abandoned the definition since the 1970s. Any similar landform lower than this height was considered a hill. However, today, the US Geological Survey concludes that these terms do not have technical definitions in the US. The UN Environmental Programme's definition of "mountainous environment" includes any of the following: Class 1: Elevation greater than . Class 2: Elevation between and . Class 3: Elevation between and . Class 4: Elevation between and , with a slope greater than 2 degrees. Class 5: Elevation between and , with a slope greater than 5 degrees and/or elevation range within . Class 6: Elevation between and , with a elevation range within . Class 7: Isolated inner basins and plateaus less than in area that are completely surrounded by Class 1 to 6 mountains, but do not themselves meet criteria for Class 1 to 6 mountains. Using these definitions, mountains cover 33% of Eurasia, 19% of South America, 24% of North America, and 14% of Africa. As a whole, 24% of the Earth's land mass is mountainous. Geology There are three main types of mountains: volcanic, fold, and block. All three types are formed from plate tectonics: when portions of the Earth's crust move, crumple, and dive. Compressional forces, isostatic uplift and intrusion of igneous matter forces surface rock upward, creating a landform higher than the surrounding features. The height of the feature makes it either a hill or, if higher and steeper, a mountain. Major mountains tend to occur in long linear arcs, indicating tectonic plate boundaries and activity. Volcanoes Volcanoes are formed when a plate is pushed below another plate, or at a mid-ocean ridge or hotspot. At a depth of around , melting occurs in rock above the slab (due to the addition of water), and forms magma that reaches the surface. When the magma reaches the surface, it often builds a volcanic mountain, such as a shield volcano or a stratovolcano. Examples of volcanoes include Mount Fuji in Japan and Mount Pinatubo in the Philippines. The magma does not have to reach the surface in order to create a mountain: magma that solidifies below ground can still form dome mountains, such as Navajo Mountain in the US. Fold mountains Fold mountains occur when two plates collide: shortening occurs along thrust faults and the crust is overthickened. Since the less dense continental crust "floats" on the denser mantle rocks beneath, the