Pico Cão Grande in New Guinea. Image credit imgur.com

Pico Cão Grande in Gulf of Guinea. Image credit imgur.com

ZIMSEC O Level Geography Notes: Extrusive volcanic landforms.

  • Magma sometimes reaches earth’s surface through a vent/hole or a fissure/crack in surface rocks.
  • When lava reaches and emerges onto the earth’s surface it is known as lava.
  • If the lava emerges from through a vent it usually builds up to from a cone shaped mound that is referred to as a volcano.
  • If it emerges out of a fissure it usually builds up a lava plain or a lava plateau.
  • Extrusive volcanic landforms are those landforms that are formed as a result of magma reaching the earth’s surface as lava.
  • These landforms include:
  1. Cinder cone
  2. Acid cones
  3. Lava flows
  4. Basic or Shield cones
  5. Parasitic or intermediate or composite cones
  6. Plug domes

Volcanic cones

  • The mount of a volcano is known as a cone.
  • It is usually made up of lava or a mixture of lava and rocks torn from the earth’s crust by the molten magma.
  • A cone may also contain layers of ash and small bits of lava known as cinders.
  • The shape and size of a cone depend on the nature of the material of which the cone is made up of and the type of eruption.
  • The conduit through which lava flows out of is called a pipe and the exit of the pipe is called a crater.
  • The diagram below shows the basic features of a volcano
The features of a volcano please note conduit is as known as a pipe. Image credit Becuo.

The features of a volcano please note conduit is as known as a pipe. Image credit Becuo.

 

1. Cinder Cone

Cinder cone is made up of successive layers of ash deposited on each eruption. Image Credit Pinterest

Cinder cone is made up of successive layers of ash deposited on each eruption. Image Credit Pinterest

  • Lava is blown a great height when it is ejected i.e. a pyroclastic eruption
  • It breaks into smaller fragments which fall back to earth and build up a cone.
  • Several ash layers of cinder and ash form on each subsequent eruption.
  • The result is a cone consisting of basaltic cinders in unconsolidated mounds and
  • Has steep sides
  • Is characterized by short duration of activity.
  • They are relatively small when compared with other volcanic cones.
  • The cones are usually geologically short lived i.e.
  • they quickly succumb to the processes of denudation.
  • Examples include Likaiu and Teleki in Kenya and the Jose Plateau in Nigeria.

2 Lava Cone

A Lava cone. Image credit Umd.edu

A Lava cone. Image credit Umd.edu

  • The slope of these cones depend on whether the lava is fluid or viscous.
  • Basic/shield/fluid lava is very fluid and mobile.
  • Because is highly mobile the lava travels some distance before it solidifies
  • because of this it produces gently sloping cones.
  • An example of a basic lava cone is Mount Nyamuragira in the DRC.
  • Mauna Loa in Hawaii has a diameter spanning 400 km and a height of just over 9 km rising from the sea bed.
  • The diagram below shows a basic/shield lava cone.
  • Note the slopes are gentle and the base covers many kilometers.
Basic Lava Cone

Basic Lava Cone

  • Acid/Viscous lava produces steeply sloping cones.
  • Viscous lava contains a lot of silicates and is sticky,
  • It travels only a short distance before it cools down.
  • Sometimes the lava can be so viscous they form a plug dome that may completely block the vent.
  • A plug dome is an isolated hill that rises from the ground as the remnant product of a volcano.
  • Usually after erosion has removed the soft material to lave behind the hard resistant rock.
  • An example of a plug is the Pico Cão Grande plug in Gulf Guinea.
  • Examples of of acid lava volcanic cones are the Hoggar Mountains in Algeria
Acid Lava Cone. Image credit ElateAfrica

Acid Lava Cone. Image credit ElateAfrica

The formation of a plug dome. Image Credit Pixshark.

The formation of a plug dome. Image Credit Pixshark.

3 Composite Cone

Composite cone. Image Credit Imagearcade.

Composite cone. Image Credit Imagearcade.

  • Is formed out alternate layers of lava and ash.
  • The volcano begins each eruption cycle with a pyroclastic eruption which forms a layer of ash.
  • As the eruption continues lava pours out forming a layer on top of the ash.
  • Lava escapes from the sides of the main cone where it builds up small conelets
  • It usually has a lot of other geological features as well hence the name composite/parasitic
  • Some of its features include: dykes, conduits, ash and lava, a crater and a pipe
  • It ranges in height from moderate to high.
  • It has alternate layers of cinder and lava and ash hence its other name is stratovolcano (layered volcano).
  • Example is Mount Kilimanjaro in Tanzania and Mount Cameroon in Cameroon.

To access more topics go to the Geography Notes page.

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