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Lecture

9.2 Shaping Planetary Surfaces


Department
Astronomy & Astrophysics
Course Code
AST101H1
Professor
C.B.Netterfield

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9.2 Shaping Planetary Surfaces
Impact Cratering
-the blasting of bowl-shaped impact craters by asteroids or comets striking a planet·s surface
-impacting objects typically hit the surface at a speed between about 40,000 to 250,000 km/hr
-craters are generally about 10 times as wide as the object and 10-20% deep as they are wide
Volcanism
-any eruption of molten rock or lava from a planet·s interior onto the surface, whether it comes from a
tall volcano or simply rises through a crack in a planet·s lithosphere
-occurs when magma (underground molten rock) finds a path through the lithosphere to the surface
-molten rock rises because
1-generally less dense than solid rock, and lower-density materials tend to rise when surrounded by
higher-density materials
2-driven upward under pressure of a chamber of molten rock being squeezed
3-molten rock contains trapped gases that expands as it rises, making it rise much faster and lead to
dramatic eruptions
-volcanic plains: runniest lavas that flow far and flatten out
-shield volcanoes: thickening before spreading out
-stratovolcanoes: thickest lava flows that can·t flow very far and therefore build up tall
-made by lower-density volcanic rock that erupts as a thicker lava
-lava plains and shield volcanoes are made of basalt (a mixture of many different minerals that erupts
from volcanoes as a high-density but fairy runny lava
-all terrestrial and some jovian show signs of volcanic plains and shield volcanoes
-outgassing: the process in which the release of pressure expels the trapped gases
-water and gas becomes trapped in the interiors as it presumably came from icy planetesimals
-can find the tallest volcano on a medium sized planet, enough geological activity to from a volcano,
with weak enough gravity to let it grow (mars)
Tectonics
-any surface reshaping that results from stretching, compression and other forces acting on the
lithosphere
-disruption of a planet·s surface by internal stress
-forms as a direct or indirect result of mantle convection
-may compress (mountains), pull apart the crust (valleys), etc.
Erosion
-blanket term for a variety of processes that break down or transport rock through the action of ice,
liquid or gas
-shaping of valleys by glaciers, the carving of canyons by rivers, shifting of sands dunes by wind
-can break things down or build things up
-sedimentary rock: piled sediment layers on the floors of oceans and seas caused by erosion
-erosional debris: erosion can create new features by deposting debris
Different planets have different geological history
Role of Planetary Size
SMALL
LARGE
-interiors cool rapidly
-harden earlier
-tectonic and volcanic activity cease after a
billion years or so
-warm interior causes mantle convection
-ongoing tectonic and volcanic activity
-most ancient craters erased
-outgassing produces an atmosphere and
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