Volcanism is a collective term for surface geological processes that result from the rise of hot material from the interior of a planet. The rising material can consist of magma (molten rock), but also of volatile substances such as gases or liquids. When such material reaches the surface it is called an eruption or eruption. When magma flows out over the surface, it is called lava. However, an eruption can be accompanied by explosions, which also throw debris and ash (called tephra or pyroclastic material) or blow into the atmosphere. A volcano can form at the site of the eruption, when the eruption is explosive in nature, a crater is formed.
When a lava flow solidifies, it becomes an igneous rock. However, solidified lava is not the only product of volcanism. A volcanic eruption can also cause ash and debris to fall from the sky, which can form thick layers in a relatively short time. This material is not very stable, so landslides or mudslides (lahars) often occur after an eruption.
On Earth, volcanism mainly occurs along the edges of tectonic plates. The nature of volcanism is determined by a combination of magma development as it rises through the crust and surface conditions.
Traces of volcanism
In an area where volcanism occurs, many landforms may have been created by volcanic activity. The best known are volcanoes, the places where lava or pyroclastic material is released during eruptions. Usually there is a place under the volcano where magma collects, a so-called magma chamber. The magma can move to the surface through a passage, such as a volcanic pipe or a crack (dike).
Usually when you think of a volcano, you think of a mountain with the shape of a cone. This type of volcano is called a stratovolcano. Stratovolcanoes have steep slopes and can have great heights. They are often the highest points in the area, such as Vesuvius (1281 m) in Italy or Merapi (2968 m) on the Indonesian island of Java. The slopes of a stratovolcano consist of pyroclastic material such as ash or rubble, interspersed with solidified lava flows. Shield volcanoes are also large mountains, but have the shape of a dome with gentle slopes. Examples are the Mauna Loa (top 4170 m above sea level) and Mauna Kea (4205 m above sea level) in Hawaii. The tops of these volcanoes are 4170 and 4205 m above sea level, but measured from the bottom of the ocean they are more than 10 km high. The volume and surface area of these shield volcanoes is much larger than that of the largest stratovolcanoes. Many shield volcanoes, like stratovolcanoes, have a crater on top of the volcano. At shield volcanoes, the lava that flows out can often collect in a lava lake in the crater. Shield volcanoes are largely made up of solidified lava flows and contain hardly any pyroclastic material.
Not all volcanoes are mountains. Some even form a depression in the landscape. An example are maars, deep craters of several kilometers or hundreds of meters in diameter, sometimes containing a small lake. A maar is often surrounded by a ring of ash and rubble.
A caldera is a large volcanic crater several kilometers in diameter. Calderas can contain more than one volcano. The largest active caldera in the world is Yellowstone in the central United States. This caldera is about 55 km wide. Not much of a caldera is visible on the surface in Yellowstone: because of its sheer scale, the caldera is only noticeable in aerial photos.
Volcanic cones are relatively small (at most a few hundred meters high) cone-shaped mounds near a volcano. They have steep flanks and always consist largely of pyroclastic material. This type of cones is dependent on the type of m