Plate Tectonics: Convergent and Divergent Plate Boundaries

Divergent Plate Boundaries

Lithosphere consists of the crust and uppermost mantle, broken into rigid plates.
Plate boundaries do not always align with continent edges.
- Active margins are on plate boundaries, while passive margins are not.
- Continents can be in the middle of plates.
Continental drift is a consequence of plate tectonics, but not the same as plate tectonics.
Earthquakes occur along plate boundaries.
Seafloor spreading occurs where oceanic crust is formed, and subduction zones are where oceanic crust is destroyed.
Plates move at different speeds (up to 15 cm/year) and directions, resulting in divergent, convergent, and transform boundaries. Focus is on convergent and divergent boundaries.

Defined by mid-ocean ridges.
New oceanic lithosphere is created in the rift valleys at the center of these ridges.
Youngest lithosphere is along mid-ocean ridges, getting older with distance.
Oceanic lithosphere consists of the oceanic crust and mantle lithosphere.
The source of new crust is the mantle asthenosphere, which rises near the surface at divergent plate boundaries where the lithosphere is thin.
The asthenosphere melts due to decreased pressure at shallow depths, forming partial melts (magmas) with a different composition from the source rock.
Basaltic magmas rise through the lithosphere and erupt.

Igneous rocks solidify from molten rock (melt).
Volcanic rocks crystallize on the Earth's surface with small or no crystals (volcanic glass).
Plutonic rocks crystallize below the Earth's surface, cooling slowly and forming larger crystals (e.g., granite).

Basaltic magma from the partially melted asthenosphere rises and pools in a magma chamber (a mush of magma and crystals).
Some magma cools within the chamber, forming gabbros.
Some magma rises further, filling cracks, solidifying, and cracking again, creating sheeted dikes.
Magma from sheeted dikes feeds volcanic eruptions on the seafloor, forming pillow lavas (underwater eruptions).

The boundary between asthenosphere and lithosphere is defined by temperature.
The thickness of the lithospheric mantle increases as the plate cools, moving away from the ridge.
The lithospheric mantle forms via cooling.

As lithosphere cools, it becomes denser and sinks lower, making the seafloor deeper further from the mid-ocean ridge.
Older oceanic crust has a thicker layer of sediment.
Spreading rates at mid-ocean ridges vary.
- Fast spreading ridges (up to 18 cm/year) have wider swaths of younger/hotter/shallower oceanic crust.
- Slow spreading ridges (less than 5.5 cm/year) have more pronounced topography with higher mountains and deeper abyssal plains.
Seawater circulates through hot rocks, picking up dissolved ions, and forming black smokers when the mineral-charged water flows back into the ocean, supporting diverse ecosystems.

Incipient rifting: broad uplift, parallel cracks form.
- Example: Basin and Range.
Rift valley formation: center block drops down.
- Examples: East African Rift Valley, Lake Baikal.
Oceanic crust and new ocean formation.
- Example: Red Sea.
Spreading continues, mid-ocean ridge forms, sides of continent thin, cool, contract, and sink.
- Example: Today’s Atlantic Ocean.
Not all continental rifts become seas or oceans; many fail.
- Example: Mid-Continent Rift System in north central US.

Downgoing slab
- Evidence:
  - Deep Earthquakes (Wadati-Benioff zone)
  - Fast regions (denser, colder rocks) imaged by seismic waves.
- Slabs sink because they are denser due to being colder and having dense metamorphic minerals formed under pressure.
Trenches
Accretionary Prisms
- Formed of sediments scraped off the downgoing slab, faulted and squeezed.
Volcanism
- Water and other volatiles in the downgoing slab lower the melting temperature of the overlaying asthenosphere, causing it to melt.
- Magmas rise through the asthenosphere and lithosphere, erupting onto the surface.

One oceanic plate subducts beneath another.
Volcanism on the overriding plate forms a chain of volcanic islands (island arc).
- Examples: Aleutian Islands, Izu-Bonin-Mariana.

Oceanic plate always subducts because continental crust is thicker, older, and less dense.
Volcanoes always occur on the continents due to the continental crust overriding the oceanic plate.
- Examples: Cascade volcanoes, volcanoes in South America.