Hawai'i Space Grant Consortium, Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, 1996

Comparison Charts of Geological Processes
N O T E S

The terrestrial planets and Earth's Moon share records of the geological processes responsible for their internal make-up and appearance. These processes are: volcanism, impact cratering, tectonics, and gradation.

Volcanism

Volcanic activity occurs on a planet or moon when sufficient heat inside the solid body causes partial melting of the interior rock. Eruptions of the molten rock, called magma, and gases onto the surface produce a variety of volcanic landforms. Surfaces of the terrestrial planets show volcanic mountains of different shapes and sizes, fields of lava flows, lava channels, and ash and cinder deposits. The forms of volcanoes and other volcanic products is controlled by such factors as the composition and fluidity of the magma, nature of the vent, eruption rate and duration, planetary atmosphere, and gravity.


Impact Cratering

Impact cratering involves the collision of solid bodies, such as a meteorite into a planet, resulting in roughly circular, excavated holes. Rock material excavated from the crater is called ejecta. This ejecta is distributed radially from the crater onto the surface of the planet as fragmental debris. The size and shape of the crater, and the form and extent of the ejecta depend upon a number of factors: impact energy, strength of the target material, rock compositions, presence of volatiles (such as water), and gravity.


Tectonics

Tectonics is a general term referring to the large-scale deformation (or change) of rock in response to forces causing faulting and folding. The forces acting upon a rock mass are generally termed tensional (pulling apart), compressional (squeezing together), or shear (parallel sliding). The magnitude and direction of the force (stress), the temperature and confining pressure on the rock, the composition of the rock, and the rate at which the rock is deformed determine how the rock changes in length, shape or volume. Common landforms resulting from tectonic processes are mountain ranges, rift zones, faults, fractured rock, and folded rock masses.


Gradation

Gradation is the set of processes that work on surface rocks to break, loosen, move, and finally deposit the smaller pieces. The processes are: weathering (the chemical decay or mechanical break-up of rocks), erosion, transportation (the removal of weathered rocks by moving water, wind, ice, or gravity), and deposition. The presence of water and atmosphere drive the chemical decay of rock. Gravity moves landslides. Moving water, wind, and ice create landforms such as river channels, deltas, beaches, sand dunes, glacial deposits and valleys. On airless, waterless planets, gradation appears to be limited to impact cratering and the pull of gravity.



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