Uplift is the process by which the earth's surface slowly rises either due to increasing upward force applied from below or decreasing downward force (weight) from above.
During uplift, land, as well as the sea floor, rises. The outer shell of the earth, the crust, divides into moving sections called plates. Uplift, forming mountains and plateaus, usually results as these plates crash into each other over millions of years. Although the plates move at roughly the speed fingernails grow, their motion still has a tremendous impact on the earth, since plates can be as big as a continent or the Pacific Ocean. Sometimes mountains rise from crust separating rather than colliding. The study of these moving plates is plate tectonics.
Collision between two pieces of continents lifts the tallest mountains. When India, formerly a large island, slammed into the south side of Asia around 55 million years ago, the Himalayas uplifted. When Africa and Europe smashed into North America 300 million years ago, the Appalachians uplifted. As the western edge of the westward-moving North America smacked into various islands over the past 200 million years, the Rockies uplifted. In these uplifts, pieces of ocean floor trapped between the approaching continents rise as well. Limestone, a rock formed on the sea floor, composes the summit of Mt. Everest, over 26,000 ft (8,000 m) above sea level.
Uplift also results when sea floor crust collides with continental crust or with other pieces of sea floor crust. Volcanic mountains (Andes, Cascades) or volcanic islands (Indonesia, Japan, Aleutians) result from sea floor colliding with and diving beneath a continent or another sea floor. As the sea floor crust descends, some of the sediment on the floor scrapes off the plunging crust and piles up to form a ridge called an accretionary wedge. For example, as the Indo-Australian plate dove beneath the Eurasian plate at Indonesia, the scraped and folded sediments form the Java Ridge off the west coast of the volcanic island of Java.
Sea floor also uplifts along mid-ocean ridges where crust separated as magma (melted rock) from inside the earth tries to reach the surface. The magma that rises from below these ridges lifts the ocean floor. Mid-ocean ridges circle the earth like the seams on a giant baseball. If the magma rises similarly beneath a continent, the land will bulge and eventually crack, potentially tearing the continent in half and creating a new ocean, as at the Red Sea.
Although compression creates most uplift, the Basin and Range region of western North America resulted from a combination of collision and then extension. A map of North America illustrates a series of parallel north-south oriented mountain ranges separated by north-south trending valleys (basins) extending from Nevada and Utah down into Mexico. These mountains formed when first the crust arched from the collision of North America with a piece of ocean floor, and then later the crust began to separate. As the top of the arches cracked (similar to how the top of a bent piece of clay cracks along its upper surface when bent), pieces of the crust dropped down to form the valleys, and other pieces formed the mountain ranges.
Finally, when a huge weight is removed from the crust, the crust will slowly rise up in a process called isostatic rebound. During an ice age, when glaciers up to 1.9 mi (3 km) thick cover continents, the weight of that ice pushes down on the crust, causing it to sink or subside. When the ice melts, the crust uplifts just as a raft in a pool rises when the swimmer gets off it. Scandinavia still responds to glaciers melting 10,000 years ago by uplifting by as much as a centimeter per year.
See also Volcano.