In the scientific community, the planet Earth is commonly understood to undergo periodic climate changes, with fluctuating temperature levels. Records of these changes can be found in the geological strata of Earth and in the chemical signatures of fossilized remains. Glaciation is the result of lowered temperatures around the planet; specifically, the movement and activity of glaciers. These are expanding ice sheets created by an accumulation of snowfall that does not have time or opportunity to melt. The existence of glaciers profoundly affects the long-term climate trends of the planet; glaciers are also responsible for land formation, such as carving out valleys and other types of erosion.
Glaciers typically form at high altitudes above the snow line, where temperatures are low enough that snow is permanent. The force of gravity carries them down mountainsides, and they are lubricated by melted water at the lowest levels of the glacier; pressure decreases the melting point of ice. Also, glaciers expand when snow falls onto them and freezes into ice. Throughout the course of a day, parts of the glacier that are at the threshold of melting will pick up and move parts of the surrounding earth. In this way, glaciation causes sediment and materials to be relocated; they are frozen to the glacier and deposited elsewhere when the ice melts.
Glaciation is one of the causes of large-scale land alteration. Erosion is the primary example of this; glaciers cause erosion in three ways: plucking, abrasion and freeze-thaw. Glacial plucking occurs when a glacier moves down a slope and the motion of the ice pulls already-fractured rock away. Abrasion is caused by rock attached to a glacier as it moves; the friction between the moving rock and underlying bedrock causes erosion of the bedrock, and this is the way most valleys are created. Freeze-thaw, meanwhile, occurs when melted water seeps into cracks in rock, where falling temperatures cause it to freeze and expand, widening the cracks.
Periods of glaciation occur with the Milankovitch cycles, or the variations in Earth's orbit around the sun over a period of 100,000 years. For example, obliquity, or the angle of the axial tilt, varies between 21.5 and 24.5 degrees, and this alters the amount of heat the polar ice caps receive. A smaller angle means less heat hits the poles and allows glaciers to form, and the reverse is true as well. Eccentricity, caused by the gravitational pull of Jupiter and Saturn, causes Earth's orbit to become more elliptical and therefore farther from the sun, resulting in cooler temperatures. Precession, or the rotation of the Earth's axis, alters the orientation of Earth relative to the sun and other planets.