The Earth's surface is comprised of numerous major and minor plates which are continuously in motion. This movement causes volcanic and seismic activity at the borders of the plates and accounts for the majority of all volcanic activity on the planet. However, there are areas of the Earth's surface where such seismic and volcanic activity occur which are not close to the boundaries of the tectonic plates, and these areas are referred to as hot spots.
The theory or model of plate tectonics developed in the 1960s explained the Earth's surface and accounted for the majority of volcanic areas. A few examples of the outliers of this model include areas such as Iceland, Yellowstone and the Hawaiian Islands. In these areas, volcanic activity is quite prevalent. In fact, the flow of magma in the Hawaiian islands is higher than at any other location on the planet, but they are not located near the boundaries of the plate tectonics. As such, areas of this type area are referred to as hot spots.
While it is easy to categorize an area as a hot spot, it is not as easy to explain exactly why these areas have such seismic and volcanic activity, especially since they are not near the boundaries of the tectonic plates. Unfortunately, what lies beneath these areas in the Earth's crust is not view-able or easily determined. Scientists can only take measurements and record data using various instruments and are not able to get below these areas and see for themselves what is actually occurring and why hot spots have formed; they can only collect and interpret data, looking to explain what is there and what they record.
The best explanation for the formation of hot spots was presented in the 1970s and proposed that they are formed from deep mantle plumes. In this theory, the heat from the core of the Earth, which is much hotter than the heat in the mantle, warms the bottom layer of an area of the mantle, which causes the heated rock to rise. The rising mantle is in the shape of a large mushroom-like head with a narrow tail that continuously supplies the head with warmer material that is less viscous than the material around it. With a constant supply of warmer material from below, the hot spot perpetuates and causes seismic and volcanic activity on or near the surface.
Though the theory has been supported by evidence and is mostly accepted, it unfortunately does not explain every hot spot. There is a lack of evidence at the well known Yellowstone as well as many other hot spots to support the deep mantle plume theory. Since its postulation, the deep mantle plume theory has been expanded upon to account for newer observations of hot spots but still does not explain all hot spots. Less than 10 of the estimated 50 mantle plume hot spots have been determined as being deep mantle plumes, with the remaining plumes not extending all the way from the core. Plumes can also be divided into shallow, intermediate and deep classifications, though the designation to each individual hot spot remains contested.
With general consensus that not all hot spots are the result of deep, intermediate or shallow mantle plumes, the explanation for the remaining hot spot anomalies has actually been attributed to plate tectonics. The first explanation for the anomalies can be explained by the stretching and cracking of the Earth's crust. Individual tectonic plates can be comprised of differing material and not homogeneous throughout. The differing materials can move or are affected differently and thus interact along their borders within the plate. One example is Yellowstone, which lies on a tectonic plate but at the border of two distinct materials within the plate, the Great Basin and an older but thicker crust. Interactions between the two distinct materials cause crack propagation in the crust and the possibility of leakage of warmer material through the cracks.
The second explanation for hot spot anomalies is the presence of older structures or a source of melt near the surface. With continuously moving plates, older areas that were once volcanically active may become cut off but remain a shallow melt. Since the plates are not homogeneous, some materials that comprise plates will melt more easily than others. This explains some hot spot anomalies having excess melt that is unexpected given the composition of other material present in the area. The excess melt can lead to increased lava production at the surface. An example of this is Iceland, where recycled ocean crusts in the upper mantle melt and increase the volume of melted material or magma in the area.