Divergent Plate Boundaries

Finz 2012

Plates move about 2 inches/year - about the same rate as your fingernails grow!

Plate Tectonics

There are 3 types of plate boundaries:1. divergent2. convergent3. transform

Tectonic Plate Boundaries

Divergent boundariesPlates move apartMagma rises, cools and forms new lithosphereTypically expressed as mid-oceanic ridges

Transform boundariesPlates slide past one another Fault zones, earthquakes mark boundarySan Andreas fault in California

Convergent boundariesPlates move toward each otherMountain belts and volcanoes commonOceanic 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.

Ocean basins form when continents split apart!

The crust of the ocean is basaltic rock on top and gabbro on the bottom.

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/education_and_outreach/aotm/11/AOTM_09_01_Divergent_480.mov

Red Sea-Gulf of Aden: An ocean basin in the making

Future ocean basin

East AfricanRift will probably stop spreading and become a “failed arm”

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

• Ridges also have – high heat flow– and basaltic flows or pillow lavas

Divergent Boundaries

• 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

• The stretching produces fractures and rift valleys.

Rift Valley

• 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 BoundaryExample: Mid-Atlantic Ridge

Continental Divergent BoundaryExample: Red Sea / E. African Rift

Continental Divergent BoundaryExample: Baja California

• Spreading ridges– As plates move apart new material is

erupted to fill the gap

Divergent Boundaries

• Iceland has a divergent plate boundary running through its middle

Iceland: An example of continental rifting

Divergent Boundary

• http://www.youtube.com/watch?v=FK1s1-OJ5BE

How rifting of acontinent could

lead to formation of

oceanic lithosphere.

e.g., Red Sea

e.g., Atlantic Ocean

e.g., East Africa Rift

Presumably,Pangea was ripped apart

by such continental

rifting & drifting.

Divergent Cross Section View