The Universe is all existing matter & space. It is incomprehensively large (beyond mental grasp). It consists of both physical (subatomic particles like electrons, protons to galactic super-clusters) and non-physical (light, gravitation, space etc.) components.

  • The universe, at present, is said to possess about 100 billion galaxies each comprising an average of 100 billion stars. In comparison, Milky Way Galaxy is believed to possess 100 billion to 400 billion stars. (1,000,000 = 1 Million = 10 Lakhs; 1,000,000,000 = 1 Billion = 100 Crore; 1,000,000,000,000 = 1 Trillion
Basic Terms
  • Cosmosanother word for universe.
  • Cosmicrelating to the universe or cosmos.
  • Cosmic rayshighly energetic atomic nucleus or other particle travelling through space at a speed approaching that of light. Direct exposure to cosmic rays can cause gene mutations resulting in cancer.
  • Cosmology: the scientific study of the large-scale properties of the universe.
  • Cosmological: relating to the origin and development of the universe.
  • Astronomy: the scientific study of celestial objects (stars, planets, comets, etc.) and phenomena that originate outside the Earth’s atmosphere (such as the solar wind, gravitational waves etc.).

The Big Bang Theory

4,045 Big Bang Theory Images, Stock Photos & Vectors | Shutterstock

The Expanding Universe

  • The Big Bang Theory is the prevailing cosmological model for the universe’s birth. It states that 13.8 billion years ago, all of space was contained in a single point of very high-density and high-temperature state from which the universe has been expanding in all directions ever since.

The Expanding Universe | SpringerLink

The Evolution of The Universe Since the Big Bang


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10-43 Sec1032The cosmos goes through a superfast “inflation,” expanding from the size of an atom to that of a grapefruit in a tiny fraction of a second.
10-32 Sec1027Post-inflation, the universe is a seething, hot soup of electronsquarks, and other particles.
10-6 Sec1013A cooling cosmos permits quarks to clump into protons & neutrons.
3 min108Still too hot to form into atoms, charged electrons and protons prevent light from shining.
3,00,000 years103Electrons combine with protons & neutrons to form atoms, mostly hydrogen & heliumLithium beryllium were formed in trace amounts. Light can finally shine.
1 billion years-200Gravity makes hydrogen and helium (primordial elements) coalesce to form the giant clouds that will become galaxies; smaller clumps of gas collapse to form the first stars.
15 billion years-270As galaxies cluster together under the influence of gravity, the first stars die and spew heavy elements into space: those will eventually turn into new stars and planets.

Big Crunch (The Death of The Universe)

  • At some point, the universe would reach a maximum size & begin collapsing. The universe would become denser & hotter again, ending in a state like that in which it started — a single point of very high density.

Accelerating Expansion of The Universe& Dark Energy

  • It is the observation that the expansion of the universe is such that the velocity at which a galaxy is moving away from the observer is continuously increasing with time (Hubbles Law) It implies that the universe will get increasingly colder as matter spreads across space.
  • The accelerated expansion of the universe is thought to have begun since the universe entered its dark-energy-dominated era  roughly 5 billion years ago.
  • Dark energy is an unknown form of energy that is hypothesised to permeate (spread throughout) all of space, tending to accelerate the universe’s expansion


A volcano is an opening in the earth’s crust that allows lava, volcanic ash, and gases to escape. There are different types of volcanoes based on the shape, nature of the eruption, frequency of eruption, etc. The common types of volcanoes are Cinder Cone Volcanoes, Composite Volcanoes (Stratovolcanoes), Shield Volcanoes, etc. The topic “Types of Volcanoes” is an important part of the UPSC/IAS Exam Geography syllabus which is discussed in this article in detail.

A Typical Volcano
A Typical Volcano

What are Volcanoes?

  • Volcanoes are landforms or mountains where molten rocks appear from the surface of the planet. The volcano mountain opens under the pool of molten rocks inside the earth’s surface.
  • A volcano is referred to as a vent or fissure in the crust of the earth from where lava, ash, rocks, and gases come out.
  • Active volcanoes are those categories of a volcano that appear in the recent past.
  • Mantle volcanoes include a weaker zone which is referred to as the asthenosphere.

