Destructive plate margins

Destructive plate margins develop where cool mantle convection currents are coming together (converging) and descending back down into the mantle. In these zones, the lithospheric plates are pushed together by the injection of magma at the mid-ocean ridges, and pulled by the frictional drag of the mantle convection currents on the base of the plate. In situations where rocks making up the plates have different densities, the plate with the greatest density will be dragged down below the less dense plate, in a process known as subduction. Destructive plate margins are so called because crust is always destroyed.


Oceanic-continental destructive plate boundaries

This type of plate margin can be seen running the length of the west coast of South America, where the Nazca plate is being subducted beneath South America. It can also be seen along the west coast of North America north of Carolina to Alaska.
The western margin of South America is made up of a considerable thickness of low density continental crust, which makes the plate buoyant and thus causes it to override or 'float' above the denser Nazca plate. The Nazca plate is composed entirely of dense basaltic rocks and peridotite, and therefore when the plates are forced together the denser Nazca plate is dragged down beneath the less dense continental plate, forming a subduction zone.

Subduction trenches
A deep submarine trench (subduction trench) marks the zone where the plates meet - an example of this is the Peru-Chile trench. Subduction trenches can reach depths of 4-5 km below the level of the adjacent ocean floor. They will progressively become infilled by sediment scraped off the top of the subducting plate, and by sediment eroded from the adjacent continental plate. The Marianas Trench in the western Pacific is the deepest subduction trench at nearly 11,000m - you could drop Mount Everest (8864m high) into the Marianas Trench and the summit would still be underwater!


Accretionary prisms and ophiolite suites
As subduction occurs, all the marine sediments and rocks lying on the top of the subducting plate are scraped off by the edge of the overriding plate and accumulate in subduction trench. This mass of compressed sediment becomes 'stuck' to the edge of the overriding continental plate to form an accretionary prism. This is a way in which continents can grow by adding new material to their margins.
The rocks making up the accretionary prism consist of a mixture of sediments such as muds and cherts (deep marine sediments) together with pillow lavas and other volcanic material scraped off the top of the subducting plate. As these sediments become deeply buried within the infilling subduction trench, they are subject to high pressure, low temperature metamorphism, and are converted to rocks known as greenschists and blueschists.

These rocks make up a distinctive association of rocks known as an ophiolite suite. The presence of ophiolite suites in ancient mountain belts indicates a past episode of subduction. The Lizard Complex in Cornwall is interpreted to be an ophiolite suite.

Fold mountains
The most obvious features of continental-oceanic destructive margins are the linear belts of fold mountains which run along the boundaries of the overriding continental plate. An example of such a belt is the Andes in South America (pictured left), and the Rocky Mountains in North America. The mountains are formed when rocks and sediment on the edge of the overriding plate gets compressed and crumpled.

 

 

Igneous Activity
Continental-oceanic margins are characterised by a distinctive set of igneous rocks. These rocks include large intrusive masses (batholiths) of granites and their associated volcanic rocks. These volcanic rocks are mainly of andesitic composition and include both lavas and pyroclastic rocks. The magmas are produced by partial melting of the subducted plate and the mantle material which directly overlies it. Melting is triggered by the release of water from hydrous minerals such as amphiboles within the subducted plate. The release of this water lowers the melting point of the plate, and affects the melting point of the overlying mantle, so that quite large volumes of melt are generated. This partial melting process generates a fairly silica-rich melt, which then rises towards the surface. During the journey from the mantle to the surface, the magma will undergo magmatic differentiation and may also become contaminated with crustal material from the continental plate, giving rise to an acidic magma which will either crystallise below the surface to form granitic batholiths or escape to the surface to form explosive volcanic activity. These volcanoes are common at oceanic-continental destructive margins, and they form a ring of active volcanoes around the Pacific margin which is known as the Pacific Ring of Fire.

Above: The Pacific Ring of Fire - so called because of the number of highly explosive
active volcanoes around the Pacific rim, all formed as a result of subduction.


Find out more about plate tectonics:

Plate tectonics make the world go round: introduction
Constructive margins: Continental (rift valleys)
Oceanic (mid-ocean ridges)
Destructive margins: Continental collision
Ocean-continent destructive margins
Ocean-ocean destructive margins (island arcs)
Conservative margins
Continental drift
Plume and hotspots
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February 2007