The types of volcanoes: All you need to know about the two major groups and more

Aside from the continuous bands of volcanoes on the ocean floor at spreading regions like the Mid-Atlantic Ridge, there are around 1,350 potentially active volcanoes on Earth. In fact, between 50 and 70 volcanoes erupt every year.

Still, each volcano is somewhat unique. From how volcanoes are formed and classified, to the most explosive ones on Earth, IE summarizes all you need to know about these attention-grabbing geological structures. We'll of course provide a few (very cool) images along the way.

What is a volcano?

Simply put, a volcano is a rupture in a planet's crust, such as Earth's, that permits hot lava, volcanic ash, and gases to escape from a magma chamber underground. Over many eruptions, the layers of these materials can build up, forming a volcano.

How are volcanoes formed?

Volcanoes can originate in various ways, but they always do so when there is magma, or partially molten rock, below the Earth's surface. Then this magma either rises through the crust's natural cracks or becomes trapped underground, where the pressure builds until it explodes violently.

In Earth's case, volcanoes occur because our planet's lithosphere is broken up into slabs of solid rock known as tectonic plates. These float on a hotter, softer, partially molten layer known as the asthenosphere.

We must first comprehend the two types of tectonic plate scenarios to understand how 'some' volcanoes form.

Convergent plate boundaries

Convergent plate boundaries are areas where lithospheric plates are moving toward one another. When these plates collide, the thinner and denser plate is overridden by the thicker and less dense plate. The subducting plate is heated as it is forced deeper into the mantle, until it eventually begins to melt. This produces magma chambers. Because the magma is lower in density than the surrounding rock material, it melts and fractures its way through the overlying rock material to the surface.

Divergent plate boundaries

When tectonic plates move away from one another, they are known as divergent plate boundaries. A crack forms as they separate, releasing pressure. This drop in pressure decreases the mantle rocks' melting temperature. Then this newly molten magma explodes at the surface. This occurs most frequently under the sea.

What are the different types of volcanoes?

There are two broad types of volcanoes, even though numerous structures can develop from erupted magma (such as calderas or lava domes). One is a stratovolcano (also known as a composite volcano), and the other is a shield volcano.

The difference between the two volcanoes stems from the viscosity (thickness), amount of gas, and composition of the magma. How the magma reaches the surface is also distinguishable.

What is a stratovolcano?

Strongly viscous (extremely thick) magma eruptions typically result in volcanoes with steep sides and slopes of 30 to 35 degrees. This is because the dense volcanic material doesn't flow very far from the point of eruption; instead, it piles up in layers to form a cone-shaped volcano called a stratovolcano.

In contrast to shield volcanoes, stratovolcanoes are the more frequent kind of volcano. Krakatoa in Indonesia, infamous for its destructive eruption in 1883, and Vesuvius in Italy, whose explosion in AD 79 devastated the Roman cities of Pompeii and Herculaneum, are two well-known examples of stratovolcanoes. The deaths from both ancient eruptions totaled in the thousands.

Both Mount St. Helens in Washington State, USA, and Mount Pinatubo in the Philippines serve as modern examples of major stratovolcanic eruptions. These have seen catastrophic eruptions in recent years, though with fewer fatalities.

What is a shield volcano?

In contrast, shield volcanoes have mild slopes of less than 10 degrees and produce more basalt fluid lavas during an eruption. A shield volcano's eruption can cause the basalt to flow far from the vent, creating broad, gentle slopes.

Tamu Massif in the pacific ocean and Mauna Loa in Hawaii are classified as shield volcanoes. These kinds of volcanoes have been found on other planets too, such as Sapas Mons on Venus and Olympus Mons on Mars.

What are the different types of volcanic eruptions?

Volcanoes containing a lot of gas and silica are explosive. They are extremely dangerous because they release ash (tiny rock pieces) and pyroclastic density currents (hot clouds of noxious gas and boulders that travel down the volcano's flanks). They also release cement-like mixtures of water and hot ash known as lahars.

Composite volcanoes, formed from repeated layers of ash and lava laid down over several eruptions, result from explosive eruptions. These eruptions happen at convergent plate borders.

Effusive eruptions typically form shield volcanoes. This is when magma has a low silica content and hence a low viscosity. Major eruptions of this kind can affect climate due to gas emissions but are less explosive and so tend to pose fewer hazards than stratovolcanoes.

