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Properties of Magma
The majority of the interior of the earth is composed of molten rock and gasses. When beneath the surface, this substance is known as magma. Lava produced by a volcanic eruption is the external surface equivalent to magma. The crust of the earth is a solid. When parts of the crust are melted into magma due to the extreme heat of the earth's interior, the density of the substance is reduced, increasing its volume. This increased volume causes the magma to be forced upward, where it erupts from a volcanic opening.
Plate Tectonics
The solid crust of the earth is composed of numerous rock plates that constantly shift and collide beneath the surface. The process of these plates shifting and colliding is known as plate tectonics. At a ridge where two plates meet, substantial amounts of friction and pressure are produced beneath the surface. This forces magma between the colliding plates, where it is ejected onto the surface through a volcano. Generally, volcanoes only occur along one of these plate ridges. This is the reason for the tendency of volcanoes to form in line along one of these ridges, such as with the Pacific Ring of Fire, which is a circle of increased volcanic activity along the perimeter of the Pacific Ocean.
Earthquakes
The shifting and colliding of plates in the process of plate tectonics is what causes earthquakes. Due to the mass and friction of two colliding plates, the crust of the earth in proximity to the colliding plates will vibrate, which is known as an earthquake. For this reason, earthquakes can help predict or pinpoint the location of future volcanic eruptions. Since similar plate tectonic conditions create the two phenomenon, the likelihood for volcanic eruptions increases in the area and time frame of an earthquake.
Eruption Reporting
It is a common misconception that the number of volcanic eruptions has steadily increased over time. Indeed the number of recorded eruptions has risen over the last century. However, this is simply due to increased recording capabilities and advancing human communications. Many volcanic eruptions are quiet eruptions, meaning they do not produce a violent explosion or ejection of material. Rather, quiet eruptions produce a steady, gentle flow of lava from the interior of the earth. Such quiet eruptions likely went unreported for much of human history. However, now that scientists extensively monitor plate tectonics and other activity in the earth's interior, many more eruptions are observed and reported.