Important Topography and Terminology Associated With Volcano (Cone)

We have learned about the volcano and its types and distribution in our previous article. In this article, various topographies formed as a result of volcanic action – Interior Topography and External Topography, volcanic cone, mountains, plateaus etc. will be known. The points mentioned in this article are frequently asked in various competitive exams. 

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A. Interior Topography

When the lava emanating from the volcano cools and solidifies inside the earth before coming to the surface, then internal land forms are formed. Of these, mainly Bacolith, Lacolith, Facolith, Lowpolyth, Sill and Dyke etc. are made. Bacolith types of rocks are seen in the deeper parts, in which granite is the main. Due to the gradual cooling, their ravens get bigger. These magma deposits are in a domed form.

The lava deposited at low depth forms sedimentary rocks. Due to different types of cooling, different forms of rocks can be seen in it. In these, the convex sloping rocks are called lacolith, the concave sloping rocks are called lopolyth. The phacolith type of deposits can be seen in the slopes and abutments of the folded mountains.

Sills and Dikes – Horizontal deposits of lava in the interior of the earth’s crust are called sills, while vertical deposits of lava are called dikes. The thin layer of cob is called sheet and the smaller form of dyke is called stock.

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B. External Topography

External Topography are formed when lava erupts from the earth’s crust through explosive eruptions or through cracks. In these, mainly plateau, cone, crater, caldera etc. are formed. Lava plateaus and lava plains are mainly formed by fissures. In India, the major plateaus are the Deccan Plateau, Brazil’s Paraná Plateau, Guatemala’s Vulcano-de-Fuegon, America’s Colombia-Snack Plateau etc.

Cones are formed as a result of the central eruption of the volcano, which are of different shapes due to the different types of volcanoes. Their details are as follows –

1. Cinder cone or Ash cone
2. Lava Cone
   • Acidic Lava Cone
   • Alkaline Lava Cone or Shield Cone
3. Compound Cone or Flaky Cone
4. Nourished or Dependent Cone
5. Volcanic Plug or Lava Scolding and Diatreme
6. Cone in cone

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1. Cinder Cone or Ash Cone

• This type of cone is mainly formed by volcanic ash, dust and fragmented materials. Liquids do not contribute to this.
• Their height is less.
• In this the slope is concave.
• The crater is wide.
• For example, the Parquetin Mountains and Jorlo Mountains of Mexico, the Vulcano de Fuego Mountains of Guatemala, the Ijalco Mountains of San Salvador, the Camaguain Mountains located on the Luzon Islands of the Philippines, etc.

2. Lava Cone

This type of cone is mainly formed by acidic or alkaline lava. Hence it can be divided into two parts.

• Acidic Lava Cone
• Alkaline Lava Cone or Shield Cone

i. Acidic Lava Cone

• This type of cone is formed by acidic lava. The lava emanating from them is very thick and viscous. It contains high amount of silica.
• These lavas cool rapidly after emanating from the volcano, forming cones with higher elevations and steeper slopes.
• Their territorial expansion is less.
• Peleon-like, Vulcano-like and Stromboli-like volcanoes can be placed in this class.
• For example – Ri Union Island, Stromboli, Krakatoa

ii. Alkaline Lava Cone or Shield Cone

• This type of cone is formed by alkaline lava. The lava emanating from them is thinner and lighter. It contains less amount of silica.
• Expansion of thin lava and delay in solidification results in the formation of wide cones.
• Their height is less.
• Volcanoes similar to Hawaiian can be placed in this category.
• For example – Monalova of Hawaiian Islands

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3. Compound Cone or Flaky / layered Cone

• When many types of lava erupt from a volcano, they accumulate layer by layer. Due to which compound or layered cones are formed.
• Acidic, alkaline, ash and other substances are mixed in the formation of this type of cone.
• When cinder and ash are also released in a volcanic eruption, this compound cone becomes very wide, shapely and tall.
• Most of the world’s tallest volcanoes fall under this category.
• Like – Fujiyama of Japan, Mayan of Philippines, Rainyar of America, Hood and Shasta etc.

