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Plate Tectonics
Most of the planet is hot, molten rock known as magma, surrounding a denser metallic core thought to be mostly iron. A thin layer of cooler, solid rock forms the crust, only a few tens of miles thick, and carried to and fro on the currents of the liquid magma beneath. The crust is not a single solid shell; it is composed of many distinct plates, each following its own local magma currents.
The continents are large "bubbles" of lighter rock float atop these plates, and are shaped by the movements in several ways. In some places, continents collide and buckle, thrusting up mountain ranges. The Himalayas are such a range, resulting from the ongoing collision between the Indian subcontinent and Asia.
Volcanism
Another process that raises landforms is volcanism, where magma erupts through weak spots in the solid crust as lava, which then cools into new rock. Depending on where and how the lava emerges, it may form large volcanic mountains, such as the islands of Hawaii. In other places, the lava oozes up and forms new sea floor at the boundary between two tectonic plates that are moving apart.
Erosion
While plate tectonics and volcanism raise new landforms, the processes of erosion wear them down. Wind, water and gravity carry away landforms in many ways. Glaciers grind down mountains, carrying enormous boulders and depositing them hundreds of miles away when they finally melt. Sand, carried by the wind, collects in dunes that move like waves across the Sahara. Rivers and streams cut through the land, carrying sediment to the sea and depositing it in the delta. Waves and tides wash shorelines, breaking down rocks and creating flat, sandy beaches.
Chemistry
Rocks are not chemically inert; they can and do react with other chemicals in the environment, changing their physical properties in various ways. Sometimes, weathering weakens rocks, making them more susceptible to the forces of erosion. In some circumstances, however, chemistry can create new geological features. As a landlocked sea evaporates, for example, deposits of salt or other minerals can become concentrated. Mineral-laden water percolating into a cave can produce stalactites and stalagmites of various forms. And, of course, the complex chemical processes known as life can affect landscapes in countless ways, from coral reefs and beaver dams to forests and man-made cities.