Earth, Moon and Mars: How They Work

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Earth, Moon and Mars How They Work
Professor Michael Wysession Department of Earth
and Planetary Sciences Washington University, St.
Louis, MO Lecture 3 Earths Rock Cycle
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Rock that crystallizes from a magma is IGNEOUS
ROCK.
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The 3 main causes of volcanoes 1. Mid-Ocean
Seafloor Spreading
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The 3 main causes of volcanoes 2. Seafloor
Subduction
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Mt. St. Helens
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Pyroclastic flow sweeps down the side of Mayon
Volcano, Philippines, 1984.
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A small lahar triggered by rainfall in Guatemala,
1989.
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Mt. Pinatubo, Philippines, 1991.
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The 3 main causes of volcanoes 3. Hotspot Mantle
Plumes
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Hawaii rises more than 5 miles above the seafloor.
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Hawaii
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Kilauea, Hawaii
Mauna Loa, Hawaii
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Pahoehoe lava, Hawaii
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Aa lava flow, Kilauea, Hawaii
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Kilauea, Hawaii
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What is a Rock?

• A solid, cohesive aggregate of grains of one or
  more minerals

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What Is a Mineral?

• A mineral is a solid, naturally occurring
  substance that has a specific chemical
  composition and a highly ordered internal
  (crystalline) structure.

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CRYSTAL - A mineral grain displaying the
characteristics of its atomic structure. -
almost 4000 different kinds of minerals -
differences result from the different elements
used and the ways they are bonded
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Mineral Composition
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Silicate Rocks
Make up 90 by weight of Earths crust

• Si and O two most abundant elements in Earths
  crust
• Covalently bonded
• Tetrahedra bond ionically with other ions
• Form chains, sheets, or 3-D structures

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Quartz Stability

• How might stishovite occur naturally?

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Quartz Stability

• How might stishovite occur naturally?
• Meteorites!

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We classify Igneous Rocks by Texture and
Composition gt Large XLs ? SLOW cool-down
(visible) ? Intrusive Rocks gt Small XLs ? FAST
cool down (microscopic) ? Extrusive Rocks
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Basalt
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Basalt
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Basalt
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Shiprock (New Mexico)
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Figure 4-12b
Devils Tower, Wyoming
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Start of Sun-driven part of Rock Cycle
WEATHERING ? Two forms Mechanical, Chemical
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Mechanical Weathering (1) Frost Wedging
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Mechanical Weathering (2) Plant growth
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Mechanical Weathering (3) Pressure release
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Mechanical Weathering (4) Large Temperature
changes (day/enight, forest fires)
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Chemical Weathering Dissolution Hydrolysis Oxidat
ion

• More significant than Mechanical Weathering (in
  volume of rock affected)
• Especially dominant in wet climates

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Chemical Weathering (1) Dissolution First
CARBONIC ACID is produced as H2O CO2 --gt
H2CO3 Then the carbonic acid dissolves calcite
as H2CO3 CaCO3 --gt Ca2 2(HCO3)-
(Carbonic acid) (calcite) (dissolved
calcium bicarbonate)
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Chemical Weathering (2) Hydrolysis Addition of
water (often as OH- hydroxyl) to a rock to form a
new rock Turns FELDSPAR into CLAYS Greatly
affects Basalt and Granite (Feldspar is the
most abundant mineral in cont. crust, and clay is
the most abundant sediment in ocean and
continental crust)
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Hydrolysis Greatly accelerated in the presence
of acidic rain
Cleopatras Needle Central Park, NYC
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Chemical Weathering (3) Oxidation Addition of
oxygen to a mineral to form a new mineral Most
common is with iron gtgt RUST Minerals start as
OLIVINE, PYROXENE, AMPHIBOLE Minerals end up as
HEMATITE and LIMONITE
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An end product of weathering is SOIL
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Soil varies as a function of latitude because of
climate differences.
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Q? Why is cutting down rain forests to grow crops
a bad idea?
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A. Tropical regions have the worst soils in the
world (all nutrients have been washed down
through the soils by high rainfall.
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MASS WASTING The removal of weathered rock and
soil.
More than 100x more destructive in U.S. than
earthquakes and volcanoes combined!!!
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Conditions just right in San Gabriels for bad
debris flows.
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Weathering/Mass Wasting Responsible for the
appearance of the land surface.
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SEDIMENTARY ROCKS Formed from accumulated
weathered sediments
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METAMORPHIC ROCKS Sedimentation gone too far!
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Anthracite Coal Seam
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Coal Formation
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Folded Sedimentary Layers
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Increasing Grades of Metamorphism
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Limestone to marble
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At Low Pressures (near surface), Rock Cracks (--gt
Earthquakes! ) At High Pressures (deep), Rock
Flows
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Flowing Rock Occurred Deep Now at Surface
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Migmatite Dikes Metamorphism gone too far !
(Melting!)
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Figure 9-5a
Deformation of Rock Layers Displays the Tectonic
History (e.g., Plate Motions)
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Figure 9-5b
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Figure 9-5c
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Figure 9-6b
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Figure 9-7a
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Figure 9-7b
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Figure 9-11a
Appalachian Mountain Folds in Pennsylvania
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Structure of a Continent
Figure 13-3
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Ages of North American Bedrock
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In many places, rocks that were once many miles
underground are now at the surface. How did they
get there?
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Erosion and Isostatic Rebound!
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Eastern Terranes
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Western Terranes
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But subduction is much more shallow than this
picture shows!
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Western Terranes Basin and Range
Heating comes from the exposure to the
asthenosphere.
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Formation of Himalayas
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Canadian Rockies
Appalachians
Alps
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Volcanism adds rock to continental volume.
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Big Eruptions Dont Happen Very Often!
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Hotspot volcanism often begins with a large
basaltic flood.
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Afar Hotspot Breaking Africa into 3 Pieces
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Columbia flood basalts
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Columbia flood basalts Beginning of Yellowstone
hotspot