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There are estimated to be more than 1,500 active or dormant volcanoes on land, while an unknown – but even larger – number of submarine volcanoes also exists. Some 600 of these have erupted in historical times, while about 50–70 volcanoes erupt each year. This makes Earth one of the most volcanically active places in the Solar System.

The vast majority of these volcanoes are located on islands or near to coastal regions (Figure 3.20). They are also generally located close to active plate boundaries, either along mid‐ocean spreading ridges or alongside subduction zones at ocean trenches (see Plate Tectonics).

The main exceptions are chains of volcanic islands which occur above hotspots in the mantle, far from any plate boundary. The most famous example of hotspot volcanism is the island chain of Hawaii. Each of the islands has been created, one by one, as the Pacific Plate passed over a stationary plume of hot magma that has been in the same place for about 80 million years (Figure 3.26). The individual volcanoes erupt for a few million years before the movement of the plate carries them away from the rising plume.

This scenario is confirmed by the ages of the islands, which become progressively older with distance from today's center of volcanic activity on the main island of Hawaii. A bend in the seamount chain was probably caused by a shift in the direction of movement of the Pacific Plate, from a northward to a more northwesterly direction, about 47 million years ago.

Mauna Loa, which makes up half the area of the island of Hawaii, is the largest volcano on Earth. However, only a small part of the huge shield volcano is visible above sea level. With an estimated volume of about 75,000 cubic km, and a base that is more than 145 km in diameter, Mauna Loa is so massive that it deforms the oceanic plate on which it sits.

Although its overall height from base to summit is about 17 km, the average gradient of its slopes is quite gentle, a mere 4–5°. Since it began to form nearly one million years ago, the enormous structure has grown from accumulated layers of fluid, basaltic lava. At its summit is a large caldera, a crater‐shaped basin created when subterranean magma drained away and caused massive subsidence.

The Hawaiian volcanoes are characterized by frequent eruptions of very hot, non‐viscous, basaltic lava which sometimes produces fountains of liquid rock and small spatter cones. Since any gas the lava contains can easily escape, explosions are rare.

Figure 3.23 Oceanic plates grow at mid‐ocean ridges, where magma rises from the mantle and creates new crust. Oceanic crust is destroyed at subduction zones, where a dense plate dives beneath a less dense plate and is slowly destroyed. Water from the melting plate helps create magma that rises to the surface and feeds nearby volcanic islands. The erratic descent of the subducted slab also sets off deep, sometimes catastrophic, earthquakes. Some volcanoes also form above hot spots in the mantle.

(USGS)

Figure 3.24 A map of ocean floor age shows the youngest areas (red) along the mid‐ocean ridges, where new rock is forming from rising magma. The oldest oceanic crust is furthest from the ridge, often along the continental margins (blue). Oceanic crust is destroyed at ocean trenches. Nowhere is it more than about 180 million years old. This contrasts with the much older continents, parts of which date back at least 3,500 million years.

(NOAA)

Figure 3.25 A series of maps showing the gradual break‐up of the Pangaea supercontinent over the last 225 million years. Note how South America and Africa drifted apart with the opening up of the Atlantic Ocean, followed by Europe and North America. India moved north to collide with Eurasia, creating the Himalayan mountains and the Tibetan plateau, but Australia remained attached to Antarctica until quite recently.

(USGS)

In other parts of the world, eruptions of shield volcanoes are associated with basaltic lava pouring quietly from long fissures, instead of central vents. The fluid lava repeatedly floods the surrounding land, creating multiple layers which build into broad plateaus. Lava plateaus of this type can be seen in Iceland, the northwestern USA, and the Deccan of India.

Eruptions are more violent where the lava is less fluid and gases can escape less easily. Pressure may build inside the volcano until the gases escape in a massive explosion, destroying part or all of the summit.

In the case of volcanoes such as Vesuvius in Italy and Mt. St. Helens in the USA, explosive activity is quite common, resulting in huge clouds of steam, gas, solid fragments, and ash which can rise several kilometers into the air and blanket the surrounding terrain. Glowing clouds of red‐hot rock fragments, known as nuées ardentes, may flow downhill, incinerating everything in their path.

The combination of alternating layers of ash and lava results in composite volcanoes or stratovolcanoes. The classical shape of such a volcano is a cone, like Mt. Fuji in Japan, but the symmetry is often spoiled by smaller cones and lava flows on the flanks.

Figure 3.26 The Hawaiian Islands and Emperor Seamount Chain formed as the Pacific Plate drifted over a hot spot in the mantle. Today, only the Big Island of Hawaii – the youngest island – has active volcanoes. Mauna Loa, which makes up half of the Big Island, is the largest shield volcano on Earth. The age of each volcanic sea mount increases with distance from Hawaii. The bend in the chain may have been caused by a change in the Pacific Plate's direction of movement.

(NGDC/USGS)

Many high volcanoes have summits covered with ice and snow, and heavy rainfall often accompanies large ash clouds. The plentiful supply of water may turn the ash deposits into mud flows (also known as lahars) which bury the surrounding landscape.

In extreme cases, the products of an explosive eruption may travel around the planet and even modify the climate (Figure 3.27). The most notable example of modern times was the 1991 eruption of Mt. Pinatubo in the Philippines, which ejected vast amounts of ash and gas as high as the stratosphere.

Pinatubo also injected about 15 million tonnes of sulfur dioxide into the stratosphere, where it reacted with water to form a hazy layer of aerosol particles composed primarily of sulfuric acid droplets. Over the course of the next two years, strong winds in the stratosphere spread these particles around the globe. The result was a drop of about 0.6°C in Earth's average temperature for a period of almost two years.