For the past hundred million years or so the thick continental tectonic plate of South America has been driven westwards a few millimetres a year, colliding with the thinner Nazca oceanic plate driving eastwards.
The continental plate overrides the oceanic plate which is slowly driving under (subducted) the thicker plate. This activity is responsible for the Andes mountains rising above the inland end of the subduction zone, and the volcanic activity, due to the heat and pressure of the collision, melts rock deep below the earth's surface. Such melted rock occasionally escapes as erupted lava and ash. The subducting ocean plate sticks and builds up strain energy that is released when the plate slips further downwards, thus causing an earthquake.
The Andes Range
The net result of the above activity is the spectacular, rugged Andes mountain range, east of which are vast stretches of tectonically inactive lowland forming the bulk of the continental plate - e.g. the Argentinian pampas, the Brazilian rainforest.
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