Isostasy

Nature is a perfect system of balances. Matter and energy exist in finite (specific) amounts and cannot be created or destroyed. The earth is a perfect example of nature's balance system. Rock particles are eroded from the mountain top, deposited in valleys or stream channels, compacted under their own weight into rock, and uplifted by mountain-building processes until they again rise to the top of the mountain.

Deeper within the earth, balancing processes also take place as major shifts in the upper part of the earth's crust change the planet's gravitational balance. Under mountain ranges, the thin crust slumps or bows deeper into the upper mantle than where the land mass is thinner across continental plains. The land masses float on the crust and mantle-like icebergs float in seawater, with more of the mass of larger icebergs below the water than smaller ones. This balance of masses of the earth's crust to maintain gravitational balance is called "isostasy."

Isostasy is not a process or a force. It is simply a natural adjustment or balance maintained by blocks of crust of different thicknesses to also maintain gravity. Isostasy uses energy to balance mass. The energy comes from the hydrologic cycle, which is the path of a drop of water that originates in the ocean, evaporates to form a cloud, falls on the mountain as a raindrop, and flows back to the sea carrying particles of rock and soil. The hydrologic cycle derives its energy from gravity and solar radiation. As water flows or a glacier slowly grinds over land, energy is lost in that now-isolated system.

Within Earth, energy comes from radioactive energy that causes convection currents in the core and mantle. Opposing convection currents pull the crust down into geosynclines (huge structural depressions). The sediments that have collected (by the processes of deposition that are part of the hydrologic cycle) are squeezed in the downfolds and fused into magma. The magma rises to the surface through volcanic activity or intrusions of masses of magma as batholiths (massive rock bodies). When the convection currents die out, the crust uplifts and these thickened deposits rise and become subject to erosion again. The crust is moved from one part of the surface to another through a set of very slow processes, including those within Earth (like convection currents) and those on the surface (like plate tectonics and erosion).

In isostasy, there is a line of equality at which the mass of land above sea level is supported below sea level. So, within the crust, there is a depth where the total weight per unit area is the same all around the earth. This imaginary, mathematical line is called the "depth of Establishing isostatic balance. (a) The weight of the ice covering Greenland pushes the land below sea level. (b) As the ice melts, the land mass rises as the pressure is removed but remains below the recently elevated sea level. (c) Normal isostatic balance is restored when the land mass is raised above sea level. Photo Researchers, Inc. Reproduced by permission. compensation" and lies about 70 mi (112.7 km) below the earth's surface.

Isostasy describes vertical movement of land to maintain a balanced crust. It does not explain or include horizontal movements like the compression or folding of rock into mountain ranges.

Greenland is an example of isostasy in action. The Greenland land mass is mostly below sea level because of the weight of the ice cap that covers the island. If the ice cap melted, the water would run off and raise sea level. The land mass would also begin to rise, with its load removed, but it would rise more slowly than the sea level. Long after the ice melted, the land would eventually rise to a level where its surface is well above sea level; the isostatic balance would be reached again, but in a far different environment than the balance that exists with the ice cap weighing down the land.