Download PLATE TECHTONICS

PLATE TECHTONICS
and formation of ocean basins
part II
Recall: “Layered
Earth”
•Hot Core (heat
source for melting)
•Chemical
composition and
physical conditions
(temp, pressure)
combine to create
“physical” layers
•Lithosphere
(=Brittle, solid) &
Asthenosphere
(=plastic, can flow)-
Another view:
• Different Layer
names based on:
• 1) chemical
properties = ~ how
much silica vs. metal
content (Core,
Mantel, Crust)
• 2) “physical
properties”= liquid,
solid, or “plastic”
(mesosphere,
asthenosphere,
lithosphere)
PLATE TECHTONICS
review from last time:
2. “Distillation”, and resulting rock
types
Light Rocks vs. Heavy Rocks
Granites and similar rocks
(silica fewer metals):
~ 2.7 g/cm3
Basalt (silica, but
lots of metals)
~ 2.8 g/cm3
Basically two types of crust
based on these rock types
•
Oceanic crust
– Mostly basalt
– More Dense (3 gm/cm3)
– Thin (~5-10 km)
•
Continental crust
– Mostly granites
– Less dense
(2.7 gm/cm3)
– Thick (~30-50 km)
PLATE TECHTONICS
review:
1. Earth's internal structure revisited
2. Distillation, and resulting rock types
3. Mantel Convection = subduction/
spreading systems
Convection Cells of oceanic crust
TODAY:
A. some more Plate Tectonic
Fundamentals
1. Why do plates move?
Convection model: basic idea we
have talked about
~ density driven
Alternate model:
ridge push/slab pull
~gravity driven
Note: two models are not mutually
exclusive- both probably operate
2) What exactly is a “Plate”
Plates are:
Broken chunks of solid
lithosphere (either ocean or
continental crust) “floating
along” on the plastic mantel
layer below (Asthenosphere).
Plates of the world
N. Amer. Plate=
Mix of Ocean & Cont.
Pac. Plate=
All Ocean floor
1) Recall: Plates can be made of ocean
crust or continental crust, or BOTH.
Plates “float” on the semi-solid mantle
(& Oceanic crust is very different than continental!)
Recall:
• This is why the
different “physical
properties” (=
liquid, solid, or
“plastic”) becomes
very important!
Result: currently about a dozen independent
plates
3) So why are there
“ocean basins”?
Ie: why are there these “deep”
areas between continental
crust and ocean crust that has
filled up with water?
The model:
lighter continental rock vs.
denser ocean crust both
floating on a common liquid
layer below
 forms the ocean basins
A Thought experiment:
1) Think about putting ice cubes in bucket of
water.
2) Then imagine putting some blocks of dense
wood in a bucket of water, and also some
blocks of Styrofoam.
All will float- right? But not at same
level!
What determines how much is above water
vs. underwater for each?
Actually, both density and volume
determine “depth” at which
something would float in water-
And also the amount if it sticking
up out of the water!
Nice website: http://discoverourearth.org/student/topography/isostasy.html
General principle is called
“Isostatic equilibrium”
Based mostly on relative density
So density and volume of crust that
is floating in liquid mantel
determine depth of ocean basins!
continents “float “higher, ocean crust “floats” lower)
Nice website: http://discoverourearth.org/student/topography/isostasy.html
Plate Elevations/ Gravity forces.
Note that oceanic plate slopes downwards from ridge to subduction zone - gradient or slope causes plate to move downhill. The
forces that are available to drive the plates include the following: ridge push, trench pull, slab pull, viscous drag at the base of the
plate, and the Eötvös force.
The graph above indicates that the fastest plates on Earth, like the Juan de Fuca, Pacific, Philippines, and Cocos Plates, are oceanic plates with
a significant
percentage of their plate boundaries being subducted. The Australian, Indian, and Arabian Plates all include a modest amount of continental
crust that is
attached to a subducting slab of significant proportions. Plates with a large amount of continental crust, like the Americas, Africa, Antarctica, and
Eurasia,
move significantly slower than most oceanic plates. Data from Gripp and Gordon, 1990, Geophysical Research Letters, v. 17, p. 1109-1112.
Next topic:
What are the Consequences
of plate boundaries?
THREE BASIC TYPES of plate
interactions
a) Two plates collide (“converge”)
b) Two plates slide along each other
(“faulting”)
c) Two plates pull apart (“diverge”)
Basic Boundaries:
Divergent, Convergent &Faulting
A. Convergent = “subduction zones”
1) ocean crust meets continental crust?
“Subduction Zone”
Ocean floor (Basalt and Sediments) is being subducted
under a continent– shoved under and re-melted-
Consequences
can have some or all in given location
“Subduction”
1. Volcanism- often explosive
- volcanic mountain ranges formed on land where re-melted sea
floor bubbles back up
2.
Trenches and “fore-arc” basins
-can be formed by sediments shoved up
CASCADE MOUNTAINS
Stretch all along coastal W. Coastfrom Southern BC (Canada) into N. California
ANDESITES
Common Rocks in Cascades
Typical Volcanic Rocks
on active margins
= Re-melted and reformed
Seafloor (basalts) + …
(whatever got subducted
with The Plate)
Cascade Volcanoes
Did you know
We have a
Volcano
“Chain”.. All
the way down
W CoastWhy?
