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:
- Cinder cone
- Acid cones
- Lava flows
- Basic or Shield cones
- Parasitic or intermediate or composite cones
- Plug domes
- 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
1. Cinder Cone
- 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
- 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.
- 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
3 Composite Cone
- 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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