The Earth's tectonic plates and volcanic activity

Earth is a restless planet. The most easily observed evidence of the fact is revealed in the continuous movement of air and water. It has to be kept in mind, however, that the ground itself is also moving, though much more slowly. The hard crust of Earth's surface layer, from about 100 kilometres (kms) thick, is composed of a set of large and small plates. Together these plates constitute the lithosphere: Lithos - 'rocks'.

The lithosphere rests on and slides over an underlying, weaker layer of partially molten rock known as the Asthenosphere : Astheno - 'weak'. Plate movement is possible because the lithosphere - asthenosphere boundary is a zone of detachment. As the lithosphere plates move across the Earth's surface, they interact along their boundaries, diverging, converging or slipping past each other. While the interior of the plates remain essentially undeformed, plate boundaries are sites of many of the principal processes that shape the terrestrial surface, including earthquakes, volcanism and orogeny - formation of mountain ranges. The outermost layer of lithosphere is called crust, having two types - continental and oceanic. The continental crust is granitic in composition, 40 kms thick, whereas oceanic crust is basaltic in composition and is about 7 kms in thickness. These crustal rocks both sit on the mantle - part of the Earth between core and crust, about 2900 kms thick and made-up of magma: molten rock and rock.

The lithospheric plates are much thicker than oceanic or continental crust. Most volcanoes form at the boundaries of Earth's tectonic plates. These plates are huge slabs of the Earth's crust and upper mantle, which fit together like piece of a puzzle. These plates are not fixed, but are constantly moving at a very slow rate. They move only few centimetres per year. Sometimes, the plates collide with one another or move apart. Volcanoes are most common in these geologically active boundaries. The two types of plate boundaries that are most likely to produce volcanic activity are divergent and convergent plate boundaries. At divergent boundary, tectonic plates move apart from one another. They never really separate because magma continuously move up from the mantle into this boundary, building new plate material on both sides of the plate boundary. The plates move apart at a divergent plate boundary, the release of pressure produces partial melting of the underlying mantle. The molten material is magma, basaltic in composition, and is buoyant. As a result, it wells up from below and cools close to the surface to generate new crust. Divergent margins are called constructive margins.

Given that Earth is constant in volume, the continuous formation of new crust produces an excess that must be balanced by destruction of crust elsewhere. This is accomplished at convergent plate boundaries, where tectonic plates move towards one another and collide. This collision forces the denser plate edge to subduct, or sink beneath the plate edge that is less dense. Where one of the plate margins is oceanic and the other is continental, the greater buoyancy of continental crust prevents it from sinking and the oceanic plate is subducted. When two plates carrying continental crust collide, neither is subducted. Instead, mountain ranges, like Himalayas are created.

Therefore, volcanoes are closely associated with plate tectonic activity. Most of the volcanic activity occurs along divergent plate boundaries, though unseen beneath the Earth's oceans. A volcano is a vent in the crust of the Earth, which issues eruption of molten rock, hot rock fragments and hot gases. A volcanic eruption is an awesome display of Earth's power. Sometimes beginning with gas rich magma rises in conduits- underground passage, magma travels to the surface as a thin and fluid lava, is then flowing out continuously or shooting up in glowing mountains. In other cases, entrapped gases, tear the magma into shreds and hurl viscous clots of lava into the air. In more violent eruptions, the magma conduit is cored out by an explosive blast and solid fragments are ejected in a great cloud of ash- laden gas that rises tens of thousands of meters into the air. During volcanic eruptions, there is sudden venting of rocks, there is flow of lava, toxic gas clouds are formed, fall of ash, mud flows, avalanches and most importantly pyroclastic (pyro - fire, clastic – broken) flows. The root zone of volcanoes is found about 100-200 kms below the surface of the Earth. In Earth's upper mantle, temperatures are high enough to melt and form magma. Magma is less dense than the solid rocks surrounding and overlying it, hence rises towards surface by buoyance force of gravity and comes out as lava.

There are about 1900 volcanoes on Earth which are considered active, means they show some level of occasional activity and are likely to erupt again. Many other volcanoes are dormant and others are considered extinct. Presently, there are twenty active volcanoes in Japan, Iceland, Italy, Hawaii, Guatemala, Chile, Alaska, Indonesia, Sicily, New Zealand, Ecuador, Philippines, Costa Rica and India.

A few important ones are: Mauna Loa, Hawai'i, it is the biggest volcano on the Earth. It is thought to have been erupting more or less continuously for 7,00,000 years now. The summit- top of volcano, of Mauna Loa stands at 4168 meters above sea level and more than 8500 meters above the ocean floor. Due to its frequent lava flows it poses great risk to surrounding communities. Eyjafjallajökull, Iceland, erupted as recently as 2010. The ash plume from eruption caused a global air traffic crisis, causing many flights to be diverted or cancelled. Mount Vesuvius, Italy is very dangerous volcano. It last erupted in 79 CE and buried the cities of Pompeii and Herculaneum under ash and lava. Mount Merapi, Indonesia - this volcano thought to have produced more lava flow than any other volcano in the world. It erupted in October 2010 and continued till end of November causing 350 human deaths and surrounding population homeless. It erupted as recently as 2018. Sakurajima volcano, Japan, has been erupting since 1955 and thousands of small eruptions happen each year. In 2013 there were 500 eruptions. In Santa Maria, Guatemala, volcano last eruption occurred in 2011 and lava flows have remained fairly constant since then. In India there are six volcanoes, three in Andaman and Nicobar Islands -- Barren Island, Narcondam and Baratang; Deccan Traps, Maharashtra, Dhinodhar Hills, Gujarat and Doshi Hill, Haryana.

The Barren Island is the only active volcano in South Asia. The first eruption of this volcano dates back to 1787. Since then it erupted ten times, most recent one being in 2017. Narcondam is dormant, Baratang is mud volcano, active since 2003, last eruption was in 2005. Dhinodhar, Doshi Hills and Deccan Traps volcanoes are extinct.

Volcanoes are some of Earth's most potent natural hazards and agents of change. Volcanic eruptions sometimes release massive amounts of sulphur dioxide, which rises into stratosphere, it reflects incoming solar radiation while absorbing outgoing land radiation, leading to cooling off the Earth's temperature. Sometimes, these 'volcanic winters' affect weather - the 1815 Mount Tambora, Indonesia , eruption cooled the average global temperature by as much as 3 degree Celsius, causing the year without summer. At times, explosive eruptions produce pyroclastic flows, a mixture of hot rock fragments & toxic gases, which moves like a liquid away from volcano with a speed more than 80 kms per hour, having temperatures upto 700 degree Celsius, knocking down, shatter, bury or burn anything in their path. Mount Vesuvius in 79 CE was such eruption which buried 13,000 people in Pompeii and Herculaneum, cities, Italy with volcanic ash and rock. The Smithsonian Institution for Global Volcanic Programme in Washington DC, has a database of world's volcanic eruptions from the past 10,000 years. Therefore, in present times the volcanologists can warn the people from impending eruptions throughout the planet.