Key Ideas
Earth’s Energy Sources: All activity in Earth’s atmosphere, oceans, and surface is powered by three sources of energy.
- Solar energy is the energy source for the atmosphere. In the greenhouse effect, some of this energy is trapped by infrared-absorbing gases in the atmosphere, raising Earth’s surface temperature.
- Tidal forces from the Moon and Sun help to power the motion of the oceans.
- The internal heat of Earth is the energy source for geologic activity.
Earth’s Interior: Studies of seismic waves (vibrations produced by earthquakes) show that Earth has a small, solid inner core surrounded by a liquid outer core. The outer core is surrounded by the dense mantle, which in turn is surrounded by the thin low-density crust.
- Seismologists deduce Earth’s interior structure by studying how longitudinal P waves and transverse S waves travel through Earth’s interior.
- Earth’s inner and outer cores are composed of almost pure iron with some nickel mixed in. The mantle is composed of iron-rich minerals.
- Both temperature and pressure steadily increase with depth inside Earth.
Plate Tectonics: Earth’s crust and a small part of its upper mantle form a rigid layer called the lithosphere. The lithosphere is divided into huge plates that move about over the plastic layer called the asthenosphere in the upper mantle.
- Plate tectonics, or movement of the plates, is driven by convection within the asthenosphere. Molten material wells up at oceanic rifts, producing seafloor spreading, and is returned to the asthenosphere in subduction zones. As one end of a plate is subducted back into the asthenosphere, it helps to pull the rest of the plate along.
- Plate tectonics is responsible for most of the major features of Earth’s surface, including mountain ranges, volcanoes, and the shapes of the continents and oceans.
- Plate tectonics is involved in the formation of the three major categories of rocks: igneous rocks (cooled from molten material), sedimentary rocks (formed by sediments in water), and metamorphic rocks (altered by extreme heat and pressure).
Earth’s Magnetic Field and Magnetosphere: Electric currents in the liquid outer core generate a magnetic field. This magnetic field produces a magnetosphere that surrounds Earth.
- Most of the particles of the solar wind are deflected around Earth by the magnetosphere.
- Some charged particles from the solar wind are trapped in two huge, doughnut-shaped rings called the Van Allen belts. An excess of these particles can initiate an auroral display.
- Earth’s magnetic field flips irregularly, but with an average of about 300,000 years between reversals. These field flips are preserved in magnetized rock on the ocean seafloor.
Earth’s Atmosphere: Earth’s atmosphere differs from those of the other terrestrial planets in its chemical composition, circulation pattern, and temperature profile.
- Earth’s atmosphere evolved from being mostly water vapor to its present composition of 78% nitrogen and 21% oxygen.
- Earth’s atmosphere is divided into layers called the troposphere, stratosphere, mesosphere, and thermosphere. Ozone molecules in the stratosphere absorb ultraviolet light.
- Because of Earth’s rapid rotation, the circulation in its atmosphere is complex, with three circulation cells in each hemisphere.
The Biosphere: Human activity is changing Earth’s biosphere, on which all living organisms depend.
- Industrial chemicals released into the atmosphere have damaged the ozone layer in the stratosphere.
- Deforestation and the burning of fossil fuels are increasing the greenhouse effect in our atmosphere and causing a global warming of the planet.