Mountains
Tension along the Earth's shifting crust, volcanism and erosion all impact mountains. According to scientist Arthur Holmes, the majority of Earth's mountains were created by the movements of the crust, also known as plate tectonics. Erosion caused by rain, wind and ice cause the changing shapes of mountains. Ongoing volcanic activity can cause mountains to form from cooling rock, and if volcanoes erupt in the ocean, it can cause island formation.
Valleys
Valleys are surface depressions where mountains or hills have created a natural trough in the surface of the earth. They can also form from water or ice erosion and become lakes, rivers or streams. There are two characteristics associated with valleys: low-land and V-shapes. The valleys in the low-land areas have an average slope while the V-shape valleys are formed between mountains and are deeper and more narrow.
Plains
Plains have no major changes in the elevation of the land. They are generally lower than the surrounding land and are found near the coast or farther inland. Coastal plains are leveled until they meet a mountain or plateau formation while inland plains can be found at higher altitudes and stretch for hundreds of miles. Plains include grasslands and deserts. Many cities and rural towns are built on plains because it is easier to build infrastructure on flat land.
Hills
Hills are comparable to mountains but are lower in elevation and have distinct summits. Hills are created by small faults in the earth's crust or by the buildup and pressurized formation of rock debris and sand. Erosion can also cause formation of hills by eroding away the soil of mountains. Hills are common in rural areas and can be seen with the inclining and declining roads that we drive on. Although volcanoes cause mountain formations, the ashes that fall around them as a result can lead to hill formations. When the rain falls, the layers of ash mix with the water and eventually freezes to become rocks.