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The Highest Points
Mountains are Earth's tallest landforms. They exist under the ocean as well as on land. Mountain ranges form when adjacent tectonic plates push against each other. As two plates push together, the crust compresses, and wrinkles occur, forming mountains. The Himalayas, containing Earth's highest point of 8,850 meters (29,035 feet) on Mount Everest, are still growing due to the Indian plate pushing under the Asian plate. A second type of mountain comes from volcanic activity when a plume of molten rock emerges from the core through the mantle to the crust. Volcanoes can be rounded on top, but most tectonic plate mountains have sharper, narrower peaks.
Rounded Tops
Hills are shorter than mountains and have rounded tops. They can form in a number of ways. Wind can push sandy soil into mounded hills. Glaciers form hills when piles of rocks and debris imbedded in the ice get left on the land as glaciers melt and retreat. Hills can form when faults, which are cracks in the Earth's crust, rise slightly upward. Erosion over millennia can wear down mountains into hills. South Dakota's Black Hills are examples of mountains uplifted during the Cretaceous Era about 65 million years ago that eroded to hills over time. Smaller lava eruptions can form hills instead of mountains.
High and Flat
Plateaus are elevated portions of mostly level land that can cover large areas. Usually they are separated from lower areas by distinct sloping sides and are created by uplift, volcanic eruptions or erosion of higher lands. The Laurentian Plateau, also called the Canadian Shield, is a huge plateau of ancient rock that covers Greenland, some of the northern United States and more than half of Canada. High plateaus, such as the Colorado Plateau, can be deserts or grasslands. Low plateaus are suited for agriculture.
Low Open Land
Coastal plains lie along shorelines of continents, and floodplains are along rivers. Coastal plains form when the rivers and streams that flow from the interior of the continent exit through coastal lands into the ocean, carrying with them sediments collected from other areas. During floods, large amounts of mud, silt and sand get added to plains. The Atlantic Coastal Plain extends from New Jersey south to Florida and west to Texas, with sediments overlying older uplifted sedimentary rocks. Inland plains, such as the Great Plains of the central United States, are grasslands that are used for agriculture. These flat areas are former ancient seabeds that became uplifted.