of this is the Himalayas in Asia.

## Earth's Atmosphere
### The Greenhouse Effect

CO2 (carbon dioxide) is increasing drastically due to burning fossil fuels and deforestation. This gas, among others, traps heat from the sun, making the Earth hotter.
The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without this atmosphere. The intensity of the downward radiation – that is, the strength of the greenhouse effect – will depend on the atmosphere's temperature and on the amount of greenhouse gases that the atmosphere contains.
Earth’s natural greenhouse effect is critical to supporting life, and initially was a precursor to life moving out of the ocean onto land. Human activities, however, mainly the burning of fossil fuels and clearcutting of forests, have accelerated the greenhouse effect and caused global warming. The planet Venus experienced runaway greenhouse effect, resulting in an atmosphere comprised of 96% carbon dioxide, with surface atmospheric pressure roughly the same as found 900 m (3,000 ft) underwater on Earth. Venus may have had water oceans, but they would have boiled off as the mean surface temperature rose to 735 K (462 °C; 863 °F).