Transcript Colliding and Spreading Plates

Created by: Genny, Danish, Aalap
 Concept that all continents were once joined in a
landmass: Pangaea
 Eventually they broke apart and drifted away, gradually
forming the world we have today
 Cause: movement of tectonic plates
Types of Plates:
Oceanic and Continental
Convergent
Divergent
Transform
Convection currents generated by the uneven
heating of magma in the Earth’s interior are what
cause Earth’s crust to break apart and move.
 Where: convergent plate boundaries
 What: two plates collide, and the heavier plate sinks
underneath the less dense plate and is subducted back
into the Earth’s mantle
 Why: the difference in density
Examples of Subduction:
 Oceanic and Continental plates collide, and the
oceanic crust subducts beneath the continental
crust due to difference in density.
 Andes Mountains
 Where: Convergent plate boundaries, usually oceanic
and continental plates
 What: As oceanic plate subducts underneath
continental plate, the sediment that breaks off
attaches to the continental plate.
 Result: expansion of continental plates, along with
pieces of oceanic crust preserved on land called
“ophiolites”
Example: Elder Creek Ophiolite located in the Coast
Range ophiolites off the coast of California
Example:
Peru/Chile border: Nazca Plate subducts underneath
the South American Plate
 Opposite of Accretion
 What: the descending plate along with parts of the
overriding plate subducts underneath
 Result: Decrease in sizes of continents
• Spreading occurs at divergent
boundaries (Plates moving apart)
• Sea-f loor spreading is when there is
a divergent boundary in the middle
of the ocean f loor
•Divergent boundaries on continental
crust causes rift valleys to form on
land
 Occurs between two oceanic plates
 As the plates move apart, magma rise through the rift
and hardens to form new crust
 Ocean floor gradually extends and size of plates increase
 The age of the rock increases as you are moving away
from the rock
 As magma piles up along cracks, a long chain of
mountains form. These are called oceanic ridges. (Ex.
Mid-Atlantic Ridge)
Age of crust (youngest = red)
 As lava cools it becomes magnetized in direction of
Earth’s magnetic field
 Scientists learned that Earth’s magnetic field has
reversed polarity many times
 Helps provide history of the rate of sea- floor spreading
(Fast=broad, Slow=squeezed)
 Rift valleys are formed at divergent plate
boundaries on land
 At the early stage of development, rivers flow into
the valley. As the valley becomes deeper, more
water flows in and eventually causes an ocean
basin
 East African Rift Valley
 Eventually Africa will split into 2 sub-continents
East African
Rift Valley
Red Sea
 Plate movements cause stress, which leads to
deformation
 Three types of deformation:
 Shearing (Opposite)
 Tension (Thinner)
 Compression (Squeeze)
 3 types of folds: Anticlines (Arch), Synclines (Down),
and Monocline (Dip)
Anticline
Monocline
Syncline
Caused by shearing. This motion results in transform
boundary
San Andres Fault in California
 Cause by tension forces. Hanging wall (top) Footwall
(bottom). Results in divergent boundaries
 Cause by compression. Same as normal fault except
walls are opposite
 Forces that “build” landforms. Examples: mountains,
islands, volcanoes, etc.
 Forces that “break down” landforms. Examples:
weathering, erosion, subduction, faults, etc.
 These forces form the landmasses we have today,
affecting spatial patterns around the globe. It
influences people’s everyday decisions as to where they
live and how they interact with the environment.