The earth's crust is made up of plates that float on molten rock in the mantle. These plates collide and separate, causing geological events. When plates collide, fold mountains like the Alps are formed through compression, and earthquakes occur along plate boundaries. When plates separate, volcanic islands and mid-ocean ridges result from magma rising through cracks between the plates. Examples are the Mid-Atlantic Ridge and volcanoes in Iceland. The Pacific Ring of Fire is a zone with many volcanoes and earthquakes caused by plate tectonics in the Pacific Ocean region.