What happens when the crests of two waves overlap

Energy of the waves are redistributed to form a resultant wave with amplitude given by the summation of individual wave's amplitude. 
If the two waves are of same frequency, speed and amplitude and travelling in opposite direction den stationary waves are form.

Magma  is composed of molten rock and is  stored  in the earth's crust. lava is  magma  that reaches the surface of our planet through a volcano vent.

Velocity=total displacement/timefrom house to store is 500 from store to neighbor is 200 therefore total displacement=500+200=700metrestherefore,700metres/910seconds=0.8metres per second