There are two basic types: explosive and non-explosive. The first appears at points where tectonic plates collide (the large blocks that form the earth’s crust) – its best example is in the volcanoes that draw the so-called Belt of Fire, around the Pacific Ocean. This type is also characterized by almost solid lava, in addition to expelling dust and a mixture of gases and water vapour. The lava from this volcano comes from the depths of the Earth, where the high temperature melts the rock of the oceanic crust and causes it to mix with sea water. “The presence of water is what makes this volcano so explosive. As the lava rises, water vapor is released from the rock and hits a cap formed by the hardened material from the previous explosion, building up pressure until it explodes. It can be compared to a bottle of fizzy drink,” says Caetano Juliani, a geologist at the University of São Paulo (USP).
Non-explosive volcanoes, like the ones in Hawaii, are right in the middle of a tectonic plate, away from the collision between them. This type arises when there is a crack in the earth’s crust through which lava can flow.
This lava is more liquid and incandescent. “It’s as if the volcano were located under a blowtorch, which makes the material in the mantle melt, the magma rising and crossing the crust”, says Caetano. There is yet another type of non-explosive volcano, which can appear at the bottom of the sea, at great depths. Volcanoes have unpleasant surprises in store for humans. Perhaps the most famous episode is that of the city of Pompeii, Italy. It was wiped off the map by Vesuvius and its 2000 inhabitants died buried by an 8 meter layer of ash in the year 79. The worst catastrophes, however, are more recent. In 1815, in Indonesia, Tambora claimed the lives of 92,000 victims. In 1985, in Colombia, Nevado Del Ruiz killed 23,000 people. Although it doesn’t have volcanoes today, Brazil once had its mountains of fire. Our oldest volcano ever discovered was spewing lava in the Amazon 1.9 million years ago.
Long before that, around 150 million years ago, there was a large fissure in South America that ran from the state of Mato Grosso to Argentina – in the region where the Paraná River flows today. From this huge crack, a quantity of lava flowed that accumulated from the city of Santos, SP, to the Andes mountain range, in the greatest volcanic activity on the planet at the time. At that time, Africa and South America were separating and, if the fissure had grown, part of the South American territory would have ended up on the other side of the Atlantic Ocean.
A – Even underwater
The non-explosive volcanoes that exist at the bottom of the oceans, although little known, are very common. They arise because of the movement of tectonic plates, which open huge fissures in the earth’s crust.
B – Mountains of lava
The fissures open the way for the advance of the lava, formed by rocks melted by the high temperatures of the interior of the planet. As the plates move, with speeds of up to 10 centimeters per year, this expelled lava solidifies and forms huge chains of volcanoes, about 2.5 kilometers below the surface of the sea
There are about 1500 active volcanoes (the red triangles on the map). Most of them were formed when tectonic plates (marked by the blue lines) meet – mainly those that surround the Pacific Ocean. Therefore, the region is called the Belt of Fire. Indonesia alone has 127 active volcanoes.
1 – Underground bump
The process of forming an explosive volcano begins when an oceanic tectonic plate collides with another plate and is forced to plunge towards the Earth’s interior.
2 – Maximum temperature
As depth increases, the temperature rises (due to the planet’s internal heat) until the rock melts. Molten rock will mix with water and form bubbles underground.
3 – Forced passage
If such bubbles are large enough, they can force their way up through the Earth’s crust. As they rise, they cool and release water vapor and other gases that were trapped in the rock.
4 – Pressure cooker
As the volcano is capped by the accumulation of solidified lava from previous explosions, the gases build up. When the pressure becomes strong enough, the volcano’s lid breaks and it explodes.
5 – Debris from the explosion
This type of volcano throws hot rock debris and dust instead of liquid lava. It is the accumulation of this material that, little by little, forms the mountain of the volcano and its crater.