Types of Volcanoes

Based on the Shape

Cinder Cones

  • Cinder cones are round or oval cones made up of tiny lava pieces blown up from a single vent.
  • Cinder cones are formed by the accumulation of largely small fragments of scoria and pyroclastics around the vent.
  • The majority of cinder cones only erupt once.
  • Cinder cones can arise as side vents on bigger volcanoes or as isolated cinder cones.
Cinder Cone Volcano
Cinder Cone Volcano

Composite Volcano

  • Composite volcanoes are steep-sided volcanoes made up of multiple layers of volcanic rocks, most of which are made up of high-viscosity lava, ash, and rock debris.
  • These volcanoes are towering conical mountains made up of lava flows and other ejecta layered in alternate layers, hence the name strata.
  • Cinder, ash, and lava make up composite volcanoes.
  • Cinders and ash build up on top of one another, lava flows over the ash, cools and hardens, and the cycle continues.
Composite Volcano
Composite Volcano

Shield Volcano

  • Shield volcanoes have long, gradual slopes formed by basaltic lava flows and are fashioned like a bowl or shield in the middle.
  • These are generated by the eruption of low-viscosity lava that can travel a long way from the vent.
  • They don’t usually blow out in a big way.
  • Shield volcanoes are more prevalent in marine environment than continental settings because low-viscosity magma is often low in silica.
  • Shield cones are found throughout the Hawaiian volcanic system, and they are also frequent in Iceland.
Shield Volcano
Shield Volcano

Lava Domes

  • Lava domes arise when erupting lava becomes too thick to flow and stacks up near the volcanic vent, forming a steep-sided mound.
  • Slow outbursts of exceedingly viscous lava form them.
  • They can sometimes be found within the crater of an earlier volcanic eruption.
  • They can erupt violently and explosively, just like a composite volcano, although the lava rarely flows far from the erupting vent.
Lava Domes
Lava Domes

Based on the Nature of the Eruption

Hawaiian Volcanoes

  • Hawaiian Volcanoes are named after the Kilauea Volcano on Hawaii’s Big Island which is known for its stunning fire fountains.
  • Fluid basaltic lava is sprinkled in jets from a vent or series of vents on the summit or side of a volcano during a Hawaiian volcano.
  • Fire fountaining is a phenomenon in which the jets linger for hours or even days.
  • The spatter formed by hot lava pouring from the fountain can either melt together to form lava flows or build hills known as spatter cones.
  • Lava flows can also emerge from vents at the same time as fountaining or after it has stopped.
  • These flows can travel kilometers from their source before cooling and hardening because they are extremely fluid.
  • The 1969-1974 Mauna Ulu eruption on the volcano’s flank and the 1959 eruption of the Kilauea Iki Crater at Kilauea’s summit are two excellent examples.
  • Lava fountains erupted to heights of nearly a thousand feet in both eruptions.

Strombolian Eruption

  • Strombolian eruptions are fluid lava bursts from the mouth of a magma-filled summit conduit.
  • Strombolian eruptions are called after the Stromboli Volcano, which is located on the Italian island of Stromboli and has many volcanic summit vents.
  • The explosions normally happen every few minutes, in either regular or sporadic intervals.
  • The bursting of huge gas bubbles, which rise upward in the magma-filled conduit until they reach the open air, causes lava explosions, which can reach heights of hundreds of meters.
  • Volcanic Products: This type of eruption can produce a range of eruptive products, including
    • spatter (hardened globs of glassy lava)
    • scoria (hardened bits of bubbling lava)
    • lava bombs (large chunks of lava)
    • ashes and
    • minor lava flows (which form when hot spatter melts together and flows downslope).
  • The debris left behind by this explosive eruption is called Tephra.
  • Small lava lakes that can form in volcanoes’ conduits are commonly associated with Strombolian eruptions.
  • They are the least violent of the explosive eruptions, yet bombs and lava flows can still be quite dangerous if they reach populated areas.
  • The lava lights brightly at night, making the explosions even more impressive.

Strombolian Eruption

Strombolian Eruption

Vulcanian Eruption

  • A Vulcanian eruption is a viscous magma explosion that is short, intense, and relatively tiny.
  • Vulcanian Eruption is named after the minor volcano on the Italian island of Volcano, which was considered to represent the vent above the forge of the Roman smith deity Vulcan.
  • This form of eruption occurs when a plug of lava in volcanic conduit fragments and explodes, or when a lava dome ruptures (viscous lava that piles up over a vent).
  • Vulcanian eruptions produce enormous explosions. The materials from the eruption travel at speeds of up to 350 meters per second and rise many kilometers into the air.
  • The materials include tephra, ash clouds, and pyroclastic density currents (clouds of hot ash, gas, and rock that flow almost like fluids).
  • Vulcanian eruptions can be monotonous and last for days, months, or even years, or they might be precursors to more explosive eruptions.

