Transcript Lecture 10 Plate Tectonics i

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
interior. Thus the surface of
the globe would be broken
… by … movements of the
fluids….
Wegener 1912: evidence
* Breakup begins about 200 mya, floods about 190 mya
Alfred Wegener 1912
• Continental drift hypothesis
• Continents "drifted" to present
positions
• Evidence used in support of
continental drift hypothesis
• Fit of continents
• Fossil evidence
• Rock type and mountain belts
• Paleoclimatic evidence
Evidence:
Precise Matching of
Continental Shelves of
Circum-Atlantic
Continents
Ranges of Triassic Reptiles
Similar
Rocks on
opposite
shores
Example, NJ and Morocco
6
Why wasn’t it popular?
• Objections to drift hypothesis
• Inability to provide a
mechanism capable of moving
continents across globe
• Wegener suggested that
continents broke through the
ocean crust, much like ice
breakers cut through ice
Attitudes change
During the 1940s and 1950s technological
strides permitted extensive mapping of
the ocean floor
Seafloor spreading hypothesis was
proposed by Harry Hess in the early
1960s http://pubs.usgs.gov/gip/dynamic/HHH.html
Seafloor spreading in detail
Harry Hess: Convection currents in
mantle
Seafloor spreading occurs along
relatively narrow zones, called rift
zones, located at the crests of ocean
ridges
As plates pulled apart, low pressure
causes mantle mellting,magma moves
into fractures and makes new oceanic
lithosphere
Hess’s Seafloor spreading (cont)
New lithosphere moves from
the ridge crest in a conveyorbelt fashion
Newly created crust at the ridge
is elevated because it is hot and
therefore occupies more volume
Area also seems to be pushed
up by mantle upwelling
Vine and Matthew’s Test
• Magnetic North and South
exchange places at irregular
intervals average = 100K years
• BUT Very high variance
• Dates when polarity of Earth’s
magnetism changed were
determined from radiometric
dating of lava.
Vine and Matthew’s Test
• Geomagnetic reversals are
recorded in the ocean
crust
• Spreading predicts
matching bands of lava
polarity on either side of
ridge.
• Vine and Matthews looked
for symmetric magnetic
stripes in the ocean crust
• Tow magnetometers
record N or S
• NOT FALSE
Ideas:Earth's Convection Cells
1963
This is the part we have been discussing. But the hottest area is the core.
Ideas: Earth's Convection Cells
Jason Morgan (1971)
In this model, Plumes
and convection cells
are aspects of the
same thing
Ideas: Earth's Convection Cells
Hawaii
MOR
Andesitic
Basaltic
Combination
A combined model.
Hot Spots and Hawaii
Hot Spots are caused by heat
from deep in the mantle.
As plates move over them, new volcanic
Seamounts are formed. Any that stick
up above the ocean’s surface as islands
are eroded away, and as they move
away from the Hot Spot, they cool,
contract, and are submerged. They are
then called Guyots
Their lavas are datable
Hot Spot and Hawaii-Emperor Chain
Speeds vary from 1 to 10 centimeters per year
Initial Flood Basalt was partly subducted
Speed = distance/time
Hey look, the
direction changed!
Hot Spots & Plate Motions
Average 5 centimeters/year
Global Positioning Satellites
LAGEOS and
GPS satellites
determine that
plates move
1-10 cm per
year, avg 5
Active Rifting of A Continental Plate
Note 3-D Triple Junction
Discussion: Eggshells
Active Rifting of A Continental Plate
Inactive Branch: Aulocogen;Subsided Passive Margins
East African Rift Zone
Active: Red Sea and Gulf of Aden. Failed Arm: Great Rift Valley (aulocogen)
Zagros Mts
Mid-Ocean Ridge System Motion
Fracture Zones and
Transform Faults
Shallow weak earthquakes
Subduction-Zone Features
Note sequence from
land to trench
Note ocean plate rocks
that don’t get
subducted in a collision
If a continent converges from
the left, what rocks will fold
in the collision? Himalayas
Continent
Back-Arc Basin
Trench
Accretionary wedge
Fore-Arc Basin
Volcanic Arc
Abyssal plains
Can be sites of thick
accumulations of sediment
Sediments thickest away
from MOR
Abyssal plains found in all
oceans
Studded by old cold
seamounts and MORs
Mélange from California Coast
Sea-floor and
land-derived
sediments,
+ some volcanics.
