Plate Boundaries 3 main types of boundaries Divergent boundary

Plate Boundaries
3 main types of boundaries
• Divergent (plates
diverge, or move away)
• Convergent (plates
converge, or come
together)
• Transform (plates slide
past each other)
Divergent boundary
• Plates move away (or
spread) from each other.
• New crust is created here.
– Magma forces plates apart.
• Example:
– Mid-Atlantic Ridge
– Great Rift Valley in
East Africa
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Geologic phenomena at divergent boundaries:
* = unique to divergent boundaries
– Iceland* (formed right on top of Mid-
Atlantic Ridge)
– Volcanoes
– Earthquakes
– Mountains
• Fault-block mountains (one side is
dropping lower as crust spreads)
– Mid-Ocean Ridges*
– Rift Valleys*
– Spreading Zones*
• Crust is spread out and stretched as plates
move apart.
Mid-Ocean Ridges
• New crust created here.
• See “Evidences of Plate
Tectonics” Notes for more
detail.
Rift valleys (divergent)
•A deep valley formed by the two plates
moving away from each other.
•Crust warps downward, spreads, eventually
“breaks” (boundary comes to surface)
Example: Great Rift Valley in East Africa
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Convergent boundary
• Plates move toward each other and
collide.
• Crust is destroyed here.
• Subduction: Where one plate sinks
underneath another and is forced
into the mantle (then melted)
Geologic phenomena at convergent
boundaries
– Volcanoes
– Earthquakes
– Mountains
• Folded mountains (crust
crumples/folds upward)
– Trenches/subduction
zones*
– Volcanic Island Arcs
(ocean-ocean
boundary)*
Types of Convergent boundaries
Continent-Oceanic
• Ocean plate collides
with a continent.
• Ocean plate sinks.
• Forms a trench.
– Trench: also called a
subduction zone,
where one plate
subducts under the
other.
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Continent-Oceanic, continued
• Examples:
– Juan de Fuca plate and
North American plate
• Juan de Fuca plate
subducts under N.A. plate;
created the Cascade
Mountain Range in
Northern CA, OR, and
WA.
• Nazca plate and South
American plate
– Subduction of Nazca
plate formed Andes
Mountains on west coast
of S.A.
Types of Convergent boundaries,
continued
Continent-Continent
• Two continents
colliding
• Pushes the crust
upward to form
mountains.
Continent-Continent Example
• The Himalayas and
Mount Everest—
formed by Indian
Plate and Eurasian
Plates colliding.
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Types of Convergent boundaries,
continued
Oceanic-oceanic
• Two ocean plates collide.
• The older one sinks under the newer one.
• Creates volcanic island arc *Not
Hawaii!*
Oceanic-Oceanic examples
– Mariana Islands
(volcanic!): Formed by
Philippine plate and
Pacific plate
– Tonga Trench in South
Pacific (Pacific plate
subducting under
Australian plate)
• Fastest moving plate: 24
cm/year!!
Transform boundary
• Plates move past each other laterally.
• Neither plate is destroyed.
• Example:
– San Andreas Fault (formed by North
American and Pacific plates)
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Geologic phenomena associated with
Transform Boundaries:
• Earthquakes
• Some warping of crust
Why Plates Move
• Convection is the driving force behind plate
movement!
How It Works:
1. Magma in the mantle is heated by the core.
2. Heated magma rises toward crust.
3. As magma moves away from heat source, it begins
to cool.
4. Cool magma sinks back down toward core, creating
a convection cell that helps move crust.
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