Isostasy –Isostasy (Greek ísos "equal", stásis "standstill") is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. –Parts of the crust will rise or subside (sink) until stability is achieved

For example: –As a mountain erodes, mass is removed and the crust beneath the mountain rises to maintain equilibrium –As sediment is deposited, the crust subsides (sinks) beneath its weight

The weight of the ice pushes down on the crust. When the ice melts the crust “pop” back up as the mantle rebounds This is an example of isostasy. The same thing happens when a mountain erodes. There is less weight pushing down on the mantle, so it rebounds.

Mountain Building

Orogony Cycle of processes that form mountain ranges Mostly associated with converging plate boundaries

Mountains

Most commonly form at converging plate boundaries Ocean-Ocean –Island arcs such as Japan and Indonesia Ocean-Continent –Volcanic mountains such as the Andes Continent-Continent –Folded and Faulted mountains such as the Himalayas

Folded Mountains

Orogenies can also happen far from plate boundaries Fault-Block Mts. –Crust is broken up due to tensional forces –Grand Tetons in Wyoming

Grand Teton Mt.

Dome Mountains Nearly circular folded mountain Individual isolated structures that occur in areas of horizontal strata

Adirondack Mts.

Mt. Marcy, NY (5344 ft. highest peak in the Adirondacks)

Dome Mountains can also be formed from hot molten material (magma) rising from the Earth's mantle into the crust that pushes overlying sedimentary rock layers upward to form a "dome" shape. Unlike a volcano, the magma typically does not reach the Earth's surface. Instead, the magma cools underneath the surface and forms the core of the mountains. Laccoliths Algonquin Peak, 5115 ft. 2 nd highest peak in the Adirondacks

An example of a dome- shaped mountain is Half Dome in the Sierra Nevada range in California. It's made of granite, and was once a large blob of magma pushed up through the Earth. Granite is much harder than other rock, and so it doesn't erode as easily as the rest of the mountain. The softer layers of sedimentary rock were washed away, leaving the hard granite dome. Earth sedimentary rock