The Earth beneath our feet may seem solid and immovable, but it is not. Earthquakes shake and reshape the Earth’s crust, triggering tsunamis, landslides and titanic shifts in tectonic plates that set off new volcanic activity, even creating new lands of basalt (black, hard volcanic rock).
On the Earth today, there are more than 1,500 active volcanoes, excluding those on mid-ocean ridges, and about 500 have erupted since historic times. They occur mainly at the margins of tectonic plates, where upswelling molten rock is brought up to the Earth’s mantle. The largest seismically active zone in the world is the ‘Pacific Ring of Fire’, a chain of active volcanoes around the edge of the Pacific Ocean, where all but three of the 25 most powerful volcanic eruptions in the past 10,000 years have occurred.
In the Russian Far East, the volcanoes of the Kamchatka are part of the northeastern sector of the Ring of Fire. They have formed because the western edge of the Pacific plate is plunging below the eastern margin of the Eurasian plate. The unstable zone above gives rise to 109 active volcanoes, the highest concentration of erupting volcanoes in the world. Plosky Tolbachik volcano is one, and its neighbor Ostry Tolbachik is another. Plosky’s most notable eruption in recent times was in 1975. A swarm of earthquakes preceded an eruption that produced the most lava of any Kamchatka volcanos.
Not all volcanos are at plate margins. The volcanoes of Hawaii are located not at the edge of a tectonic plate, but in the middle. They sit over a hot spot, an upswelling of molten rock from the mantle, in the Earth’s crust. As the plates move imperfectibly over the hot spot, volcanoes push up from the seabed to form volcano islands; in fact, they are the highest mountains in the world. Measured from base to summit, Mauna Kea in Hawaii’s Big Island is over a kilometer taller than Mt. Everest, although much of it underwater. Its neighbor Kilauea is currently over the hot spot, and is the most active of Hawaii’s volcanoes. As the tectonic plates move eastwards, however, the islands move away from the hot spot. Eruptions slow and then cease altogether, and the ocean gradually erodes the islands away.
Hawaii’s volcanoes are shield volcanoes. They produce low-viscosity, fast-flowing basaltic lava that spreads a great distance from the vent, forming a broad volcano with gently sloping sides. Iceland’s volcanoes are basaltic too, but they sit astride a hot spot on the Mid-Atlantic Ridge. Evidence of ancient eruptions can be seen in the island’s rocks. The Svartifoss waterfall for example, tumbles over dark, hexagonal columns of solidified basaltic lava. The same rock formations can be seen at the Giant’s Causeway in Northern Ireland.
Some of the most hauntingly beautiful volcanoes are stratovolcanoes. The symmetrical cone shape of this type of volcano has inspired artists and poets alike, but their splendor belies a sinister history. They tend to go off with a bang, producing thick, viscous lava that does not flow easily and gases cannot escape. The gas pressure builds inside the volcano until, and usually with little warning, the volcano erupts with a violent explosion that releases enormous amount of magma, ash and hot gases. Pico do Fogo in the Cape Verde Islands is a stratovolcano. The active in the eastern Atlantic erupted in 2014, accompanied by strong earthquakes and lava that flowed for about 1 km. It is believed to be one of the most powerful eruptions the island has ever experienced.
While stratovolcanoes and shield volcanoes account for most major eruptions, there are minor events, such as fissure eruptions, that can have a big impact on our lives. These occur when magma finds a long fissure in rocks and erupts at the surface in fountains of lava that form “curtains of fire.” Icelandic volcanoes, pushing up from the Mid-Atlantic Ridge, tend to erupt in this manner.
One of the world’s most dramatic of volcanic features is the “gateway to hell” – a lava lake. It forms when so much magma is pushed up through the vent so fast that it pools in the crater and remains molten, constantly circulating like a gigantic witch’s cauldron. The lakes appeared in the crater of Hawaii’s Kilauea, where lava fountains burst from the surface, but the most spectacular one in recent years has been in the crater of Mount Nyiragongo, an active stratovolcano in the Democratic Republic of Congo. This lava lake is about 600 m deep, making it the largest in recent times. Similar lava flows from volcanoes elsewhere generally move at speeds of 6 mph and a person could easily outrun them, but Nyiragongo’s lava travels at 60 mph! The volcano has been dubbed the most dangerous volcano on Earth.
On an even larger scale is the caldera. If the magma chamber beneath a volcano erupts, the chamber roof can collapse, forming a large bowl-shaped hollow with very steep walls. This is the caldera, and it can be many kilometers across. One of the most closely watched calderas contains Yellowstone National Park. This area of intense geothermal activity with an extraordinary number of geyser, mudpots and hot springs is a vast caldera complex, the leftovers of a super-volcano that has erupted in at least three known caldera-forming eruptions at least 600,000 years ago.
A geothermal area a little different to that of Yellowstone can be found in the Danakil Depression of Ethiopia. Here, the source of underground heat is not a dormant super-volcano or a simple hotspot, but a place on the Earth’s surface where the crust is splitting, a triple junction where the edges of three tectonic plates meet.
