Download Plate Tectonics

Sea Floor Spreading
&
Plate Tectonics
Ch. 2 part 2
Objectives
1.
2.
3.
4.
The basics of plate tectonics
Convergent vs. divergent boundaries
Diagram the geological provinces
Active vs. passive continental margins
Sea Floor Spreading
• Huge pieces of oceanic
crust are separating at
the mid ocean ridge
• Rifts are created:
– Cracks in the crust
• Releases pressure from
mantle
• Magma pushes up
• Animation
• Paper/desk demo with
numbers
Plate Tectonics
• What is the general idea behind plate tectonics?
– Earth’s lithoshpere broken into plates
– Plates all moving in different directions
Plate Tectonics
• What is the lithosphere?
– The crust and top part of mantle
– Lithospheric plates float on asthenosphere
• 100km or 60 mi thick
Plate Tectonic Boundaries
• Three types
• Draw them
Movement
• Plates moving in different directions
• This is continental drift
• What happens when plates separate (we looked at
this already) or collide?
Sea Floor Spreading: Divergent
Boundaries
• I’ll draw this on board--you
copy
• Forms mid-ocean ridges
• Move 2 cm -18 cm per year
– Fingernails grow 6 cm year
• New lithosphere is created
• What happens to old
lithosphere?
• Destroyed in trenches
• Otherwise . . .
• . . .earth would be
continually expanding
Subduction: Convergent Boundaries
• Subduction:
– Downward movement of lithospheric plate into mantle
– Causes earthquakes, volcanoes, and trenches
• Three types
– Oceanic --> <-- Oceanic
– Oceanic --> <-- Continental
– Continental --> <-- Continental
• Animations
Subduction
• Oceanic-Continental
• Which plate always descends into the other?
• Oceanic --> denser. What does this cause?
Oceanic --> <--Continental
•
•
•
•
I’ll draw on board---you copy
Trenches
volcanoes
Mountain ranges
Oceanic--> <--Oceanic
• Trenches
• Volcanoes
• Forms island arcs:
I draw on board--you copy
– Curved chain of volcanic islands along a trench
– Examples:
• Aleutian and Mariana Islands
Older, colder plate becomes
denser than underlying hot,
weak asthenopshere. Upper
sediments become melted -->
magma forms --> heads towards
surface
The Curve
• Why are trenches and
island arcs curved?
– Due to the earth’s
spherical shape
– Notice Aleutian islands
and Mariana trench
• What do they call this
“ring” in the Pacific?
Aleutian Islands
Cleveland Volcano, Aleutians
Mt. Saint Helens Erupts: 1980
• Play Mt. Saint Helens video clip
Ring of Fire
(Play Ring of Fire clip)
Hot Spots and the HI islands
• What are hot spots? (play Hot Spot video clip and
Typical Hawaii Eruption clip)
• When will Lo'ihi break the surface?
Continental--> <-- Continental
• I draw on board---you
copy
• No subduction
• Continental plates light,
resist downward motion
• Rocks buckle and fold and
are “welded” together
• Mountain range formed
• What example is in this
picture?
• The Himalayas
Himalayas from space
Himalayan Collision
Transform or Shear Boundary
• Draw diagram below
• Plates slide past each
other
• What is a classic, local
example of this?
Quiz a partner
• What are the three types of plate tectonic
boundaries? Give examples of each and the
result of each
– Convergent
• Oceanic-oceanic (trenches, volcanoes, earthquakes & island
arcs)
• Oceanic-continental (trenches, volcanoes, mt. ranges)
• continent-continent (earthquakes, mountain ranges)
– Divergent
• Sea floor spreading  forms mid ocean ridges
– transform
• earthquakes
Plate Overview Vid Clip
Fault lines in the Bay Area
Yes, there is
more than just
one . . .
San Andreas Fault
• Map and Pic of 1906 quake
1906 Quake
Quake of 1989
Hayward Fault
• Let’s look at this fault
through Google Earth (in
demo section—stop at UCB
football stadium)
• Covers some of the
densest urban terrain in CA
• Article on Hayward Fault
Formation of a New Ocean in
Africa?
• 3 plates pulling apart
from each other at the
Afar Triangle (shaded
part)
– Arabian plate and two
parts of the African plate
(splitting along East
African Rift Zone)
– Rift: 36 mi long, 26 ft wide
• Red triangles are
volcanoes
• More on Ethiopia's Afar
region
Afar Triangle video clip
What Causes the Motion of Plates?
• Slab Pull:
– Lithosphere cools --> denser
– Sinks into mantle --> pulls rest of plate with it
Convection (heat from core causing mantle to swirl) was original hypothesis
Satellite View
• Let's zoom in
Continental Drift
Continental Drift
• The continents once
united in a
supercontinent called
Pangaea about 180
mya.
• They have since been
moving into their
present locations
• What about before 180
mya?
750 mya to present
BBC Interactive Timeline
Plate Tectonics
500 million years in the future?
• If the plates continue to move in the directions they
move today for another 500 million years, propose a
configuration of oceans and continents at the end of
that time.
Constantly Changing Earth
•
•
•
•
Constantly changing SST and Sea Level
More ice --> lower sea level
Last ice age about 18,000 years ago
There will be more on this later in the year, but here is a
quick clip . . .
If the sea level rises . . .
Geological Provinces
•
Two main regions in the sea floor:
1. Continental Margin
2. Abyssal plain
Continental Margin
1. Continental Shelf:
•
Submerged; almost flat (400-600 ft)
2. Continental Slope
•
Steep; edge of continent (10,000-16,500 ft)
3. Continental Rise:
•
Formed by sediments building up at base of slope
Deep Sea Fan
• Sediment
accumulating at
canyon’s base
CA Continental Margin
(Monterey Bay)
Active vs. Passive Margins
• Active Margin:
– Shelf colliding with
another plate
– Little or no shelf; steep
– earthquakes, volcanoes
• Passive Margin:
– Trailing edge
– Little activity
Width of margin: East vs. West coast of South
America
Hydrothermal vents
• What are plates doing
at center of ridges?
– Pulling apart
• Water seeps down
cracks, heats up, and
emerges
• How hot?
– Up to 650º F!!
• Does anything live
down there?
Black Smokers
• Water dissolves many
minerals as it seeps into
crust
• Emerges hot--> rapidly
cools and solidifies
• Forms a chimney-like
structure
• Show Hydro Vents movie
clip (5 minutes)
Question #1 p. 39
• How can you tell which way the plates are moving?
• Trenches indicate plates colliding
• Mid-ocean ridge indicates plates separating