Subduction Zones
Geosyncline
What We Actually See
What Does This Look Like?
Andes – True Scale
Continent-Continent Collision
Continent-Terrane Collision
Where Does Ocean Crust Go? Hugo Benioff, 1954 1. How we know plate tectonics happens
Benioff’s Interpretation 1. How we know plate tectonics happens
Benioff’s Interpretation Updated 1. How we know plate tectonics happens
How Plates Move
A Subduction Zone
Subduction and Metamorphism
Why Mountains are High
Where the Plates Meet
Terrane Accretion
Terranes in Western North America
Subduction Zone Rocks
Steinmann Trinity (1905) Serpentinite Pillow Basalt Radiolarian Cherts Characteristic of ophiolite settings Other Distinctive Rocks Graywacke and Flysch Blueschist
Bedded Chert
Serpentinite
Serpentinite
Magnesite Mines in Serpentinite
Pillow Lava
Pillow Lava
Pillow Lava
Blueschist
Graywacke
Flysch
Melange
Melange
Eclogite
Molasse, Switzerland
Molasse and the High Alps
Wisconsin 2 Billion Years Ago
1900 MY Penokean Orogeny
Before the Penokean Orogeny
First Collision
Second Phase
Marshfield Terrane Collides
Assembling North America
Assembling North America
1700 MY Granite and Rhyolite
Mazatzal Orogeny
Cactus Rock
Baraboo Interval Quartzites
“Baraboo Interval”
Yavapai Orogeny
Baraboo Quartzite
Mazatzal-Yavapai Events
1450 MY Wolf River Batholith
Wolf River Batholith
1100 MY Mid-Continent Rift
The Grenville Orogeny Begins
The Grenville Orogeny
A Subduction Zone The story begins where two oceanic plates converge and one sinks into the earth's mantle, a process called subduction. In these diagrams light green indicates basaltic oceanic crust, dark green is a layer of the earth's mantle about 100 kilometers thick that forms the base of the plate. These two units collectively are called the lithosphere. Brown represents deeper mantle and red represents young igneous rocks. An ocean trench marks the place where the descending plate sinks into the mantle.
When the descending plate reaches about 100 kilometers deep it begins to be heated.
The descending plate has been soaking in sea water for many millions of years and is wet. Heating drives water into the surrounding mantle.
Water doesn't affect just the earth's surface but its deep interior as well. Water lowers the melting point of rocks and causes the adjacent mantle to begin melting. Magma rises upward.
How Arcs Grow
Exotic Terranes Continued subduction builds a larger volcanic chain and the weight of the volcanoes causes the crust to sag. Erosion off the volcanic arc sheds sediment onto the flanks. Eventually, convergence of the plates may bring a submarine volcano, or seamount, into the subduction zone.
When the seamount enters the subduction zone, something has to give.
Often the seamount is thrust onto or beneath the arc.
When the seamount reaches the subduction zone, it may be shoved under the other plate or may break off and be thrust onto the other plate. Sediment accumulating in the trench also may get shoved onto the arc as well. This process is called obduction.
At times, a sliver of oceanic crust may break off and ride onto the other plate as well to form an ophiolite.
Submarine volcanic plateaus may also collide with the volcanic arc.
The submarine volcanic plateau begins to be thrust onto the arc The submarine volcanic plateau begins to be thrust onto the arc. It is too thick to be subducted so it will either be thrust onto the arc, or it will cause the subduction zone to relocate somewhere else.
Here a submarine volcanic plateau has been added to the arc Here a submarine volcanic plateau has been added to the arc. Seamounts, ophiolites and volcanic plateaus are all made of igneous rock but ophiolites, in particular, have a distinctive structure that sets them apart.
Eventually so much material can be added to the arc that the subduction zone clogs and a new subduction zone forms. Repeating this process over 100 million years can build up a very sizable land mass like Cuba, the other Greater Antilles, or Costa Rica and Panama.