Cascade Volcanoes
Mt St Helens
(Oregon)
May 18, 1980
Cascade Mountain Range
We’ve also got a
“local” volcano
within weekend
driving range!
Lassen
Lassen Peak:
the last volcano in the cascade range
Like a mini-Yellowstone- but 5 hrs away..
And still active..
(Last blew: 1914)
2) ocean crust meets ocean
crust
“Subduction”
Typical consequence:
“Island Arc and Trench” system
“Subduction”
Back-Arc basin +Trench
Kurile
Island Arcs and trenches
Aleutian
Japan
Izu Bonin
Marianas
Caribbean
Marianas
Trench + Island
system behind:
classic Island
Arc/Trench
3) Continental crust meets
Continental crust
Result: Massive mountain
building
Himalayas today
B. Plates Moving past each other=
“Transform Faulting”
Essentially relieves stress in crust, when forces of plate
movement don’t line up perfectly ( & rare they would
ever line up perfectly..)
Transform faulting often seen on
mid-ocean ridges
Can see major transform faults clearly on global map of mid-ocean rid
Transforms on global ridge system
A detailed map of East Pacific
Rise- spreading
center
E-pac
rise
Do these kind of faults also occur
on land?
Anyone know of any especially relevant examples to
where you are now living?
Local San Andreas Fault:
Huge transform fault
Wider view- major faults and
“fractures” along w.coast
*Only currently active
subduction zone is off
Wa. State / Canada
*Each of these major
faults is relieving
pressure built up by
uneven plate movement
*FYI: eventually, Baja
and S. Cal WILL become
a very big island..
(very good news for Riverside County
property values. If you can wait ..)
San Andreas Fault- near San Luis Obisbo
Downtown Santa Cruz- Pacific Ave in 1989
(After Loma Prieta Quake)
San Francisco
C. Divergent boundary
= Sea floor spreading
(new ocean crust formed)
Major spreading centers occur at “mid ocean
ridges”- main zones for formation of New
Ocean crust
Global map of mid-ocean ridges: seafloor spreading
creates the MAJOR
feature
of open ocean deep sea floor
RIDGE
SYSTEMS
How ridges create ocean floor
hydrothermal Vents
Hydrothermal activity
on top of these
underwater mountain
ranges represent some
of most wild and
unexpected
ecosystems in ocean !
Q: Are there spreading centers
on land?
Yes: “Rift valleys”
Major Examples
today of
continents pulling
apart to form
new oceans in the
future are in:
1) Iceland and
2) Africa (the
“great rift valley”
Rift valley
evolution- a
new sea will
soon be
created..
C. Some final “Big Picture”
consequences of Tectonics
Major global consequence is:
leads to “Recycling” of
Ocean Floor!
(but not continents!)
C. Some final “Big Picture”
consequences of Tectonics
1. Global Earthquake Patterns and
Volcanoes
2. Recycling of seafloor – age of ocean
floor vs. continents
3. Over Geologic time:
“dance of the continents”
Look at Major plate boundaries
Now look at map of global earthquakes.
(look familiar?)
“Ring of FIRE”
Global earthquake locations tend to go with volcanoesmost can be predicted by plate boundaries
(A “ring of fire” surrounds the Pacific ocean)
The “ring of fire”
Note that major parts of “ring of fire” are volcanic
island chains (island arc systems)
2) Subduction and age of ocean
floor
(Seafloor recycling)
Very oldest sea floor rock is ~ 200 MY- most much
younger. In contrast, continental rocks can be
BILLIONS of yrs old..
Dark Blue:
Older than
2.5 BILLION
yrs
Yellow: older than 500
Million yrs
Why?
Can you use ocean floor ages to
tell how can how fast plates
are spreading over time?
Age of Ocean Floor
Age of seafloor
Young crust (zero yrs!) is at mid ocean
ridges- OLDEST is just before subuction
Old
Young
Can show that ridges spread at very
different rates.. Some fast, some much
slower
Fast sea-floor spreading
(10-15 cm/year)
Slow
Slow sea-floor
sea-floor spreading
spreading
(a
(a few
few cm/year)
cm/year)
3) Over Geologic Time..
“Dance of the continents”
Continent shapes, and ocean
basins (what oceans exist!) are
constantly changing-
Evolution of
continents / seas over
just last 250 Million
years..
(not very long on
geologic time scale!)
Finally- an important
exception :
Sometimes you find
volcanism on ocean floorbut far away from plate
boundaries..
See any plate boundaries in
center of pacific?
Anyone heard about any volcanic islands
there?
“Hot Spot” Island Chains
Stationary “hot”area in mantel- plate moves over it.. Volcanic islands
“punch through” at regular intervals over geologic time
HAWAIIAN ISLANDS/ sea mount chain!
Chain
of Islands
Hawaii
Note: very different
types of volcanic
eruptions
• Hot spots: Basalt –
relatively low silica
concentrationtypically flows
“gently”
• Subduction zonesViscous- full of gas/
steam (subducted
seafloor!) explodes
Can see change in apparent plate motion!
Emperor seamount chain from
Chain of Islands
space
“Loihi”-
is next Hawaiian island
on its way up..
Next time:
MARINE PROVINCES