Subduction occurs as a result of:
a) Gravity pulling the older and denser lithosphere downward.
b) Horizontal plate accommodation.
c) Upwelling of hot mantle material along the trench.
d) Lubrication from the generation of andesitic magma.

a) Gravity pulling the older and denser lithosphere downward.

Explanation:

Earth's crust is made of plates which float on mantle . Plates are of two types

1 ) oceanic plates which are older and heavier 2 ) continental plates which are lighter . Continents are situated on continental plates. Due to relative motion , oceanic plate, because of its  heavy weight,  goes under lighter continental plate . This is called subduction.  

a high pressure system is a dense air mass that is usually cooler and drier than the surrounding air. a low pressure system is a less dense air mass that is usually wetter and warmer than the surrounding air.

in general, areas that experience high atmospheric pressure also experience fair weather. low pressure systems can cause the formation of clouds and storms. air usually flows from areas of high pressure to areas of low pressure..