Vulcanian Eruption

Vulcanian Eruption

Plinian Eruptions

  • Plinian eruptions are the largest and most violent of all volcanic eruptions.
  • They are frequently linked with particularly viscous magma and are caused by the fragmentation of gassy magma.
  • They release massive quantities of energy and produce eruption columns of gas and ash that can reach 50 kilometers in height and travel at hundreds of meters per second.
  • Hundreds of thousands of kilometers away from the volcano, ash from an eruption column can float or be blown.
  • The eruption columns resemble a mushroom (similar to a nuclear explosion) or an Italian pine tree;
  • Plinian eruptions can be highly devastating, obliterating the entire summit of a mountain, as happened in 1980 at Mount St. Helens.
  • They can erupt miles away from the volcano, releasing ash, scoria, and lava bombs, as well as pyroclastic density currents that raze forests, strip soil from bedrock, and demolish everything in their path.
  • These eruptions are frequently climactic, and a volcano with a magma chamber drained by a massive Plinian eruption may go dormant.

Vesuvian Volcanoes

  • Vesuvian Volcanoes have magma that is ejected from a start cone vent and are particularly violent and explosive.
  • After a long period of quiet or modest activity, the eruption begins.
  • The vent has a tendency to be emptied to a great depth.
  • The lava sprays explosively, and the gas cloud rises to a tremendous height before depositing the tephra.

Based on the Frequency of Eruption

Active Volcano

  • Activeve volcanoes are volcanoes that are either erupting or on the verge of eruption.
  • There are around 500 active volcanoes on Earth, excluding those submerged beneath the oceans.
  • Every year, approximately 50 to 70 active volcanoes erupt, most of them being around the pacific “ring of fire”.
  • Mount Etna (Italy), Hawaiin Islands (Pacific Ocean), Mauna Loa (Pacific Ocean), Mount Vesuvius (Italy), and Barren Island (India) are some examples of Active Volcanoes around the world.

Dormant Volcano

  • A dormant volcano is one that is not erupting at the present but has erupted in the past and is expected to erupt again.
  • The distinction between active and dormant volcanoes can be hazy;
  • Some volcanoes can last thousands of years without erupting, thus they are theoretically predicted to erupt in the future, but it could take many lives.
  • Another of the Big Island’s five volcanoes, Mauna Kea, last erupted 3,500 years ago, but it is predicted to erupt again, but no date has been set.
  • People living in the neighborhood of dormant volcanoes are frequently complacent and unprepared when an eruption occurs.
  • This was the case in 1980 with Mt. St. Helens.

Extinct Volcano

  • Extinct volcanoes are considered to be dormant and unlikely to erupt again.
  • Example: Kohala, the Big Island of Hawaii’s oldest volcano, hasn’t erupted in 60,000 years and isn’t expected to erupt again.
  • However, because many Hawaiian volcanoes are in the process of rejuvenation, this classification isn’t completely accurate.
  • Aconcagua of the Andes is a typical example of an extinct volcano.

Based on the Characteristics of Lava

Basic Lava

  • Basic Lava is the hottest lava, with temperatures around 1,000 degrees Celsius and high fluidity.
  • They are dark-colored basalt with high iron and magnesium content but low silica content.
  • They emerge from the volcanic vent silently and aren’t particularly explosive.
  • They move easily at a speed of 10 to 30 miles per hour due to their tremendous fluidity.
  • They have a wide range of effects, spreading out as thin sheets across long distances before solidifying.
  • The resulting volcano has a large circumference and is gradually descending, forming a flattened shield or dome.
  • Along the constructive boundaries, shield lava flow is prevalent.

Acid Lava

  • Acid Lava flows are viscous and have a high melting point.
  • They’re light-colored, low-density, and have a high silica content.
  • They move slowly and rarely travel a long distance before hardening.
  • As a result, the resulting volcanic cone is stratified and steep-sided (hence the name stratovolcano).
  • The quick solidification of lava in the vent obstructs the outpouring of the lava flow, resulting in loud explosions and the release of numerous volcanic bombs or pyroclasts.
  • Lava can be so viscous that it forms a lava plug in the crater, as seen at Mt. Pelée in Martinique (an island in the Lesser Antilles, Caribbean Islands).
  • The majority of andesitic lava flows occur along destructive boundaries (convergent boundaries).