Forebulge
Encyclopedia
In geology
, a forebulge is a flexural bulge in front of a load on the lithosphere
. This load causes the lithosphere to flex by depressing the plate beneath it. Because of the flexural rigidity of the lithosphere, the area around the load is uplifted by a height that is 4% of that of the depression under the load. This load and resulting flexure place stress on the mantle
, causing it to flow into the area around the loaded area. The subsidence of the area under the load and the uplift of the forebulge continue until the load is in isostatic
equilibrium, a process which takes on the order of 10–20 thousand years. Because of the coupling with the mantle, the rate of forebulge formation and collapse is controlled by mantle viscosity
.
s during continental glaciations. Because of the removal of the ice sheets, these formerly-glaciated areas are currently rising in a phenomenon known as post-glacial rebound
. Because of the coupling of the mantle with the plates, data from post-glacial rebound
are used as a direct probe of the viscosity of the upper mantle. As the ice melts and the land under it rises by isostatic recovery, the forebulge also subsides. Forebulge subsidence is the reason why the Netherlands
and parts of southern England have been slowly sinking in the present day.
One estimate is that the centre of the North Sea
rose by about 170m (558 feet) during the Ice Age because of forebulging.
s lead to loading and flexure of the lithosphre. This flexure produces a significant forebulge that divides a forearc
basin and a back arc basin.
of a thin elastic beam or plate (i.e., the lithosphere). The magnitude of the flexure goes as an exponentially decaying sinusoid. The lithosphere is therefore anomalously high at a flexural half-wavelength past the load; this flexural wavelength is determined in large part by the elastic thickness of the lithosphere.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, a forebulge is a flexural bulge in front of a load on the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
. This load causes the lithosphere to flex by depressing the plate beneath it. Because of the flexural rigidity of the lithosphere, the area around the load is uplifted by a height that is 4% of that of the depression under the load. This load and resulting flexure place stress on the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
, causing it to flow into the area around the loaded area. The subsidence of the area under the load and the uplift of the forebulge continue until the load is in isostatic
Isostasy
Isostasy 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. This concept is invoked to explain how different topographic...
equilibrium, a process which takes on the order of 10–20 thousand years. Because of the coupling with the mantle, the rate of forebulge formation and collapse is controlled by mantle viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
.
Glacial
One cause for forebulge formation is loading of the continental lithosphere by ice sheetIce sheet
An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier...
s during continental glaciations. Because of the removal of the ice sheets, these formerly-glaciated areas are currently rising in a phenomenon known as post-glacial rebound
Post-glacial rebound
Post-glacial rebound is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostasy...
. Because of the coupling of the mantle with the plates, data from post-glacial rebound
Post-glacial rebound
Post-glacial rebound is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostasy...
are used as a direct probe of the viscosity of the upper mantle. As the ice melts and the land under it rises by isostatic recovery, the forebulge also subsides. Forebulge subsidence is the reason why the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
and parts of southern England have been slowly sinking in the present day.
One estimate is that the centre of the North Sea
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
rose by about 170m (558 feet) during the Ice Age because of forebulging.
Tectonic
Collisions between tectonic plates and island arcIsland arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
s lead to loading and flexure of the lithosphre. This flexure produces a significant forebulge that divides a forearc
Forearc
A forearc or forarc, also called arc-trench gap is a depression in the sea floor located between a subduction zone and an associated volcanic arc. It is typically filled with sediments from the adjacent landmass and the island arc in addition to trapped oceanic crustal material...
basin and a back arc basin.
Physics
The existence of a forebulge is predicted by the solution to the equations for the flexureBending
In engineering mechanics, bending characterizes the behavior of a slender structural element subjected to an external load applied perpendicularly to a longitudinal axis of the element. The structural element is assumed to be such that at least one of its dimensions is a small fraction, typically...
of a thin elastic beam or plate (i.e., the lithosphere). The magnitude of the flexure goes as an exponentially decaying sinusoid. The lithosphere is therefore anomalously high at a flexural half-wavelength past the load; this flexural wavelength is determined in large part by the elastic thickness of the lithosphere.