Key Ideas and Terms

5-1 Most of Earth’s surface is covered with flowing water that radically changes the landscape

• Seventy-one percent of Earth’s surface is covered with liquid water, but water is found in greater amounts elsewhere in the solar system.
• Ninety-seven percent of Earth’s water is in the form of seawater, with most of the remaining freshwater locked up as frozen water near the poles or underground. Only a very small portion of Earth’s freshwater is easily accessible on Earth’s surface.
• The continuous movement of water around our planet in different forms through rising evaporation and falling precipitation, including flows in the atmosphere, across the surface, under the ground, and along ocean currents, is known as the water cycle.
• Water is unique in that it expands when frozen, can hold tremendous energy without changing phases, and has a molecular structure that allows it to dissolve substances, flow as a continuous body, and easily attach itself to many surfaces.

5-2 Earth is surrounded by a thin, multilayered envelope of gas that has changed since life became prominent

• Photosynthesis is the chemical energy process by living organisms that consumes carbon dioxide (CO₂) and water and releases oxygen (O₂).
• Respiration is the chemical energy process where modern animals consume oxygen and release carbon dioxide.
• A planet’s atmosphere is the layer of gas surrounding the planet, and the atmospheric pressure is the weight of all the air above a particular height.
• Earth’s atmosphere is divided into layers where the lowest is the troposphere, where most weather occurs, surrounded by the ozone-containing stratosphere, which in turn is surrounded by the mesosphere and thermosphere.
• The fraction of incoming sunlight that a planet reflects is called its albedo.
• The greenhouse effect occurs when a planet’s greenhouse gases in its atmosphere prevent some of the energy emitted by Earth’s warm surface from escaping into space.

5-3 Volcanoes and earthquakes reveal energy from a molten interior driving Earth’s surface to shift positions

• Earth has a solid inner core and a molten outer core surrounded by a mantle surrounded by a thin crust.
• Earthquakes release energy in the form of surface waves, compressional P waves, and transverse S waves.
• Plate tectonics describes how the process of convection drives the movement of Earth’s surface plates on the lithosphere.
• Where plates separate, undersea mountain ranges called oceanic rifts are caused by seafloor spreading.
• Where plates collide, cool crustal material from one of the plates sinks back down into the mantle along a subduction zone.

5-4 Earth’s magnetic field emanating from its spinning, molten interior creates a protective shield from the Sun’s harmful radiation

• Earth’s magnetic field is created from electric currents moving in Earth’s outer core.
• The region of space around a planet in which the motion of charged particles is dominated by the planet’s magnetic field is called the planet’s magnetosphere, which captures the solar wind’s charged particles.
• When charged particles excite atoms in Earth’s atmosphere and cause them to glow, the aurora, also called the northern lights (aurora borealis) or southern lights (aurora australis), are observed.

5-5 A rapidly growing population is altering our planetary habitat

• All life on Earth subsists in a relatively thin layer called the biosphere.
• Without the high-altitude ozone layer to absorb ultraviolet light from the Sun, solar ultraviolet light would beat down on Earth’s surface with greatly increased intensity. This layer is thinnest over Antarctica and is known as the ozone hole.
• The recent dramatic increase in atmospheric CO₂ concentration has produced an equally dramatic increase in Earth’s average surface temperature, known as global warming.