A general term for the discrete downslope movement of rock and soil masses under gravitational influence along a failure zone. The term "landslide" can refer to the resulting land form, as well as to the process of movement. Many types of landslides occur, and they are classified by several schemes, according to a variety of criteria. Landslides are categorized most commonly on basis of geometric form, but also by size, shape, rate of movement, and water content or fluidity. Translational, or planar, failures, such as debris avalanches and earth flows, slide along a fairly straight failure surface which runs approximately parallel to the ground surface. Rotational failures, such as rotational slumps, slide along a spoon shaped failure surface, leaving a hummocky appearance on the landscape. Rotational slumps commonly transform into earthflows as they continue down slope. Landslides are usually triggered by heavy rain or melting snow, but major earthquakes can also cause landslides.