Download PowerPoint - MrStapleton.com

Plate Tectonics
• Earth’s internal heat creates convection
currents.
• Convection currents in the mantle drive
the movements of the Earth’s crustal
plates.
• Motions of the Earth’s plates create a
variety of landforms and phenomena.
Some parts of the Earth’s mantle are especially hot. Hot
material is generally less dense, and less dense things rise.
This creates a rising “convection current.”
Rising mantle currents split when they reach the top of the mantle.
As they separate outward, this mantle material cools and eventually
sinks back to the bottom of the mantle. In this way, patterns of
circulating cells are created.
Tectonic Feature Background
Information:
• Ocean Crust is more dense than
Continental Crust. (That’s why it sits lower
and gets covered with water.) It is also
typically darker in color.
• Ocean Crust material is very similar to the
material in Earth’s mantle.
• “Convergent” means “moving together”
• “Divergent” means “moving apart”
Typical Ocean Crust (Basalt)
is dense and dark due to iron
and magnesium content.
Typical Continental Crust
(Granite) Light in color and
less dense, due to feldspar
and silica content.
Sinking Mantle
currents (cooler
areas) create
convergent
boundaries
Rising Mantle currents
(hotter areas) create
divergent plate
boundaries
Ocean/Continent Convergent Plate Boundary:
When they collide, an ocean plate dives beneath a continental plate
(because ocean crust is more dense). Notice that some of this diving crust
melts and rises back up to create volcanoes on the continent.
Chain of
volcanoes on
continent
Trench
Earthquakes
from friction
between plates
Magma melts easily
and rises easily due
to high water
content of ocean
crust
Ocean/Ocean Convergent Plate Boundary:
When two ocean plates collide, one dives beneath the other. Notice that
some of this diving crust melts, and rises back up to create a line of
volcanoes (“island arc”) on the upper plate.
Trench
Earthquakes
from friction
between plates
Chain of volcanic
islands on upper
plate
Magma melts easily
and rises easily due
to high water
content of ocean
crust
Ocean/Ocean Convergent Plate Boundary: another view
Continent/Continent Convergent Plate Boundary:
When two continental plates collide, neither plate is as dense as the mantle
below. Therefore, neither plate dives below the other. They smash together,
pushing crust up and down, and creating a tall mountain range. The
Himalayas were formed in this way.
•Tall, non-volcanic mountains
Earthquakes caused by
crumpling and cracking of
plates due to collision
Continent/Continent Convergent Plate Boundary: an example
Continent/Continent Divergent Plate Boundary:
Magma pushes up from below, creating a spreading convection current. This
is occurring in Eastern Africa. Over time, this turns into a…
Ocean/Ocean Divergent Plate Boundary:
Ocean crust is made of the same material as the mantle. As these pictures
show, when a continent splits apart, magma from the mantle rises up and
fills the gap. When this magma hardens, it becomes ocean crust. Since
ocean crust is dense, it sinks low in the mantle and becomes covered by
water. A new ocean is born.
Earthquakes
Mantle Material
hardens to
create new
ocean crust
Mid-Ocean Ridge
Rift Valley
Ocean Hotspot:
A minor mantle current creates a “plume” or “jet” of magma, called a
“hotspot.” The plume shoots straight up through a moving plate. As the
plate moves over the plume, this creates a chain of volcanic islands, the
youngest of which is right over the hotspot.
Newest
Island.
Directly over
hotspot
Hawaii Hotspot:
Typically, the newer islands (closer to hotspot) are larger. Volcanoes cool as
they become farther from the hotspot. This causes them to shrink and sink
below sea level.
Which way is the plate moving?
Older islands cool as they move
away from hotspot. Cooling causes
them to contract and sink.
Hawaii
Hotspot
Transform Plate Boundary:
At this type of boundary, one plate shears across another. [The San
Andreas fault, in CA, is a famous example]
Earthquakes from
friction due to
shearing
Transform plate boundaries form along jagged convergent or divergent
plate boundaries.
Development of Theory of Plate
Tectonics
• 1620 – Sir Francis Bacon noticed that the shores
of the Americas fit together with Europe and
Africa
• 1915 – Alfred Wegener postulated “Pangaea”
• 1928 – Arthur Holmes suggested that convection
currents in the mantle could drive plate
movements.
• 1960s – Ocean floor mapping and other
evidence caused most scientists to accept
the Theory of Plate Tectonics.
1620 – Sir
Francis
Bacon
noticed that
the shores of
the Americas
fit together
with Europe
and Africa
1915 – Alfred Wegener postulated “Pangaea”. He showed that, when the
continents are placed together, traces of fossil remains match up.
Further evidence for Wegener’s Pangaea: when the continents are placed
together, glacial remains also match up.
Wegener’s theory was widely dismissed because he could not
suggest a “driving force” that would be capable of moving entire
continents.
1928 – Arthur Holmes suggested that convection currents in the
earth’s mantle could provide the driving force for plate tectonics
1960s – Ocean floor mapping showed a “Mid-Ocean Ridge” in the middle of
the Atlantic.
The earth’s magnetic poles have reversed many times
during the past. When lava hardens to form rock, the
polarities of magnetic materials in the rocks are locked in
position.
This banded magnetic
pattern suggested that
bands of new rock harden
from lava at the ridge and
then get pushed outward
by newer bands of rock
(with opposite magnetism)
Dating of ocean bedrock shows that new
rock is “born” at the mid-ocean ridge, and
then gets pushed outward by newer rock.