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.

1. Cinder Cone

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

• 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

• 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

The formation of a plug dome. Image Credit Pixshark.

3 Composite Cone

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.

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