http://www.ucmp.berkeley.edu/geology/tectonics.html. Plate Tectonics 2 Making oceans and continents. Pangea* seen at about 225 mya. Collision of Laurasia and Gondwana. Sir Francis Bacon 1620. Benjamin Franklin 1782 The crust of the earth must be a shell floating on a fluid
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Plate Tectonics 2Making oceans and continents
Collision of Laurasia and Gondwana
Sir Francis Bacon 1620
Benjamin Franklin 1782
The crust of the earth must
be a shell floating on a fluid
interior. Thus the surface of
the globe would be broken
… by … movements of the
Wegener 1912: evidence
* Breakup begins about 200 mya, floods about 190 mya
Alfred Wegener 1912
Pangaea about 200 mya
Example, NJ and Morocco
Sonar revealed Trenches,
record North or South pointing minerals
Maps of Magnetic Stripes in Oceanic Crust
Example from the past 4 million years
Pattern is irregular so useful for corellation
Hot spots form chains.
The big Island of Hawaii is a composite of five volcanoes. Kohala is the oldest. Kilauea is very active because it is closest to the hot spot, which is to the southeast of the big island.
Worldwide, plate speeds vary from 1 to 10 centimeters per year
Before satellites, we measured plate speeds as the distance between two islands divided by the age of the youngest basalts
Flood Basalt was subducted
Hey look, the direction changed!
Average 5 centimeters/year
Determining plate speeds for continents
Just find position wrt distant stars, then watch fixed objects on earth move .
150 mya Atlantic is already open
110 mya Displaced (Exotic) Terranes from S. Am. hits W. N.Am.
60 mya another terrane forms Cuba, Hisp.
About 50 mya Southern Ocean forms
20 mya Himalayas forms
About 5-3.5 mya Central America forms
Note 3-D Triple Junction
Inactive Branch: Aulocogen;Subsided Passive Margins
Active: Red Sea and Gulf of Aden Failed Arm: Great Rift Valley (aulocogen)
Discussion: Fault Block Mountains, HA normal fault, rain shadows, divergent margin. global cooling & grasslands
Humans as tall savannah specialists, voice
Story: The drunk and the lamp post
Fracture Zones and
Shallow weak earthquakes
Note sequence from land to trench
Note: over here are some ocean plate rocks that don’t get subducted in a collision
We will see some on the field trip, as well as the volcanic arc
If a continent converges from
the left, what rocks will fold
in the collision?
Rocks in the Himalayas
Reverse faults at convergent margin
+ some volcanics.
When stuffed down trench into
zone, result is
Source:Betty Crowell/Faraway Places
High Angle Normal faults of Rift Escarpment
Active and unstable continental margin
Craton : the stable portion of the continental crust versus regions that are more geologically active and unstable
exposed by glaciers
Collisions with Volcanic Island Arcs and microcontinents
hard to subduct. Erosion resistant parts
Pieces are volcanic island arcs, and microcontinents
Moved along transform faults, then accreted.
Anecdote Western California
Aesthenosphere shallow convection model
Deep mantle/core convection model – Plumes cause MOR’s – Morgan
Trench an entrance to Subduction Zone, Ridges and Rises are Mid-Ocean Ridges
Back Arc Basin
Volcanic Island Arc (Japan)
See previous slide
Form deepwater carbonate oozes, depths less than 4 km
below carbonate line
Passive Margin MOR Passive Margin
Circulation of hot water in cracks at mid-ocean ridge dissolves metals (Copper, Iron, Zinc, Lead, Barium) which are re-precipitated as (for example) sulphide ores. Hydrothermal waters are capable of metamorphism.
Ocean Floor layers:Ophiolite Suite
- Magma that creates new ocean floor originates from partially melted mantle rock (peridotite) in the asthenosphere
Some Serpentine is formed
due to hot water (called Hydrothermal)
End Plate Tectonics 2