Lecture Plate Boundaries

A boundary in which two plates move apart, resulting in upwelling of material from the mantle to create new seafloor. Ex. Mid-ocean ridges

A linear zone along which the continental lithosphere stretches and pulls apart. The crust will start to thin out and create cracks. Can lead to a rift valley, and if it keeps rifting and gets low enough, water can go through. Its creation may mark the beginning of a new ocean basin. If it continues to rift it can lead to a mid-ocean ridge rifting

A boundary in which two plates move together, resulting in oceanic lithosphere being thrust beneath an overriding plate, eventually to be reabsorbed into the mantle. It can also involve the collision of two continental plates to create a mountain system. Ocean - continent Ocean - ocean Continent - continent

A long, narrow zone where one lithospheric plate descends beneath another.

The oceanic lithosphere is subducting beneath the continental lithosphere back into the mantle Since the oceanic lithosphere is denser than the continental lithosphere This process triggers melting in the wedge of mantle rock directly above the descending plate. As the plate plunges downward, heat and pressure drive water out of the hydrated (water-rich) minerals in the subducting slab, triggering melting in the mantle (flux melting). Generates some magma mixed with unmelted mantle rock, being less dense leads to it rising towards the surface, and depending on the surface, can lead to a volcanic eruption

Continental volcanic arc Trenches

Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent. Examples include the Andes and the Cascades.

A narrow, elongated depression of the seafloor A form where oceanic plates bend and sink into the mantle Deep trench is cold, dense slabs that subduct at steep angles Shallow trench is warm, buoyant slabs that subduct at shallow angles

Trench Volcanic island arc

Where two oceanic slabs converge, one descends beneath the other, initiating volcanic activity by the same mechanism that operates at all subduction zones. The oceanic lithosphere, which is older, thicker, and denser, subducts. Water released from the subducting slab of oceanic lithosphere triggers melting in the hot wedge of mantle rock above. In this setting, volcanoes grow up from the ocean floor rather than upon a continental platform. Sustained subduction eventually results in a chain of volcanic structures large enough to emerge as islands. The newly formed land consists of an arc-shaped chain of volcanic islands. Ex. Aleutian Chain in Alaska

Whereas oceanic lithosphere tends to be dense and readily sinks into the mantle, the buoyancy of continental material generally inhibits it from being subducted, at least to any great depth. Consequently, a collision between two converging continental fragments ensues. This process folds and deforms the accumulation of sediments and sedimentary rocks along the continental margins. The result is the formation of a new mountain belt (range) composed of deformed sedimentary and metamorphic rocks that often contain slivers of oceanic lithosphere. This is what happened with Indian plate and the Euroasian plate leading to the creation of the Himalayas

Mountain range

A boundary in which two plates slide past one another without creating or destroying lithosphere.

The Mid-Atlantic ridge with its zigzag pattern, roughly reflects the shape of the sone of rifting that resulted in the breakup of Pangaea Ex. San Andreas Fault

Long, narrow scar like features in the seafloor that are roughly perpendicular to the offset ridge segments. They include both the active transform fault and its preserved space

Slab pull or ridge push Slab pull dominantes

A mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along.

A mechanism that may contribute to plate motion. It involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity.

Ocean trenches Forearc region Volcanic arcs Back arc region

Sediment from pyroclastic debris, eroded sediments, and ocean-floor deposits Before the volcanic arc Steep subduction equals a narrow forearc Shallow subduction equals broad forearc

Experiences tensional forces, causing crustal stretching and thinning Behind the volcanic arc Forms basins that fill with volcanic ash and sediment

Subduction and roll back of an oceanic slab creates flow in the mantle that pulls the upper (non-subducting) plate toward the retreating tench Slab suction causes the overlying plate to become elongated and thinned, often resulting in the formation of a basin and spreading center behind the volcanic arc

Magma solidifies underground

A large body of igneous rock that forms when magma solidifies deep in the crust, usually an assemblage of multiple plutons