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LANDFORMS ASSOCIATED WITH PLATE BOUNDARY ACTIVITY

FOLD MOUNTAINS

• upland areas formed by the buckling of the earth's crust - associated with convegence boundaries (destructive and collision margins)
• Fold mountains at destructive margins occur where oceanic crust is subducted underneath the continental plate, forcing the over-riding plate to crumple and uplift - the resulting folding and faulting results in the formation of a mountain chain roughly parallel to the oceanic trench - e.g. the Andes Mountains.
• Fold mountains at a collision zone occur as the colliding plates resist subduction and uplift occur with folding and faulting - e.g. the Himalayas.

OCEANIC TRENCHES

• These are often found in a linear pattern located just offshore;
• Deep oceanic trenches form at destructive margins where oceanic lithosphere is subducted into the asthenosphere below;
• the long and narrow oceanic trenches are formed during and mark the place where the ocean floor is deepest (about 10km below sea level)
• Example - the collision of the Nazca plate and South American plate has produced the Peru-Chile deep sea trench off of the west coast of South America

A detailed account of ocean trenches including named examples from Wikipedia

see also this easy to read article on ocean-trenches

ISLAND ARCS

• like oceanic trenches, these tend to form a linear pattern, although one which is "arc-shaped";
• an island-arc is a chain of volcanic islands that form at a destructive boundary where there is the convergence of two oceanic plates;
• during subduction, the subducting plate is melted (due to interior heat, friction and pressure) and creates a magma source;
• the light, less dense magma rises to the surface and erupts to form sub-marine volcanoes
• eventually these volcanoes grow through successive eruptions to above the height of sea-level forming a chain of volcanic islands which is known as an island-arc.

Check out this excellent animation of Island Arc Formation (link from Wycombe High School)

See the following article on Island-arc formation from platetectonics.com

OCEAN RIDGES

• These are underwater mountain ranges which develop where divergence takes place (as two plates move apart)
• Ocean ridges, occur in a linear pattern, usually through the middle of oceans (e.g. the Mid-Atlantic Ridge)
• As magma rises at this point (the rising limb of a convection cell), the rising magma causes the lithosphere to dome up creating the ridge and where tensional forces, creates weaknesses in the crust, magma erupts forming volcanoes which further build up the ridge;
• The East-Pacific Ridge is an anomaly from the pattern of mid-ocean ridges - which can be explained by the fact that the rising limb of the convection cells in the asthenosphere is located in this position;

View the location of ocean ridges on this map from the USGS - note the anomalous position of the East Pacific Ridge.

See the following article on ocean-ridges.