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Volcanism
• Volcanism is the process by which material is
expelled from the interior of a planet
• Eruptions bring new rock materials to surface
¾ Esp. basalt
• Volcanic structures:
¾ Cinder cone: conical hill built up around a
vent ejecting cinders, ash and boulders
¾ Shield volcano: Gently sloping volcanic
mountain, built by fluid lava flow
¾ Composite volcano: Large volcanic mountain
with irregular profile, built by eruption of
viscous and fluid lavas
¾ Caldera: volcanic crater, caused primarily
by collapse
¾ Diatreme: volcanic crater caused primarily
by eruptive, explosive activity
• Caused because magma is slightly less dense
than the surrounding rock (upper mantle)
¾ A magma chamber connected by a thin
crack to the surface is overpressurized by
the surrounding rock
Surface Features
• Faults
¾ Thrust faults and strike-slip faults are
due to compression
¾ Normal faults and grabens are due to
stretching (rift zones)
• Mare
¾ Lava-covered plains
¾ Dark colour is due to basic rock
• Wrinkle-ridge
¾ Usually found in mare (not recognized
on earth)
¾ compressional ridges formed during
final stages of lava filling?
• Rilles
¾ Long valleys, either sinuous or linear.
¾ May be due to lava flow, or graben
formation
Plate Tectonics
• Tectonics, generally, is the study of how a planet’s
surface responds to global stresses.
• On the Earth the lithosphere is broken into plates which
move because of convection in the mantle.
• The plate tectonic model is well supported by a variety of
observational data:
¾ Ocean floor spreading: In the mid 1960’s geologists
found clear evidence of sea floor spreading and crust
formation along the mid Atlantic ridge.
ƒ Data show hot spots along the ridge and a steadily
increasing of rock age in both directions away
from it.
¾ Fossil and Rock Evidence: Very similar fossil records
are found for widely separated places such as the west
coast of Africa and the eastern coast of South
America. In the Himalayan mountains geologists have
found remnants of sea creatures.
¾ Plate Motion Measurement: Sensitive detectors in
many locations on the Earth have measured the change
in distance between one sensor and others. This has
allowed determination of the rate of speed at which
the plates are moving – a few cm/y.
• As plates move they can separate, collide or slide along
each other. When plates collide they can either push
against each other and raise the crust (i.e. Himalayas) or
move one under the other (Andes) to produce mountains
and often volcanoes.
• When plates slide along each other friction resists
motion until too much stored force is present and the
slide abruptly (southern California) again producing
earthquakes.
• The current set of continents was originally together
~200Myr ago in a land mass called Pangaea.
• Before that the geological record suggests a previous
mass called Rodinia which began to break up ~750Myr ago.
• This continuous recycling of Earth’s crust has changed its
surface almost beyond recognition from first formation.
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Atmospheric effects
Winds can significantly alter the surface, especially in
dry regions (Earth deserts, Mars)
Thin Martian winds can carry small dust grains, which
bounce over the surface, dislodging larger grains
(saltation)
Wind will deposit dust on the leeward side of craters
¾ Carried dust may be darker or lighter than that
already on ground, leaving wind streaks.
Winds can pile fine sand into ripples over a wide range of
sizes, forming dune fields
¾ Dune field in the north Martian polar cap is larger than
the Sahara and Arabian deserts combined
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Geochemical Cycles
History of rock material after it first appears on the
surface, from deep magma sources
Small, airless planets:
¾ Surface lava may be shock damaged, pulverized,
reburied, or welded into breccia
¾ Remelting
¾ Slow process
Planets with atmospheres (Earth, Venus, Mars)
¾ Cycle may be faster
¾ Magmas may produce atmospheric, hydrospheric, and
rock products
¾ Rocks may break down into sediments, be
metamorphosed, or remelted
¾ Only well understood for Earth
Urey cycle controls the amount of CO2 in the atmosphere
¾ In the presence of water, carbon dioxide forms
carbonic acid
¾ This acid can react with silicate rocks to form
carbonate rocks. Also can be converted by biological
organisms, into carbonate shells
¾ Thus, much of the carbon dioxide on Earth is locked up
in solid material
¾ The lack of water on Venus means all the CO2 is in the
atmosphere.
Exercises
1. Calculate the depth of the magma chamber at
Mauna Loa (17 km high). The magma has a
densty of 2770 kg/m3 and the surrounding rock
an average density of 3270 kg/m3.
Assuming pressure equilibrium,
h ρR − ρM
=
z
ρM
hρ M
z=
ρR − ρM
(
17 km )(2770 )
=
500
= 83km
This is actually an overestimate because the pressure of
the surrounding rock varies with height.