Earth’s outer shell is made of two materials whose different densities and thicknesses give rise to two distinct “levels” on the planet’s surface.
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Hypsometric Curve: Basically, a chart that shows the proportions of surface area at every elevation on a planet.
Crust: Earth’s outermost layer, made out of two distinct materials – oceanic crust (which is denser) and continental crust (which is less dense).
Lithosphere: The rigid outer layer of Earth, including the crust and the hard, un-bending part of the upper mantle.
Subduction: The process of an ocean plate crashing into another plate and getting forced to dive down into Earth’s mantle.
Isostasy: Describes the way earth’s crust sort of floats in the underlying mantle. Continental crust is less dense and thicker, and floats higher than the oceanic crust, which is denser and thinner. Geologists talk about things like “isostatic rebound,” which is what happens after an ice age, when the ice melts off a continent and the continent lifts up, like a floating raft in a pool after someone gets off (though continents rise more slowly).
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References:
Albarede, F. (2009) Volatile accretion history of the terrestrial planets and dynamic implications. Nature, Vol 461.
Calogero, Meredith. Personal Communication, 2018.
Eakins, B.W. and G.F. Sharman. Hypsographic Curve of Earth’s Surface from ETOPO1, NOAA National Geophysical Data Center, Boulder, CO, 2012
from: https://www.ngdc.noaa.gov/mgg/global/etopo1_surface_histogram.html
Hawkesworth, C. J. & Kemp, A. I. S. (2006) Evolution of the continental crust. Nature, Vol 443.
Rosenblatt, P.C , & Thouvenot, P.E. (1994). Comparative hypsometric analysis of Earth and Venus. Geophysics Research Letters, Vol 21, pp 465-468.
Stern, R.J., Gerya, T, & Tackley, P.J. (2018) Stagnant lid tectonics: Perspectives from silicate planets, dwarf planets, large moons, and large asteroids. Geoscience Frontiers, 9.