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Chapter 10:Planetary surfaces
Volcanism and Tectonics
Dating craters
• Apollo missions returned rock samples from more than half a
dozen locations on the Moon’s surface, both maria and highland.
• Radioactive dating of these samples provides ages which can then
be compared with the number density of craters in each region.
Mission
Location
Sample Age(y)
A17
Mare Serenetatis basalt
A17
Nectaris highlands
4.3x109
A15
Apennine, PreImbr highland
4.3x109
A15
Imbrium Basin, rim
A14
Fra Mauro, Imbrium basin
A12
Copernicus: ray+rim
A12
A11
#D>10
#D>1
3.3-3.7x109
≥1000
98000
3.9x109
95
40000
3.9x109
130
≤0.9x109
10
2000
Oceanus Procellarum basin
3.3x109
20
2000
Mare Tranquilitatis
3.7x109
50
12000
• The numbers in columns 4 and 5 are surface density for craters with
diameters >10km (col.4) and >1km (col.5); the surface densities are in units
of 10-6/km2 so 2 means 2x10-6.
Volcanoes
• Volcanism is the process by which material is brought from the
interior of the planet to the surface
Volcanic structures
Cinder cone
Lava plain
Composite volcano:
Mount Hood, Oregon
Shield volcanoes: Mauna Loa, Hawaii, and Olympus
Mons, Mars
Volcanic craters
Diatreme
Caldera
Volcanoes in the Solar System
Mare
• Lava-covered plains
• Dark colour is due to basalt (igneous rock)
• Moon mare are exceptionally flat because magma was especially
hot (1400-1600 K) and therefore fluid.
Mare Crisium on the Moon
Caloris Basin on Mars
A simple volcano model
• Consider a magma chamber, at a depth z embedded in rock of
density rR.
• Assume the hydrostatic pressure on this chamber is equal to the
pressure exerted by the weight of the magma above it:
 The magma has a lower density rM, and extends a height h above the
ground.
• The pressure P at the depth of the magma chamber is
• So
h rR  rM
z

rM
P  rM g ( z  h)  r R gz
Mauna Loa
Calculate the depth of the magma chamber at Mauna Loa (17 km
high). The magma has a density of 2770 kg/m3 and the
surrounding rock an average density of 3270 kg/m3.
h rR  rM

z
rM
Outgassing
• Volcanoes release gas, as well as molten rock
• Can contribute significantly to the composition of the atmosphere.
Faults
Thrust fault: compression
Horsts and Grabens:
stretching
Wrinkle ridge
• Usually found in mare lava plains
• Arise from tectonic stresses associated
with the cooling and contracting of the
lava that flooded the maria
Rilles
Sinuous rille: winding valley,
resembling a channel cut by
a river or lava flow
Linear rille: straight-sided, like a
graben
Tectonics
Plate tectonics
• Rodinia – the giant continent
assembled from fragments ~1.2 Gyr
ago
• began to break up ~750Myr ago
• eventually reassembled >200Myr ago
“Pangaea”
• its breakup led to our continents
today
• model: bands of alternating colour also
alternating magnetic polarity
• also crust age increases with distance
from rift
• observations of Earth’s crust along midocean ridge near Iceland support plate
tectonic model
Mid-atlantic ridge
Tectonic activity on Mars
The Acheron Fossae region on
Mars, an area of intensive
tectonic (continental ‘plate’)
activity in the past.
Shows how the rifting
crosses the older impact
crater with at least three
alternating horsts and
grabens.
Break
Atmospheric effects
Saltation: wind can carry
small particles, which
bounce on surface and
dislodge larger particles
Wind Erosion
Some regions of Mars’ surface look strikingly like Earth deserts,
due to wind erosion.
Earth desert
Chryse Panitia, Mars
Wind streaks
As wind sweeps across the
Martian plains, dust may be
deposited on the leeward
sides of craters
Dune Fields
Sand dunes on Mars
Sand dunes in
Namiba
Geochemical cycles
• On planets with atmospheres, surface rock may be tranformed
Urey Reaction
• A geochemical link between rocks and the atmosphere
• On Earth, CO2 from volcanic gases dissolved in rainwater and oceans
CO2  H 2O 
 H 2CO3
• This formed a weak carbonic acid, which can to form carbonate rocks.
MgSiO3  H 2CO3 
 MgCO3  SiO2  H 2O
• Similarly, living organisms
make calcium carbonate
shells
• Subducted and reconverted
to CO2.
• This could not occur on
Venus (no water), so
atmosphere is rich in CO2.
Chemistry
• The hot atmosphere of Venus (750 K) drives unusual chemical
reactions
 Atmosphere reacts with rocks to produce volatile HCl, HF, sulfuric
acid
 Even mercury and lead may be produced
• Any water would have been used up in oxidizing iron minerals or
hydrocarbons
2 FeO  H 2O 
 Fe2O3  H 2
CH 2  2H 2O 
 CO2  3H 2
Red Mars
The red soil of Mars is due to the oxidation of iron atoms in minerals
 Occurs in the intermittent presence of water
 The same process that rusts (wet) iron on Earth