Volcanoes why they occur

However, when one plate pushes underneath the other, this forces molten rock, sediment and seawater down into the magma chamber. The rock and sediment are melted into fresh magma, and eventually overfill the chamber until it erupts, releasing sticky and thick andesitic lava, at temperatures from to degrees Celsius.

Decreasing temperatures can cause old magma to crystalise and sink to the bottom of the chamber, forcing fresh liquefied magma up and out — similar to what happens when a brick is dropped in a bucket of water. A decrease in external pressure on the magma chamber may also allow for an eruption by minimising its ability to hold back increasing pressures from the inside. This is often caused by natural events, such as typhoons, that decrease rock density, or by glacial melting on top of the chamber lid, which alters molten rock composition.

The volcanoes found in the Hawaiian islands are of this sort. Ultimately, the size of an eruption will depend on the thickness of the magma, the density of gases it contains and the amount of new magma being pushed into the magma chamber. Basaltic lava allows gas to escape easily, resulting in smaller eruptions, while andesitic and rhyolotic lava makes it harder for the gas to escape, leading to larger eruptions.

Lava is often thought to be the main danger of a volcanic eruption, but this is not the case. Numerous hazards result from eruptions and they can have a range of consequences. The most dangerous are the pyroclastic clouds, which destroy anything in their path. Other hazards include ash clouds, ash rain, mudslides, earthquakes, tsunamis, odd weather patterns and glacial flooding. Originally published by Cosmos as Why do volcanic eruptions occur? Cosmos is published by The Royal Institution of Australia, a charity dedicated to connecting people with the world of science.

Financial contributions, however big or small, help us provide access to trusted science information at a time when the world needs it most. Please support us by making a donation or purchasing a subscription today. The magma rises up to fill in the space. When this happens underwater volcanoes can form. Magma also rises when these tectonic plates move toward each other. When this happens, part of Earth's crust can be forced deep into its interior.

The high heat and pressure cause the crust to melt and rise as magma. A final way that magma rises is over hot spots. Hot spots are exactly what they sound like--hot areas inside of Earth. These areas heat up magma.

The magma becomes less dense. When it is less dense it rises. Each of the reasons for rising magma are a bit different, but each can form volcanoes. There have been a lot of volcanoes on other planets in the past. Some places in our solar system have active volcanoes erupting right now! Venus and Mars are covered with extinct volcanoes. Although there are several factors triggering a volcanic eruption, three predominate: the buoyancy of the magma, the pressure from the exsolved gases in the magma and the injection of a new batch of magma into an already filled magma chamber.

What follows is a brief description of these processes. As rock inside the earth melts, its mass remains the same while its volume increases--producing a melt that is less dense than the surrounding rock. This lighter magma then rises toward the surface by virtue of its buoyancy. If the density of the magma between the zone of its generation and the surface is less than that of the surrounding and overlying rocks, the magma reaches the surface and erupts.

Magmas of so-called andesitic and rhyolitic compositions also contain dissolved volatiles such as water, sulfur dioxide and carbon dioxide. Experiments have shown that the amount of a dissolved gas in magma its solubility at atmospheric pressure is zero, but rises with increasing pressure.

For example, in an andesitic magma saturated with water and six kilometers below the surface, about 5 percent of its weight is dissolved water. As this magma moves toward the surface, the solubility of the water in the magma decreases, and so the excess water separates from the magma in the form of bubbles.

When the volume of bubbles reaches about 75 percent, the magma disintegrates to pyroclasts partially molten and solid fragments and erupts explosively. The third process that causes volcanic eruptions is an injection of new magma into a chamber that is already filled with magma of similar or different composition. This injection forces some of the magma in the chamber to move up in the conduit and erupt at the surface. Although volcanologists are well aware of these three processes, they cannot yet predict a volcanic eruption.