2.5 Flashcards
Convergent plate boundaries may demonstrate different features and generate different types of magma (and volcanic activity) and seismic activity depending on…
Magma and Subduction Zones
whether the plates colliding are:
* oceanic + continental plates
* two oceanic
* two continental plates
Subduction zones are commonly associated with volcanic activity caused by…
Flux Melting
magma rising from the region of the subducting plate
If mantle rocks move upwards at divergent boundaries (+ decompression melting), what happens at convergent boundaries>
- At a subduction zone, relatively cold oceanic crust is sinking into increasingly greater depths of the mantle.
- The combination of cold material and rising pressure should inhibit melting.
The solution to cold material + rising pressure inhibiting melting
- WATER
- The oceanic crust begins interacting with seawater shortly after its formation.
- Over time, this interaction alters the minerals in the rocks, with some silicate minerals hydrating as water is incorporated into their crystal structures.
- Common minerals that form as a result of this process include Amphiboles and Chlorite mica.
- This relatively low-temperature metamorphic process transforms basalt on the ocean floor into rocks known as Greenschists and Amphibolites.
What happens to remaining sediments on a plate when it subducts?
As the oceanic slab descends into the mantle, increasing pressure compacts any sediments that may remain on the plate, driving off the water they contain into the overlying mantle wedge
Under the new temperature and pressure conditions in the mantle, the once-hydrous minerals in the oceanic lithosphere are converted into
- anhydrous forms like pyroxene and garnet
- This process transforms the plate into a rock called an eclogite, that is denser than the mantle.
- This increase in density may be an important factor in the slab-pull mechanism
What does Eclogite formation also lead to?
Eclogite formation releases more water, which rises into the mantle above the descending oceanic slab.
The water squeezed out of the compacting sediments and from eclogite acts as a…
- flux: a substance that causes the depression of the freezing point of materials.
- For example, adding salt to roads during winter can prevent water from freezing when temperatures drop below 0°C. In the mantle, water has the same impact.
- The release of water from the descending slab lowers the melting point of mantle peridotite and allows it to partially melt and generate magma.
Why is flux melting important for magma?
The release of water from the descending slab lowers the melting point of mantle peridotite and allows it to partially melt and generate magma.
The partial melting of Peridotite by flux processes at convergent boundaries produces magma richer in…
Volcanism at convergent boundaries
Silica
Higher silica content in magma
Higher silica content increases the viscosity (thickness/density) of the magma due to the silicate minerals forming larger framework structures
What does the viscosity of magma mean for explosivity of eruptions?
This has implications for the explosivity of eruptions
- Low-viscosity magma tends to produce eruptions where volatiles dissolved in the magma can form bubbles that escape easily, producing effusive eruptions
- For high-viscosity magma, the volatiles cannot escape as easily, resulting in a build-up in pressure and explosive eruptions
The magma produced at convergent boundaries rises to form WHAT?
- Well-spaced volcanoes above the point on the descending oceanic slab where flux melting occurs (flux melting in the mantle wedge (underneath the mantle) goes through the crust and creates a volcano)
- These chains of volcanoes are called volcanic arcs
Why do volcanic rocks formed at convergent boundaries show much greater variation in composition compared to the basalts produced at divergent spreading ridges?
- This is partly due to the magma rising through and assimilating components of the overlying tectonic plate.
- This is especially true where magmas rise through continental crust, which is more variable and contains more FELSIC materials than the MAFIC oceanic crust..
- Evident across the Aleutian Island arc, where the Pacific Plate is subducted below the North American Plate.
The three boundary types are:
Convergent Plate Boundaries
- ocean-ocean convergence
- ocean-continent convergence
- continent-continent convergence
Ocean-ocean convergence - which plate subducts? What is this associated with?
- When two oceanic plates collide, the older, colder, and therefore denser plate will generally be subducted beneath the younger, warmer plate
- Subduction is typically associated with the formation of an oceanic trench
Ocean-ocean convergence - how do earthquakes occur?
- Unlike at divergent boundaries, subduction zones generate a range of earthquakes from shallow to deep
- These earthquakes occur due to slip along the descending slab or movement along faults within the subducting plate.
- This zone of seismicity is known as the Wadati-Benioff zone and served as further evidence for the theory of seafloor spreading.
Ocean-ocean convergence - what major earthquakes can occur?
- Megathrust earthquakes happen when the interface between the two converging plates becomes stuck, leading to a sudden release of accumulated stress.
- This release can generate earthquakes with magnitudes over 9, causing substantial uplift of the ocean floor and triggering catastrophic tsunamis
Ocean-ocean convergence - produces volcanism?
- The type of volcanism associated with ocean-ocean boundaries can be variable but typically ranges from basaltic (low silica content) to andesitic (slightly higher silica content)
- As a result, volcanic behaviour can also range from effusive (lava flows and fountains) to more explosive eruptions.
Ocean-ocean convergence - accretionary wedges
- Forms when sediments, not carried downward with the subducting plate, are scraped off at the leading edge of the overriding plate.
- As this process unfolds, compressional forces cause the sediments to accumulate into a wedge-shaped mass that is faulted by low-angle reverse faults, known as thrust faults
- Additionally, this compressional environment leads to the folding of these sediments as subduction continues.
Ocean-continent convergence - WHY?
- The density of the continental lithosphere (2.8 g/cm³) prevents it from being subducted beneath the denser oceanic lithosphere (3.0 g/cm³).
Ocean-continent convergence - volcanism/magma
As magma (also known as diapirs) rises through the continental lithosphere, it has a greater chance of becoming more saturated with silica compared to magmas that ascend through the oceanic lithosphere. Consequently, the volcanic materials (and associated activity) produced can vary greatly, ranging from MAFIC lava-like basalt to very silica-rich FELSIC lavas like rhyolite.
Ocean-continent convergence
Ocean-continent convergence