Additionally, they do not emit much ash as they are composed primarily of lava. These eruptions occur in the middle of tectonic plates (hot spots) and at diverging plate boundaries.

What are calderas and lava domes?

These volcanic structures fall outside the two main categories of volcanoes.

Sometimes the roof of a magma chamber may collapse during a massive, explosive eruption. This violently empties the chamber, creating a depression or bowl with extremely steep sides on the surface. These are calderas, some of which can span tens of miles.

Calderas can also develop when an eruption destroys a single stratovolcano's summit (the tip). Literally, the top of a stratovolcano is blown off!

Lava domes are formed when extruded lava cannot easily flow away from the vent because its high viscosity means it's not very fluid. Instead, the lava builds up into a giant mass, resembling a dome, on top of the vent.

The Soufriere stratovolcano in St Vincent has a well-known lava dome growing on the SW edge of its main crater. Additionally, the central lava dome of Mount Myoko in Japan serves as an example.

Where are the most active volcanoes?

Seventy-five percent of Earth's active volcanoes—more than 450—are located along the "Ring of Fire," a band encircling the Pacific Ocean. This is not quite a ring, as the name suggests, but rather a horseshoe-shaped belt about 40,000 kilometers long and up to about 500 kilometers wide.

Some volcanoes occur in the interior of plates in areas called "hot spots." Hotspots are volcanic regions where it is believed that the underlying mantle is abnormally hot in comparison to the surrounding mantle. Volcanoes in these zones are independent of tectonic plate boundaries and with examples being the Hawaii, Iceland, and Yellowstone hotspots.

What was the largest volcanic eruption in recorded history?

The 1815 eruption of Mount Tambora, in Indonesia, was one of the largest in recorded history. One of the main products of the eruption was sulphur dioxide (SO₂) gas, which acts as a short-term global coolant. So many millions of tons of SO₂ were released during the eruption that average global temperatures fell by around 3 degrees Celcius (°C). Because of this, 1816 became known as the "Year without Summer."

What are the largest volcanoes on Earth?

Picking the world's largest volcanoes is more complex. So, this is a summary of the world's tallest (from base to summit), highest (elevation of summit above sea level), and most massive (by volume).

Tallest volcano (base to summit): Mauna Kea, Hawaii

Mauna Kea is a dormant shield volcano located in Hawaii. Volcanoes that have been dormant are those which have not erupted in a long time but are expected to erupt again in the future. While the volcano's summit is 13,796 feet (4205 meters) above sea level, its base is around 19,685 feet (6000 meters) below sea level. This makes Mauna Kea around 33,000 feet tall - the tallest on Earth (Everest stands at 29,035 feet).

Highest volcano (elevation of summit above sea level): Ojos del Salado, Argentina-Chile border

In the Andes Mountains, on the boundary between Chile and Argentina, lies a stratovolcano known as Ojos del Salado, which is considered active. There are two summits on the volcano; the higher summit is in Chile, with an elevation of 22,615 feet (6893 meters) above sea level. This makes Ojos del Salado the highest volcano on Earth.

Most massive volcano by volume: Mauna Loa, Hawaii

The largest active volcano by mass on our planet is the shield volcano Mauna Loa in Hawaii, which gradually rises to a height of more than 4 km (2.5 miles). Its undersea flanks drop another 5 km (3 mi) to the ocean floor, and its colossal mass depresses the ocean floor by an additional 8 km (5 mi). As a result, the volcano's summit (top) is located roughly 17 km (10.5 mi) above its base.

The Tamu Massif volcano is the only volcano larger than Mauna Loa - however, this is extinct. It is submerged underwater in the northwest Pacific Ocean, sitting atop a triple junction of mid-ocean ridges. Tamu Massif has the largest footprint of any volcano, covering an area roughly equivalent to New Mexico at around 120,000 square miles (310,800 square kilometers). It is also heavier than any other single volcano on Earth that is currently known.

Still, there has been some debate as to whether Tamu Massif is a single volcano at all. In 2019, the same scientist who discovered and classified the structure as a single volcano (2013) disclosed that Tamu Massif might be part of the mid-ocean ridge system instead.

This turnaround came after a study team from the US, China, and Japan examined magnetic field data over the Tamu Massif. They discovered that magnetic anomalies, which are changes in the Earth's magnetic field brought on by magnetic rocks in the crust, were similar to those created at mid-ocean ridge plate borders.