When stuffed down
trench into
Low TemperatureHigh Pressure
zone, with plenty of
ion-rich water, result
is Blueschist
Metamorphic Facies
Source: Betty Crowell/Faraway Places
More Terms
Canadian Shield,
North America’s
crystalline core
exposed by glaciers
Terms: Shield + Platform = Craton,
Continental Platform
Coastal Plain
Exotic (Displaced) Terrains
Collisions with Volcanic Island Arcs and microcontinents
Continental Crust
is thick and silicarich, so low
density. Buoyant
and hard to
subduct. Also
Erosion resistant.
Made of volcanic
island arcs, backarc
basins and
microcontinents
Moved by
transform faults,
then accreted
Sutures
Anecdote Western California
Paleogeography Reconstructions
• Orientation of magnetic minerals gives
latitude (north or south of equator)
• Radiometric dates of ocean floor basalts,
plus distance from ridge, gives
paleolatitude for last 200 million years
• Explains continental movements
Origin of Pangaea
Origin of Pangaea
http://www.odsn.de/odsn/services/paleomap/animation.html
Last 150 Million Years
100 mya Exotic terrane hits western N. Am.
50 mya Opening of the South Atlantic
40 mya Separation of Australia and Antarctica
35 mya India hits Asia forming Himalayas
3.5 mya Formation Isthmus of Panama isolates N. Atlantic
Mapping the ocean floor
Three major topographic units
of the ocean floor
•
•
•
•
Continental margins (few 100’s of meters below surface)
Deep-ocean basins ……(Abyssal Plain 4 km below surface)
Mid-ocean ridges ……( top is 1 km below surface)
Trenches
(may be 11 km below surface)
Ocean surface depths
Trenches 11 km deep. Abyssal Plain 4 km deep, MOR’s average 3 km high so 1 km deep
Bathymetry of the Atlantic Ocean
A few hundreds
Abyssal Plain 4 km
1 km average
Atlantic – Passive Margins – Note no trench – no subduction
Abyssal Plain
A passive continental margin
Found along coastal areas that surround
large oceans. Coast is far from central MOR
Not near active plate boundaries
Little volcanism and few
earthquakes
Example: East Coast of US
(turbidites,
graded bedding)
(Microfossil Ooze)
sand
Very thick sediments
muds
High-angle Normal Faults
Of Divergent Margin (Initial Rifting)_
Carbonate and
silica ooze
An active continental margin
Continental slope
descends abruptly into
trench
Example: Pacific Ocean
margin. Note
Accretionary Wedge at
trench
Seafloor sediment
Abyssal Plain is mantled with sediment
Sources
• Turbidity currents on continent margins
• Sediment that slowly settles to the bottom
from above – fine mud and plankton
Thickness varies
• Thickest in trenches – accumulations may
exceed 9 kilometers.
• Thinnest at MOR – new ocean floor
Foraminifera (a.k.a. Forams)
http://www.geomar.de/zd/labs/stab-iso/forams.jpg
Form deepwater carbonate oozes, depths less than 4 km
Chert
sample
Only Silica below
carbonate line
So Depth of pure Chert
>4 km
Small Phytoplankton (tiny floating
plants)
Diatoms
(siliceous
Ooze SiO2)
The structure of oceanic crust
Black Smoker
on cracks near magma
MORs characterized by an elevated ridge
Closely spaced normal faulting
Mantle flow below pulls the crust apart
Newly formed basalt ocean floor fills in
cracks
Ophiolite Suite
Some Serpentine
due to hot water
circulation
2 km
3-6km
3-6km
Hydrothermal Metamorphism
Black Smokers
http://collections.ic.gc.ca/geoscience/images/detail/F92S0220.jpg
Circulation of hot water in cracks at mid-ocean ridge dissolves metals which are reprecipitated
as sulphide ores
Outcrop of
pillow lava
End Plate Tectonics 2