The Earth's crust is not an unbroken shell. It is a jigsaw of about 14 huge rocky pieces called plates that continually push and slide against each other.
• when two plates collide, the rocks they are made of can get squashed and pushed up into the air. Some mountains were formed that way.
• earthquake are caused by the plates of the Earth's crust sliding against each other.
• The surface of the Earth is changing all the time as rocks wear down and plates move.