Plate convergence and destruction

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Topics

Trenches

Earthquake distribution

Explosive volcanism

Types of subduction zone

Continental collision

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What happens when plates collide/converge

One plate slides beneath the other, process called subduction

This happens at subduction zone, the plates are referred to as over riding and down going

Subduction is a 3D process, the process carries on at depth

The down-going plate is destroyed , these boundaries are sometimes referred to as destructive

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When plates converge

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Standard features of a subduction zone

Trench

Earthquakes in configuration, at different depths and magnitude

Volcanism, often explosive

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Trenches

A massive submarine depression or canyon

The down-going plate plunges deep into the earths interior

At the junction with the over-riding plate there is a deep trench, the deepest on Earth is the Marianas Trench in the Western Pacific

Peru Chilli trench, 6000 km in length max depth 8km, the oceanic Nazca plate is being subducted under the South American plate. The crust on the over-riding plates deforms building up int the Andes, the area has active volcanoes, the Earths tallest active volcano is found in the Andes , summit of nearly 7km

Trenches are the first standard feature of a subduction zone

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Converging Plates

Oceanic and continental lithosphere, the Nazca is subducted due to it being more dense. This carries on at depth as the plates are in relative motion to each other along the plunging line of contact

Earthqhakes can be used to map at depth on the subduction zone, they are signals on the fault plane

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Earthquake Distribution

Earthquakes are shallow nearer the trench at the edge of the down-going plate and deeper on the over-riding plate, from 0 to 300 km

showing this on a graph indicates the inclined seismic belt

also known as Wadati-Benioff zone

Deepening earthquakes in the direction of travel of the plate that is being subducted along the subduction zone

The deepening earthqake foci is the second stanadrd feature of a subduction zone

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Seismic signals stop at around 700km

Earthquakes are common all around The Pacific margin, from South America, round anti-clockwise

Some seismic belts are not associated with continental subdution margins, like the island chains of the Aleutians, Japan, Phillipines, Fiji and New Zealand

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Explosive volcanism

The third standard feature of subduction zones

Tend to errupt violently with gas rich viscous magma

Mount Pinatubo in 1991, 2nd largest erruption of the 20th C

Global impact, releasing 10 cubic kms of magma, most of it ash, the amount of sulpherous gas relased caused a 0.5 degree C cooling for 2 years following the erruption

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Types of Subduction Zone

continental - oceanic convergence

Nazca-South American plate convergence

oceanic- oceanic convergence

causes an island arc like the Lesser Antilles arc in the Carribbean

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Previous image also called active continental margin

Lesser ANitilles Arc, Montserrat volcano woke up in 1995 after no recorded events in human history, destroyed the capital

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Continental collision

Continental lithosphere is too bouyant to subduct, instead one plate overrides the other leading to thickening and deformation of the crust forming mountain belts, this is a process called orogeny

The Himalayas is a site of active continental collision today

Earthquakes are more widely distributed and do not show a systemic pattern of deepening