Because of convection currents, the Indo-Australian plate is moving North at 5cm per year whilst the Eurasian plate is moving South-east at a slower rate. These two continental crusts coming together is whats known as a collision margin. Because both plates are low density and buoyant, no subduction occurs, therefore no volcanic activity takes place. Trapped between the two continental plates there is oceanic sediment which is pushed upwards as plates converge. In this case, it was the Tethys sea, which originated at the of the break up of Pangea. The sediment from the Tethys sea was thrust upwards into the Himalayas at a height of 9000m. Earthquakes occur but because they are often deep beneath the earth their impact is limited.
The Mariana islands were formed where the Pacific Plate and the Philippine Plate (both oceanic crust) converged. Subduction of one of the oceanic plates occurs because one plate is likely to be older and denser than the other. The volcanic activity the subduction causes (the same as in oceanic/continental destructive margins) leads to the formation of a chain of volcanic islands- like the Mariana Islands. Marianas Trench is 10.911 km deep and 1580 miles long.