On Earth, 500-600 Volcanoes exist. A volcano is a relief of the terrestrial crust (submarine or air) formed by the ejection and stacking material from the rise of magma in the form of lava ash.

Volcanic activity can take several forms; each of them depends on the quantity of gases present in the magma and on the percentage of silica. The magma composition is therefore the key. The gases for example facilitate the explosive activity, whereas the silica makes the magma very viscous and therefore less mobile.

The magma derives from partial melting of the mantle. It is through chimneys or ducts that molten rock reaches the surface and the accumulation of these rocks can reach thousands of feet thick forming the mountains and even islands.

A second major site of active volcanism is along the axis of the oceanic ridge system. Here it is present another kind of volcanism, which is the one generated by the hot spots. This corresponds to deep climb of magma, which pierces the lithosphere. The hot spot can be either on top of ocean ridges or next to them and usually are responsible for the generation of a series of islands.

Here we will have a collage of images taken with Sentinel-2. In some of these images, it is worthwhile to note the presence of the caldera. A caldera is a circular depression or nearly at the top as large as the volcano. Calderas form as a result of a cataclysmic explosion of the volcano, where a giant magma chamber is emptied in a few hours forming a large depression, whose width depends on the size of the magma chamber. 
The eruptions cause rapid evacuation of the magma chamber and collapse of the summit overlooking the volcano.

Tenerife Island

A chain of seven volcanic islands composes Canary Islands. Tenerife is the largest island, which is surrounded by Mount Teide. Pico de la Tiede is a volcano with a crater of 70 m diameter. Eruptions in history: 1430, 1492, 1704-1705 (Siete Fuentes, Fasnia, Guimar volcano), 1706 (Montana Negra), 1798 (Pico Vejo or Chahorra), 1909 (Chinyero).

Taal

Taal is only 50 kilometers south of Manila on the island of Luzon in the Philippines. The green dark area in the center of the image is Taal Lake, which nearly fills the 30-kilometer caldera. The caldera rim consists of deeply eroded hills and cliffs. Eruptions in history: 1572, 1591, 1608, 1634, 1635, 1641, 1645; 9 during XVIII cen., 6 during XIX cen., 13 between 1903 and 1994.

Aeolian Inlands

The Aeolian Islands are a volcanic group off the northeast coast of Sicily. The group therefore forms an island back arc with the two southernmost, Vulcano and Lipari, followed by Salina, Filicudi, and Alicudi.  The isolated island on the right corner of the image is Panarea and further north Stromboli is located. The volcanic activity of this area is due to the subduction of the African plate under the European one. It is still active in Lipari, Vulcano and  Stromboli, whereas for the remaining islands the volcanic activity ended between 5000 and 20000 years ago.

Sakurajima

Just few kilometers across Kagoshima Bay from the city of Kagoshima lays Sakurajima, one of Japan’s most active volcanoes. The volcano is made of a new volcanic cone inside a very wide Aira caldera forming the northern half of Kagoshima Bay. Once an inland, Sakurajima joined to the Osumi Peninsula during the major eruption of 1914. Eruptions in history: 708, 716-718, 764, 766, 778, 3 during XVI cen., 15 during XVIII cen., 2 during XIX cen., 10 from 1914 to 1950, continuous explosive activity of the southerner crater since 1955.

Vesuvio and Campi Flegrei

Vesuvio is a volcanic cone of 1281 m height and a crater about 500 m in diameter, which is growth within the caldera of the older edifice of Mt. Somma. Vesuvio is well known for its eruption in AD 79, which destroyed the roman cities of Pompei and Ercolano. Eruptions in history: 79AD (Pompei), 1631-1632, 1637, 1652, 1654-1689, 1695, 1696-1698, 1701, 1704, 1706-1707, 1712-1724, 1730, 1764-1767, 1835-1839, 1841-1850, 1855-1861, 1875-1905, 1913-1944.

Next to Vesuvio, there is a presence of the Campi Flegrei. Campli Flegrei is a complex volcanic area characterized by a peculiar landscape including several volcanic land forms. Eruptions in history: 1198, 1538.

Fernandina

Fernandina island is the most pristine of the Galapagos Islands and it is most famous for its continuing series of volcanic eruptions. La Cumbre Volcano is a shield volcano similar to those found in the Hawaiian Islands. Its summit caldera is approximately 6.5 km wide. Eruptions in history: 1813-1814, 1817, 1819, 1825, 1846, 1888, 1926-1927, 1937, 1958, 1961, 1968, 1972-1973, 1977-1978, 1981,1984, 1988, 1991, 1995, 2005, 2009.

Fogo

Fogo, discovered most likely in 1456, is a circular island with a diameter of 24 km, composed of a deep-sea shield volcano. The surfaced part has a height of 2829 m and it is asymmetrical: the mountaintop is located easterly. Inside the caldera, it is possible to observe numerous old cinder cones, which are aligned on radial fractures. Eruptions in history: 1500, 1769, 1785, 1799, 1816, 1847, 1852, 1857, 1909, 1951, 1995.

Augustine

The Augustine volcano is placed in Alaska and forms an inland in the Cook Inlet. The island is part of a long active volcano axis. The summit ridge is characterized by cathedrals of lava, and  it shows a big breakthrough in the northern side, through which the volcanic flows fall. Eruptions in history: 1812, 1883, 1935, 1936-1964, 1971, 1976-1977, 1986.