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
Geologic phenomena at divergent boundaries:
* = unique to divergent boundaries
– Iceland* (formed right on top of MidAtlantic 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
• Undersea mountain
ranges that are also a
plate boundary.
• Formed by magma
flowing to surface,
forming new crust.
• Forces plates apart =
seafloor spreading
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)
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.
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.
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.
• Example:
– San Andreas Fault (formed by North
American and Pacific plates)
Geologic phenomena at transform boundaries
• Earthquakes
• Mountains (folded mountains)
– Warping of crust
• Displacement of Mid-Ocean Ridges
Evidence supporting Plate
Tectonics
• Seafloor topography (the physical
conformation of the ocean floor)
– Mid-ocean ridges (undersea mountains and
boundaries; proof that there are plates and
they can move)
– Trenches (proof that there are plates and
that they are moving)
– Island chains (Hawaii!!): can help track
movement of a plate over a hotspot
– Magnetic reversals
Island Chains/Hot Spots
• Island chains: line of volcanic islands and
seamounts caused by a hot spot.
– Example: Hawaiian Islands
• Hot spot: long-lasting,
extra hot region in
mantle; causes
volcanism
– Can track movement of
a plate over a hot spot
– Volcanoes form and
move away from hot
spot as plate moves.
– New volcanoes then
take place of old ones.
Magnetic Reversals
• Earth’s magnetic field
changes polarity (reverses)
every 500,000 years or so.
– Geographic North
becomes Magnetic South.
• These magnetic reversals are
preserved in rock formed at
mid-ocean ridges
– We can see how ocean floor
has grown over time and
moved away from mid-ocean
ridge.
• What happens:
– At mid-ocean ridges, when molten rock cools
and solidifies, iron-containing minerals align
with current magnetic field.
– Records orientation of magnetic field at the
time it solidified.
– Creates “bands” of normal magnetic polarity,
then reverse, then normal, then reverse, etc.
• Allows us to “track” the growth of the ocean floors
– Shows their movement over time.
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