Volcanic Landforms: Extrusive and Intrusive Landforms – Class 9

Volcanic landforms are divided into extrusive and intrusive landforms based on whether magma cools within the crust or above the crust.

Basically, Volcanic Landforms are divided into two broad categories:
1. Extrusive Landforms
2. Intrusive Landforms

In this module, we will learn about these landforms.

1. Extrusive Landforms
Extrusive landforms are formed from material thrown out to the surface during volcanic activity. The materials thrown out include lava flows, pyroclastic debris, volcanic bombs, ash, dust, and gases such as nitrogen compounds, sulfur compounds, and minor amounts of chlorine, hydrogen, and argon.

Some important Extrusive forms:

(i) Ash and Cinder Cones
What are Ash and cinder cones? A cinder cone, also called an ash cone, deposits around a volcanic vent, formed by pyroclastic rock fragments (formed by volcanic or igneous action), or cinders, which accumulate and gradually build a conical hill with a bowl-shaped crater at the top. These are formed while explosive eruptions.
The growth of ash or cinder cone begins around a crater and they don’t have much height.

(ii) Lava Dome
A lava dome is a circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. It is formed when viscous lava solidifies quickly and forms a steep-sided cone.

Do you know! Around 6% of eruptions on Earth are lava domes forming.

(iii) Lava Shields
A broad volcano built up from the repeated non-explosive eruption of basalt to form a low dome or shield, usually having a large caldera at the summit is called lava shields. The Hawaiian volcanoes are excellent examples of basalt domes or shields. For example Mauna Loa and Mauna Kea in Hawaii.

(iv) Composite or Strato cone
Composite volcanoes, also called stratovolcanoes, are cone-shaped volcanoes built from many layers of lava, pumice, ash, and tephra. Because they are built of layers of viscous material, rather than fluid lava, composite volcanoes tend to form tall peaks rather than rounded cones. They are conical or central type volcanic landforms. Along with andesitic lava, large quantities of pyroclastic material and ashes find their way to the surface.

Andesitic lava along with pyroclastic material accumulates in the vicinity of the vent openings leading to the formation of layers, and this makes the mounts appear as a composite volcano or a stratovolcano (divided into layers). The highest and most common volcanoes have composite cones.

Mount Stromboli (the Lighthouse of the Mediterranean), Mount Vesuvius, Mount Etna, Mount Fuji, etc. are some examples.

2. Intrusive landforms
Intrusive Landforms are formed when magma cools within the crust. Some important Intrusive landforms are:

(i) Batholith
These are large granitic rock bodies formed due to the solidification of hot magma inside the earth. They appear on the surface only after the denudation processes remove the overlying materials. Batholiths form the core of huge mountains and may be exposed on the surface after erosion.

(ii) Laccoliths
Laccoliths are large dome-shaped intrusive bodies connected by a pipe-like conduit from below. These are intrusive counterparts of an exposed domelike batholith. The Karnataka plateau is spotted with dome hills of granite rocks.

(iii) Lopoliths
When lava solidifies in shallow basins in the shape of a saucer, it is called lopoliths. Lopoliths are generally concordant with the intruded strata with a dike or funnel-shaped feeder bodies below the body.

(iv) Sills
The near horizontal bodies of the intrusive igneous rocks are called a sill. It is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock.

(v) Dykes
When the lava makes its way through cracks and the fissures developed in the land, it solidifies in vertical columns called Dykes.

(vi) Phacoliths
Phytoliths are concordant plutonic bodies that lie parallel to the bedding plane or foliation of folded country rock. They occur along the crests of anticlines or the troughs of synclines in folded sedimentary strata. These are dome-shaped. They are formed when acid lava solidifies on an anticline or on the base of a syncline.

Questions

1. Which type of Extrusive landform is formed when viscous lava solidifies quickly and forms a steep-sided cone?
(i) Lava Sheild
(ii) Strato Cone
(iii) Cinder Cones
(iv) Lava Domes

2. Growth of ash or cinder cone begins around what?
(i) Volcano
(ii) Mountain
(iii) Spine
(iv) Crater

3. When Lava solidifies in shallow basins in the shape of a saucer it is called
(i) Sills
(ii) Dykes
(iii) Batholith
(iv) Lopoliths

4. Mt Etna is an example of which type of landform?
(i) Lava dome
(ii) Cinder Cones
(iii) Strato cone
(iv) Lava Sheilds

5. These are large granitic rock bodies formed due to the solidification of hot magma inside the earth.
(i) Laccoliths
(ii) Composite cone
(iii) Sills
(iv) Batholith

Exercise: Fill in the blanks.

1. The crater lake serves as a great source of perennial rivers.
2. volcanic eruptions can change the heat balance of the Earth and the atmosphere, causing global warming.
3. Volcanic eruption of Mt. Karakota in 1883 caused the destruction of almost the entire island.
4. The highest and most common volcanoes have composite cones.
5. Extrusive landforms are formed from material thrown out to the surface during volcanic activity.

State whether the following statement is True or False.


1. Volcanic eruptions cause only destruction and are not at all helpful for humans. False
2. Volcanic eruptions cause hurricanes. False
3. Intrusive Landforms are formed when magma cools within the crust. True
4. Mt Fuji is an example of Dyke. False
5. Sometimes remnants of volcanoes after erosion help in preserving traces of Old civilization. True

Read More: Effects of Volcanic Eruption: Constructive and Destructive – Class 9

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