4. Nourished or Dependent Cone

• When many small sub-branches come out in addition to the main tube in the volcano, then other cones are formed on the slope of the main cone. These small cones are called nurturing cones or dependent cones.
• For example – Shasta mountain of America.

5. Volcanic Plug or Lava Scolding and Diatreme

• When lava emanating from a volcanic eruption is highly acidic and silica-rich, it accumulates in the form of a solid band in the volcanic canal and cavity. The solid lava deposited on its face is called a volcanic plug or scol.
• According to the above, the lava belt frozen in the duct of the volcano is called Volcano neck.
• Volcanic plugs and volcanic holes are called diatremes.
• For example, the Ship Rock in New Mexico, USA, is a prime example of a diatreme, while the Black Hills and Devil’s Tower in the same region are examples of volcanic clefts.

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6. Cone in Cone

When there is a small volcanic eruption in a dormant volcano after a long interval, then a small cone is formed inside the cone. This is called the cone in the cone. e.g. Fujiyama

7. Crater

• The funnel shaped structure usually formed on top of a volcanic cone is called a crater.
• When water gets filled in it, then it is called Crater Lake. Eg – Lonar lake of Maharashtra.
• When each subsequent eruption of a volcano is of less intensity than the previous eruption, then many smaller craters are formed inside a crater. The group of small craters formed in this way is called Nested Crater.
• For example, there are three small craters located in Mount Tal of the Philippines.

8. Caldera

• This is a more detailed version of the crater itself.
• When the land of the crater sinks or explodes again, the size of the crater becomes large. Due to which a more extensive area is formed by the caldera.
• A very large caldera is called a super caldera. For example – Lake Tova on the island of Sumatra
• For example – Orogan Lake of America, Era Caldera of Japan etc.

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Other Important Points and Terminology

1. Solftara

Even after the ash and smoke have stopped coming out of the volcano, when various types of gases keep coming out of it for many days. Then this state of the volcano is called Solfatara. Its name is derived from the Solfatara volcano located on the edge of the Mediterranean Sea, from where sulfuric smoke keeps coming out. Mainly sulfurous smoke keeps coming out of Solfatara.

2. Fumaroles

The final stage of the volcanic activity is called the fumaroles. Many types of gas and smoke keep coming out of it. Sulfur-rich smoke is called solfatara. For example, the Katmai mountain range located in Alaska of America (which is called the Valley of Thousands Dhuar), Koh Sultan of Iran and Dhaura in New Zealand’s White Island etc.

3. Subtropical or Geyser

Such sources of hot water, from which hot water and steam keep coming out in the form of fountains from time to time. are called geysers. It is considered a secondary form of volcanic activity. It is different from hot springs. For example, Faithful and Excelsior geysers located in Yellowstone National Park in America, Grand Geyser of Iceland and Geysers in New Zealand.

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4. Different Types of Rocks Emanating From Magma

Many types of fragments come out with magma during volcanic eruption, which have been given different names. 

These are the following –

i. Pyroclasts or Tephra – These are large rock fragments emanating from magma.

ii. Volcanic Bomb – When molten lava is thrown up to a height during a volcanic eruption, it takes time to come down. During this time, it becomes fiercely round, oval or pear-shaped, it is called a volcanic bomb.

iii. Lapili – When the size of a volcanic bomb is like a pea, it is called lapilli.

iv. Scoria – The pieces smaller than the lapilli are called scoria.

v. Pumice – These are light and porous boulders made from lava foam.

vi. Tuff – The rock formed by the solidification of volcanic dust, ash, lapilli etc. is called tuff.

vii. Brescia – The organized sharp rocky piece of scoria is called Brescia.

viii. Pele’s Hair – Sometimes small red bodies of lava with gases rise up like a fountain and due to strong wind, its parts are pulled like long shiny threads, which the people of Hawaiian Islands called Pele’s hair .

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