TOPIC: General Studies 3
- Environmental hazard and Disaster
A volcano is an opening in the Earth’s crust that allows molten rock, gases, and debris to escape to the surface. Volcanism is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called a vent.
Movement of molten rock in the mantle, caused by thermal convection currents, coupled with gravitational effects of changes on the earth’s surface (erosion, deposition, even asteroid impact and patterns of post-glacial rebound) drive plate tectonic motion and ultimately volcanism.
Volcanoes tend to exist along the edges between tectonic plates, massive rock slabs that make up Earth’s surface. About 90 percent of all volcanoes exist within the Ring of Fire along the edges of the Pacific Ocean.
Types of Volcanos
- Active Volcano: About 1,900 volcanoes on Earth are considered active, meaning they show some level of activity and are likely to explode again.
- Dormant Volcano: Many other volcanoes are dormant, showing no current signs of exploding but likely to become active at some point in the future. Volcanoes go dormant because
- Magma from the Earth’s mantle can no longer reach the volcano. If a volcano doesn’t have a source of magma, it won’t erupt
- Earth’s plates are constantly shifting above volcanic hotspots
- When the angle of subduction changes and magma begins to rise through the mantle in a different location. The tectonic plate continues to shift above the hotspot, and eventually the volcano is shut off from the magma chamber beneath. The magma finds a new source to the surface, creating a new active volcano. The older volcano stops erupting and becomes dormant.
- Mauna Kea, one of the five volcanoes that make up the Big Island of Hawaii
- Clear Lake in California
- Yellow stone national park
- Tongariro in New Zealand
- Mt Kilimanjaro, Tanzania
- Mt Ararat, Turkey
- Mt Fuji, Japan
- Extinct Volcano: Have all the features of volcano but not erupted in historic times
Factors that lead to volcanism:
- Plate Tectonics: The majority of volcanoes occur where two lithospheric plates converge and one overrides the other, forcing it down into the mantle to be reabsorbed.
- Ocean floor spreading: A major site of active volcanism is along the axis of the oceanic ridge system, where the plates move apart on both sides of the ridge and magma wells up from the mantle.
- Weak Earth Surface: Because of high pressure in the earth’s interior, the magma and gases escape with great velocity as the pressure is released through eruptions where opportunity is provided by weak zones along the earth’s surface.
- Faults: Whenever extreme pressure builds in the mantle, along fault lines an eruption is likely to happen next. The earthquakes, for instance, may expose fault zones through which magma may escape.
- Magma crystallization: Decreasing temperatures can cause old magma to crystallize and sink to the bottom of the chamber and this movement can force fresh liquid magma up and out – similar to dropping a brick in a bucket of water.
- A decrease in external pressure: A decrease in external pressure can trigger an eruption as it may minimize the volcano’s ability to hold back by increasing the pressures inside the magma chamber
- Plate movement: Volcanism may occur because of plate movement over a “hot spot” from which magmas can penetrate to the surface. Ex. Islands of Hawaii
The gases and dust particles thrown into the atmosphere during volcanic eruptions have effects on climate.
Effects of Volcanism
Volcanic eruptions produce hazardous effects for the environment and climate
- Ashes can stay in the stratosphere for about two to five years, and within this period, there are chemical reactions that destroy the stratospheric ozone molecules.
- Volcanoes contribute about 18%-20% of Chlorine entering the atmosphere, this also destroys ozone.
- Volcanoes also cause global warming by releasing greenhouse gases such as water vapour and carbon dioxide into the atmosphere.
- Lava can kill plants and animals. This also contributes to global warming by reducing carbon sink.
Unlike other natural disasters such as floods, wildfires and earthquakes, volcanoes can have some positive effects.
- Most of the particles spewed from volcanoes cool the planet by shading incoming solar radiation. The cooling effect can last for months to years causing cooling over large areas of the Earth.
- Some ash and lava breakdown become soils that are rich in nutrients and become good areas for crop planting activities and growth of the forest.
Volcanism and landform evolution
Craters: Crater is a bowl-shaped depression at the top of a volcano caused by past volcanic eruptions. A volcanic crater is relatively small, usually spanning about a half a mile in diameter or less, and can fill with water to form a crater lake.
Caldera: If a volcanic eruption causes the magma chamber to empty, the volcano can implode, forming a larger depression known as a caldera. eg. Yellostone caldera.
Volcanic domes: Volcanic Domes result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent.
Basalt plateau: Basalt plateau is formed due to outpouring of extremely large volume of low viscosity basaltic magma from fissure vents. The basalts spread huge areas of relatively low slope and build up plateaus. eg. Deccan plateau, Columbian plateau.
Dykes – When magma solidifies in vertical fissures. Eg. Many Scottish Islands, such as Mull and Skye
Sills – Horizontal bodies of intrusive igneous rocks. Eg. Great Whin Sill (which carries part of Hadrian’s Wakk) and Drumadoon on the Isle of Arran
Batholiths – Magma that cools in the deeper depth & develop in the form of large domes. Eg. Sierra Nevada Batholith, California
Laccoliths – These are large dome shaped intrusive bodies having a pipe like conduit from below. Eg. Eildon Hills on the Scottish Borders
Humans pollute more than volcanoes
- The Deep Carbon Observatory (DCO), a 500-strong international team of scientists, noted that Human activity churns out up to 100 times carbon each year as all the volcanoes on Earth.
- Manmade emissions in 2018 alone topped 37 gigatonnes.
- By comparison, the CO2 released annually by volcanoes hovers around 0.3 and 0.4 gigatonnes
- Modern manmade emissions were the “same magnitude” as past carbon shocks that precipitated mass extinction.
Hawaii’s Kilauea volcano
- The Kilauea volcano, the most active in Hawaii, remained highly unstable.
- Highly toxic sulfur dioxide gas pouring from some fissures continued to contribute to “extremely dangerous” conditions.
- The high levels of sulfur dioxide are a threat to all who become exposed.
Important Value Additions:
- Kilauea is a currently active shield volcano in the Hawaiian Islands, and the most active of the five volcanoes that together form the island of Hawaii.
- Kilauea is considered one of the world’s most frequently active volcanoes.
Do you know?
There are five active volcanoes in Hawaii. They are:
- Mauna Loa
Mauna Loa is an active volcano and is due for an eruption.
India’s only active volcanoe is located in Barren island of Andaman, which is also the only confirmed active volcano in South Asia.
Connecting the Dots:
- Discuss the distributional pattern of volcanoes in the world.