Grades 6–8 · Earth & Space
How can colliding continents build mountains?
The Himalayas grew where the Indian and Eurasian continental regions collided. Plates continue to move, but continental crust is relatively buoyant. Instead of one whole continent simply sinking away, the collision zone crumples, faults and thickens. Some crust also underthrusts beneath neighboring crust.
In the diagramContinental collision can thicken crustFolded rock layers above · separate area model belowBeforeAfter shortening100 km wide30 km thick80 km wide37.5 km thickEach rectangle: 3,000 km² in cross-sectionThickness includes deep crust; it is not peak height.
Rock layers can fold, and large slices can move along thrust faults over other slices. These processes shorten the horizontal distance across the collision zone while building a thicker stack of crust. Deformation takes place over long periods, with some motion occurring suddenly during earthquakes.
The lower rectangles give a separate geometric account. An invented crustal slice is initially 100 kilometers wide and 30 kilometers thick: its cross-sectional area is 3,000 square kilometers. If it narrows to 80 kilometers while that area stays the same, its thickness becomes 37.5 kilometers. This is an illustration of thickening, not a calculation of a mountain’s height.
Real thickened crust extends deep below a range as well as rising above its surroundings. Erosion removes material and reshapes the surface. Volcanic and fault-block mountains can form by other routes, so continental collision explains one important kind of mountain building.
About this explanation
Starting knowledge: Know that tectonic plates move slowly and that a cross-section shows a slice through Earth.
Sources
- U.S. Geological Survey — The Himalayas: Two continents collide
- U.S. Geological Survey — Geology of Great Smoky Mountains National Park