Download Impact cratering

10/17/2011
Planetary Geology II
The Terrestrial Planets
Recall…
• Planets look very different despite
undergoing very similar formation processes.
• Result of geological processes
• Layering by structure/strength
• Interior heating
• Cooling
• Magnetic fields
Seismology
• S-Waves and P-Waves
– Shear and Pressure waves
• Travel along the Earth’s surface and through
the interior after an Earthquake
• Shear waves cannot travel through liquids.
• Pressure waves are bent at boundaries
between materials of different density.
Seismology
Seismology
• We can use the motion of these waves
through the interior of a planet to draw
conclusions about their structure
– Measured on Earth and Moon
Planetary Surfaces
• Now that we know about the processes happening
in the interior, we can talk about the surfaces.
• Planetary surfaces are incredibly flat.
– Grain of sand on a normal globe
• Four major geological processes on the surface
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–
–
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Impact cratering
Volcanism
Tectonics
Erosion
Impact Cratering
Impact Cratering
• Largely the result of the impact of
planetesimals during the epoch of planetary
formation.
• Heavy Bombardment
• More small impact craters than big ones.
• Impact speeds around 40,000 – 250,000
km/hr (10-70 km/s)
• Craters usually ten times as wide as the
impactor, 10-20% as deep as they are wide.
Impact Cratering
• The features or shapes of craters can tell us
about the geology of a planet at the time of
impact.
– Most craters are round regardless of impact
direction
• Capable of radically altering the surface of a
planet over a short period of time.
Craters on Mars
Impact Cratering
Craters on Mars
Craters on Earth
Craters on the Moon
Volcanism
• Volcanism is the term for any process
through which molten material rises to the
surface from the interior of a planet.
– Magma when submerged, lava on the surface
• Molten rock rises for a number of reasons
– It is of lower density than solid rock.
– Most of Earth’s interior is NOT molten, so molten
rock can be squeezed up to the surface.
– Contains trapped gases that expand as it rises.
• The expansion of trapped gases leads to
eruptions.
Volcanic Eruptions
• The type of eruption depends on how
viscous the lava is, or how easily it flows.
– Also affects the geological formation created by
the eruption.
• “Runny” lava can flow far before cooling and
solidifying, while “thick” lava will tend to
collect in one place.
Volcanic Features
• Volcanic Plains
– Formed by the runniest lava
– Example: Lava plains on the Moon
Volcanic Features
• Shield Volcanoes
– Formed by lava of intermediate viscosity
– Examples: Olympus Mons (Mars) or Hawaiian
Islands
Volcanic Features
• Cone Volcanoes (Stratovolcanoes)
– Formed by the thickest lava
– Example: Mount Hood
Lava
• Important not to confuse density and
viscosity.
• Lava plains and shield volcanoes made from
basalt.
– Higher-density, but runny
• Stratovolcanoes made from thicker, lowerdensity lava.
Outgassing
• Volcanism produces many solid features, but
a more important aspect is outgassing
– Outgassing is the release of gases trapped in
molten rock during an eruption.
• The majority of the gases in the
atmospheres of Venus, Earth and Mars are
the result of outgassing.
– Likely a large portion of the water on Earth, too.
Tectonics
• In geology, tectonics is the creation of
surface features by compression, stretching,
or other forces acting on the lithosphere.
• Tectonic features can arise in a variety of
ways.
– The weight of a volcano may bend or crack the
lithosphere.
– Rising magma may create a bulge.
– Most features are the result of convection in the
mantle.
Tectonics
Tectonics
Plate Tectonics
• Ongoing stresses from convection fractured
the Earth’s lithosphere into many sections or
plates.
– The process by which the plates move against,
around, and over or under each other is called
plate tectonics.
Erosion
• Erosion is the breakdown and transport of
materials through the actions of ice, liquid,
or gas.
Geological Age
• Notice that, of the four geological processes,
only impact cratering is external.
– We can use the number of impact craters to
estimate the geological age of a surface.
• Geological Age: the time since a surface
was last significantly modified by a
geological process.
– A geologically young surface contains many
features that have formed recently.
– A geologically old surface will look the same
today as it did billions of years ago.
Geological Age
• To understand how impact cratering tells us
the geological age of a surface, compare the
surfaces of the Earth and Moon.
Differences
• All of the geological processes act in the
same manner on all of the terrestrial bodies,
so why do they appear so different?
– All comes back to the three fundamental
properties: size, rotation rate and distance from
the Sun.
• Volcanism and Tectonics
– Basically absent on the Moon and Mercury
because of their small size, Mars had much more
in its past.
Differences
• Erosion
– No atmosphere on Moon and Mercury (too small)
– Despite its thick atmosphere, Venus rotates too
slowly (no or very little wind) and is too close to
the Sun (no precipitation).
– Earth has an atmosphere and most of the water
has condensed onto the surface.
– Mars has a very thin atmosphere.
Differences
• Impact Cratering
– Inherently random, but larger objects show less
evidence of cratering.
– More volcanism and tectonic processes erase the
craters, leaving only the more recent craters
(about a billion years old or younger)
Next time…
Geologies of the Moon and Mercury
Quiz 7
1) Which types of seismic waves can travel through
the Earth’s lithosphere?
a)
b)
c)
S-Waves only
P-Waves only
Both S-Waves and P-Waves
2) Which of the following is not a geological process
a)
b)
c)
d)
Impact Cratering
Convection
Tectonics
Volcanism