destructive plate margin landform

Question 1

how are fold mountains formed?

Answer 1

an ocean exists between a two continents (geosyncline)
the geosyncline is filled with sediment which over time is compressed into rock
over time, as continents converge, the ocean sediment is folded upwards into mountains and ocean disappears
when the two pieces of continental crust collide neither get subducted as they are too buoyant. one piece is folded over the other.
e.g. himalayas

Question 2

how are trenches formed?

Answer 2

oceanic lithosphere is denser than continental so gets subducted beneath the continental lithosphere
ocean trenches are formed due to a linear depression on the sea floor which runs parallel to the plate boundary

Question 3

how are volcanoes and island arcs formed? oceanic x2 and continental

Answer 3

oceanic lithosphere is denser than continental therefore oceanic gets subducted
the oceanic lithosphere melts due to heat from asthenosphere and friction with continental lithosphere
(also generates earthquakes in benioff zone)
magma created by melting of oceanic crust rises towards the surface escaping through cracks/fissures in continental crust
lava is viscous andesitic lava which creates explosive eruptions and composite volcanoes
chain of islands (island arc) is created by volcanic activity - gradually building up layers of lava