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Plate Boundaries
Types of Plate Boundaries
• Divergent Boundaries
• Convergent Boundaries (Subduction and Collision)
• Transform Boundaries
Transform Boundaries
A transform boundary
is two plates that are
sliding past each other.
• Features: offset the
segments land.
• Examples: San Andreas
Fault, California
Convergent Boundaries
• A convergent
boundary is between
two plates that are
moving toward each
other.
• There are 2 types:
Subduction boundaries
& Collision boundaries.
Subduction Boundaries
• A subduction boundary is when one plate
plunges beneath another plate.
– The plunging plate is subducting beneath the
overriding plate.
• Subduction boundaries can occur between:
1. Oceanic Plate – Oceanic Plate
2. Oceanic Plate – Continental Plate
Subducting Oceanic Plates
Formations:
• Deep-sea trench
• Volcanic island arc (a chain of volcanic
islands).
Example: Aleutian Islands, Alaska
Oceanic Subduction Under Continental
Formations:
•Deep-sea trench
•Mountain chain and volcanoes inland
Example: Cascade Mts. & Mt Shasta
Collision Boundaries
• A collision boundary is when two continental
plates collide and become welded into a single,
larger continent.
Formation: Giant mountain ranges
Example: Himalayas & Mt. Everest
Divergent Boundaries
• A divergent boundary is between two plates
moving apart. This is where sea floor spreading
occurs.
Divergent Boundaries
• Most divergent
boundaries are
located along the
ocean floor. They
also have rift valleys
(deep valleys at the
center of a midocean ridge).
Divergent Boundary Examples
•
•
•
•
Mid-Atlantic Ridge
East Pacific Rise
Southeast Indian Ridge
Southwest Indian Ridge
BRAIN BREAK!!!
Causes of Plate Movement
Remember that the asthenosphere (a layer in
the upper mantle) provides the lithospheric
plates with a surface on which they can move.
Instead of being rigid, the asthenosphere is
pliable because the rock materials there are
hotter than those in the lithosphere.
Causes of Plate Movement
1.
Mantle Convection Hypothesis (driving force)
2. Slab Pull Hypothesis (driving force)
3. Ridge Push Hypothesis (weak force)
Mantle Convection
• Magma that is hotter and less dense than its
surroundings rises upward at a mid-ocean ridge, forming
a convection cell on each side of the ridge. As the
convection current moves away from the mid-ocean
ridge, it drags the lithospheric plate with it.
Plate Movement
Tectonic plates move at varying rates, with some moving at
only 1.3 cm (0.5 inches) per year and others moving more
rapidly, up to 10 cm (4 inches) per year.
Newer rocks found along
the ridge are formed by
hot, molten rock rising
between the spreading
plates. This pushes the
older rocks farther away
from the ridge on both
sides.
Sea Floor
Spreading and
the Age of the
Ocean Floor
Mid-ocean ridges are
found in both the Atlantic
(the Mid-Atlantic Ridge)
and the Pacific (East
Pacific Rise) oceans.