Volcanic activity is characterized by release of magma and hot gases from the subsurface through cracks or openings in the earth's crust. This release can be a violent eruption (of viscous magma) or slow outpouring (of non-viscous magma). The heat from within earth causes rocks in the upper mantle (one of the earth's layers) or lower crust (earth's topmost layer) to melt, and they tend to rise up towards the surface. Volcanic activity is a result of plate tectonics, i.e., movement of tectonic plates (parts of earth's crust) relative to each other. This motion causes earthquakes, and they create cracks through which magma flows out. Earthquakes accompanying the volcanic activity are called volcanic tremors and are used for detection of pending volcanic activity. Volcanoes are also caused by plate movement over hotspots; Hawaiian volcanoes are an example of this.