Chapter 10:Planetary surfacesChapter 10:Planetary surfaces

Volcanism and Tectonics

Dating cratersDating 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)

#D>10

#D>1

A17 Mare Serenetatis basalt 3.3-3.7x109

A17 Nectaris highlands 4.3x109 ≥1000

98000

A15 Apennine, PreImbr highland

4.3x109

A15 Imbrium Basin, rim 3.9x109 95 40000

A14 Fra Mauro, Imbrium basin 3.9x109 130

A12 Copernicus: ray+rim ≤0.9x109 10 2000

A12 Oceanus Procellarum basin

3.3x109 20 2000

A11 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.

VolcanoesVolcanoes

•Volcanism is the process by which material is brought from the interior of the planet to the surface

Volcanic structuresVolcanic structures

Cinder coneComposite volcano:

Mount Hood, Oregon

Shield volcanoes: Mauna Loa, Hawaii, and Olympus Mons, Mars

Lava plain

Volcanic cratersVolcanic craters

CalderaDiatreme

Volcanoes in the Solar SystemVolcanoes in the Solar System

MareMare

•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.

Caloris Basin on MarsMare Crisium on the Moon

A simple volcano modelA simple volcano model

•Consider a magma chamber, at a depth z embedded in rock of density R.

•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 M, and extends a height h above the ground.

•The pressure P at the depth of the magma chamber is•So

gzhzgP RM )(

M

MR

z

h

Mauna LoaMauna 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.

M

MR

z

h

OutgassingOutgassing

•Volcanoes release gas, as well as molten rock•Can contribute significantly to the composition of the

atmosphere.

FaultsFaults

Thrust fault: compression Horsts and Grabens: stretching

Wrinkle ridgeWrinkle ridge

•Usually found in mare lava plains•Arise from tectonic stresses associated

with the cooling and contracting of the lava that flooded the maria

RillesRilles

Sinuous rille: winding valley, resembling a channel cut by a river or lava flow Linear rille: straight-sided, like a

graben

TectonicsTectonics

Plate tectonicsPlate 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 mid-ocean ridge near Iceland support plate tectonic model

Mid-atlantic ridgeMid-atlantic ridge

Tectonic activity on MarsTectonic 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.

BreakBreak

Atmospheric effectsAtmospheric effects

Saltation: wind can carry small particles, which bounce on surface and dislodge larger particles

Wind ErosionWind Erosion

Some regions of Mars’ surface look strikingly like Earth deserts, due to wind erosion.

Chryse Panitia, MarsEarth desert

Wind streaksWind streaks

As wind sweeps across the Martian plains, dust may be deposited on the leeward sides of craters

Dune FieldsDune Fields

Sand dunes on Mars

Sand dunes in Namiba

Geochemical cyclesGeochemical cycles

•On planets with atmospheres, surface rock may be tranformed

Urey ReactionUrey Reaction• A geochemical link between rocks and the atmosphere

OHSiOMgCOCOHMgSiO 223323

• On Earth, CO2 from volcanic gases dissolved in rainwater and oceans

• 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.

3222 COHOHCO • This formed a weak carbonic acid, which can to form carbonate rocks.

ChemistryChemistry

•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

23222 HOFeOHFeO

2222 32 HCOOHCH

Red MarsRed 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