Transcript Divergent Plate Boundaries

Divergent Plate Boundaries
Finz 2012
Plate Tectonics
Plates move about 2
inches/year - about the same
rate as your fingernails grow!
There are 3 types of
plate boundaries:
1. divergent
2. convergent
3. transform
Tectonic Plate Boundaries
Divergent boundaries
Plates move apart
Magma rises, cools and forms new lithosphere
Typically expressed as mid-oceanic ridges
Transform boundaries
Plates slide past one another
Fault zones, earthquakes mark boundary
San Andreas fault in California
Convergent boundaries
Plates move toward each other
Mountain belts and volcanoes common
Oceanic plates may sink into mantle along a
subduction zone, typically marked by a deep ocean
trench
Divergent Plate Boundaries:
Divergent plate boundaries are where seafloor spreading occurs,
producing new oceanic crust. Material from mantle intruded into
fractures as plates are move apart.
The crust of the ocean
is basaltic rock on top
and gabbro on the
bottom.
Ocean basins form when continents split apart!
Early evidence of seafloor spreading
Old mountain belts show us where
continents used to be connected
Old mountains belts (Appalacians
and Caledonides) now separated
but if continents are fit together,
mountain chains form a continuous
belt
More recent evidence of seafloor spreading
1. Symmetry of magnetic stripes (defined by polarity of magnetic
minerals in basaltic rock of seafloor)
The pattern of normal and reverse
polarities on either side of a divergent
boundary can only be explained if new
crust was being formed and repeatedly
split apart as magnetic field reversed
But how does seafloor spreading
(divergence) start?
Hot plume in mantle upwarps
lithosphere of continent
Cracks develop forming rift valleys
Rift zones allow further spreading to
produce an ocean because the water
moves into the low area created from
the divergence
•http://www.iris.edu/hq/files/programs/edu
cation_and_outreach/aotm/11/AOTM_09
_01_Divergent_480.mov
Red Sea-Gulf of Aden: An ocean basin in the making
East
African
Rift will
probably
stop
spreading
and become
a “failed
arm”
Future
ocean
basin
Divergent boundaries
• Perhaps the best known of
the divergent boundaries is
the Mid-Atlantic Ridge.
• The rate of spreading along
the Mid-Atlantic Ridge
averages about 2.5
centimeters per year
(cm/yr), or 25 km in a million
years.
• The mechanism that drives
seafloor spreading was
thermal convection cells in
the mantle
• hot magma rises from
mantle to form new crust
• cold crust subducts into the
mantle at oceanic trenches,
where it is heated and
recycled
Mid-Atlantic Ridge
Oceanic Crust Is Young
Divergent Boundaries
• Ridges also have
– high heat flow
– and basaltic flows or pillow lavas
• Pillow lavas
have
– a distinctive
bulbous shape
resulting from
underwater
eruptions
Divergent Boundaries
• Divergent boundaries
• Beneath a
continent,
– magma wells
up, and
– the crust is
initially
• elevated,
• stretched
• and thinned
Rift Valley
• The stretching produces fractures and
rift valleys.
• Example: East
African Rift
Valley
Narrow Sea
• As spreading proceeds, some rift
valleys
– will continue to lengthen and deepen until
a narrow linear
sea is
formed,
– Examples:
• Red Sea
• Gulf of
California
Modern Divergence
– View looking down the
Great Rift Valley of
Africa.
• Little Magadi
soda lake
Oceanic Divergent Boundary
Example: Mid-Atlantic Ridge
Continental Divergent Boundary
Example: Red Sea / E. African Rift
Continental Divergent Boundary
Example: Baja California
Divergent Boundaries
• Spreading ridges
– As plates move apart new material is erupted
to fill the gap
Iceland: An example of continental
rifting
• Iceland has a divergent
plate boundary running
through its middle
Divergent Boundary
• http://www.youtube.com/watch?v=FK1s1
-OJ5BE
How rifting of a
continent could lead
to formation of
oceanic lithosphere.
e.g., East Africa Rift
e.g., Red Sea
e.g., Atlantic Ocean
Presumably,
Pangea was
ripped apart by
such continental
rifting &
drifting.
Divergent Cross Section View