CONVERGENT BOUNDARIES

Ocean Trenches

Linear, narrow, steep-sided

Associated with subduction zones

Deepest parts of ocean

Mariana Trench, 11,022 m (36,161 ft)

Majority in Pacific Ocean

Three Plate Boundary

•East Pacific Rise – Spreading Center

•Central America Trench – Collisional Zone

•Connectors between ridge segments are transform faults (A mid‑ocean ridge is not one straight crack but a series of ridge segments separated by offsets. A transform fault is the long fracture that links two neighboring ridge segments and lets the two sides slide sideways past each other as the plates move)

Transform Faults (PHOTO)

• What a transform fault is: the active part of the break that connects two ridge segments where plates slide past each other; along the transform fault the two sides move in opposite directions so it’s a plate boundary and can produce earthquakes.

• What a fracture zone is: the extended, often inactive lineation beyond the transform fault; it’s a scar in the oceanic crust that marks the offset in the ridge but beyond the ridge the two sides move in the same direction so there is no ongoing slip or earthquakes there

CONVERGENT BOUNDARY: SUBDUCTION OR COLLISION

Subduction:  recycling of crust - a portion subsides into mantle whereas another portion returns to the surface as andesitic magma.

Volcanic arcs: stratovolcanoes

•Island arc; chain of islands, e.g., Japan

•Continental arc; volcanic mountain range, e.g., Andes Mountains

Earthquakes cluster along Benioff zone.

Volatile

“Short-tempered”, readily wants to be a gas; not explode.

Ex: alcohol (evaporates readily)

Convergent Boundary (PHOTO)

Convergent Plate Boundary where Oceanic Lithosphere is Subducting Beneath Continental Lithosphere (PHOTO)

MT Hood

Convergent Plate Boundary Involving Two Slabs of Oceanic Lithosphere

Continental Collisions Occur Along Convergent Plate Boundaries When Both Plates are Capped with Continental Crust

Active Volcanoes in… (Based on PowerPoint)

In Aleutian Chain, Alaska

ALSO, Maug Volcano (Izu-Bonin Mariana Subduction zone)

Fundamental Silica CYcle

•Basalt rises from the mantle (MORB- Mid-Ocean Ridge Basalt)

•Horizontal transport picks up silica

•Subduction results in generation of two fractions:

•one that sinks back into the mantle, and

•one that becomes produces magma at volcanic arcs.

Mantle rocks contain about 44% silica.

MORB ~ 50% silica.

Think layer of sediment ~  80% silica

Andesite (strato)volcano ~ 60% silica.

Himalayas (Photo)