Geol 33 Environmental Geomorphology

J Bret Bennington

Tectonic Processes

Geomorphologists divide the processes that alter the surface of the Earth into two categories:

exogenic - weathering, erosion - driven by atmosphere, hydrosphere, and gravitational potential energy. With the exception of gravity-driven processes, powered by nuclear fusion in the Sun.

endogenic - uplift and subsidence of the crust - driven my heat flow and density differences in the mantle, powered by nuclear fission in the interior of the Earth.

Endogenic processes include:

Volcanism - the upward flow of magma to the surface.

Orogeny (mountain building) - the uplift and subsidence of crust along continental margins due to plate tectonic collisions.

Epeirogeny - the uplift and subsidence of broad regions of crust due to changes in isostatic equilibrium.

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Isostacy

Isostacy is a term that describes the dynamic relationship between the Earthâs outer, rigid lithosphere (the plates) and the underlying plastic asthenosphere (part of the upper mantle).

Brief review of the structure of the shallow interior of the Earth

Compositionally, the outer Earth is subdivided into the mantle and the crust. They differ in their mineral composition and therefore in their respective densities.

Crust: (two types) silica-rich (SiAlic) continental crust is relatively light with an average density of about 2.65 g/cc and iron-rich (MaFic) oceanic crust is relatively dense at 3.0 g/cc. The average thickness of oceanic crust is about 6-8 km and it is relatively uniform. Continental crust is usually much thicker and much more variable, ranging from 25-75 km.

Mantle: ultramafic rock (very rich in iron, low in silica) with a density of between 3.25 and 3.3 depending on its temperature.

Structurally, the story is more complicated. Moving from the solid interior of the mantle toward the outside of the Earth, pressure and temperature both decrease. At about 400 km from the surface the pressure on the mantle eases enough that it becomes partially molten and plastic - behaving more like a liquid than a solid. This region is called the asthenosphere. The average density of the asthenosphere is about 3.25 g/cc.

The crust floats on the asthenosphere because it is less dense (2.65 - 3.0 g/cc). The crust is also cold relative to the asthenosphere. Where the cold crust contacts the mantle it cools the asthenosphere, causing mantle rock to weld to the base of the crust. This combination of crust and solid, welded mantle rock form the rigid plates of the lithosphere.

• The lithosphere floats on the underlying asthenosphere and behaves as a block of material suspended in a liquid. This fact is of considerable importance to understanding tectonic and large scale geomorphic processes.

Isostacy and Archimedesâ Principle

Archimedes first described the mathematical relationship for solids floating in fluids. Basically, a less-dense solid will displace a volume of fluid equal to the mass of the solid. Expressed as an equation:

with the thickness of each substance measured from the base of the solid. Rearranging this equation allows for an expression of the height at which a solid will float above the surface of a liquid. This height is a function of the thickness of the solid and the density difference between the solid and the liquid.

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Hypsographic Curve of the Earth

If the altitude or depth of the Earthâs surface relative to sealevel is plotted against the cumulative percentage of the Earthâs surface, a relationship called a hypsographic curve is obtained.

The hypsographic curve shows that most of the Earthâs surface is found to occupy two levels - one at an altitude of less than 1 km, the other at a depth centered around 4 km. Extreme altitudes an extreme depths are rare. These two mean surface levels represent the averge isostatic equilibria of continental and oceanic crust.

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The Isostatic position of oceanic crust

The steady downward depth trend of 60% of the Earthâs surface is caused by the fact that the depth of the oceans varies with the age of the ocean floor. The deepest part of the oceans does not lie in the middle of the ocean basins, rather it occurs toward the margin of the basin in proximity to oceanic trenches where subduction zones are found.

Ocean floor subsidence, subduction and plate motion

Depth measurements combined with samples of the oceanic crust show that the ocean floor subsides as it ages - the deepest parts of the oceans are found where the crust is oldest. Why? Recall from plate tectonic theory that oceanic crust forms at zones of upwelling mantle heat and material along midocean ridges. When the oceanic crust first forms it is hot enough that no mantle adheres to it - essentially the crust is in direct contact with the lithosphere.

As the oceanic crust cools it cools the mantle beneath it, which begins to weld to the base of the crust. As ocean floor continues to age the solid lithosphere beneath it thickens. As the proportion of dense mantle rock to less dense crustal rock increases, the density of lithosphere as a unit increases:

Age = 0, crust = 6 km, mantle = 0 km, r lithosphere = 3.0 g/cc bouyant

Age = 10 Ma, crust = 6 km, mantle = 32 km, r lithosphere = 3.25 g/cc neutral

Age = 100, crust = 6 km, mantle = 100 km, r lithosphere = 3.28 g/cc sinking

This is why ocean floor does not remain on the surface for much longer than 100 Ma - by this time the lithosphere becomes more dense than the underlying asthenosphere and is driven by gravity to begin to subduct.

The sinking of old lithosphere must exert a powerful force, pulling the rest of the plate along behind it away from the midocean ridge.

Crustal thickening

Any process, such as orogenesis, that thickens the crust will result in uplift. Likewise, any elevated region of the continents must be supported by a thick mass of crust below. The idea that mountains such as the Himalayas have deep "roots" was first seriously considered when the first sensitive gravity surveys demonstated that high mountains are associated with negative gravity anomolies. The large mass of relatively light continental crust projecting deep into the mantle beneath mountains displaces more dense mantle material, resulting in less mass and less gravity than expected.

Isostatic loading and unloading

Loading and unloading refer to any process that adds or removes mass from the surface of the crust. Loading causes subsidence and unloading causes uplift or rebound. This leads to the somewhat counter intuitive observation that eroding rock from the top of a mountain will cause the mountain to rise and that adding sediment to a basin will cause the basin to subside, making room for additional sediment. Transgressions and glaciers both cause crustal loading and subsidence, while regressions and melting lead to rebound.