Extrusive Volcanic Landforms

These are formed from material thrown out during volcanic activity. The materials thrown out during volcanic activity includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon.

Volcanoes at convergent plate boundaries
  • Ocean-Ocean plate collision and Ocean – Continent plate collision: In subduction zones where oceanic plates dive under continental plates.
  • As the oceanic plate subducts beneath the surface, intense heat and pressure melts the rock.
  • Molten rock material, magma, can then ooze its way toward the surface where it accumulates at the surface to create a volcano.

Types of volcanoes found due to convergent plate boundaries:

Composite Cones or Strato volcanoes:

    • They are conical or central type volcanic landforms.
    • Along with andesitic lava, large quantities of pyroclastic material and ashes find their way to the ground.
    • They are accumulated in the vicinity of the vent openings leading to formation of layers, and this makes the mounts appear as composite volcanoes.
    • The highest and most common volcanoes have composite cones.
    • Example: Vesuvius, Mt. Fuji, Stromboli (Lighthouse of the Mediterranean) etc

Cinder cone (Tephra cones):

    • Cinder cones are small volume cones consisting predominantly of tephra that result from strombolian eruptions.
    • They usually consist of basaltic to andesitic material.

Calderas:

    • After the eruption of magma has ceased from the cones, the crater frequently turns into a lake at a later time.
    • Water may collect in the crater. This lake is called a ‘caldera’.
    • Example: Lake Toba in Sumatra, Crater Lake in Oregon, USA.
Volcanoes at Divergent plate boundaries
    • Divergent boundaries within continents initially produce rifts which eventually become rift valleys.
    • Example: In Africa’s East African Great Rift Valley– Mt.Kilimanjaro, Mt. Kenya, Mt. Cameroon.
    • Most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges.
    • Example: Mid- Atlantic ridge where there is a constant sea-floor spreading and formation of new plate boundaries. Iceland is a remarkable location in that a section of the North-Atlantic mid-ocean ridge is exposed on land.
    • Divergent boundaries also form volcanic islands which occur when the plates move apart to produce gaps which molten lava rises to fill.

Types of volcanoes found due to divergent plate boundaries:

Conical Vent and Fissure Vent:

      • A conical vent is a narrow cylindrical vent through which magma flows out violently. Conical vents are common in andesitic (composite or strato volcano) volcanism.
      • A fissure vent, also known as a volcanic fissure or eruption fissure, is a narrow, linear volcanic vent through which lava erupts, usually without any explosive activity.
      • The vent is often a few meters wide and may be many kilometers long. Fissure vents are common in basaltic volcanism.

Shield Volcanoes or Lava domes:

      • These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. They are not steep.
      • They become explosive if somehow water gets into the vent; otherwise, they are less explosive.
      • Example: Mauna Loa (Hawaii).

Mid-Ocean Ridges

      • These volcanoes occur in the oceanic areas. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins. The central portion of this ridge experiences frequent eruptions.
      • The lava is basaltic in nature.
      • Cools slowly and flows through longer distances.
      • The lava here is responsible for sea floor spreading.
      • Example: Mid-Atlantic Ocean ridge; extension is seen in the Iceland.
Volcanoes due to Hot Spots
      • Hot spots are places where a chamber of magma has accumulated at depth beneath the surface.
      • The volcanic islands of Hawaii are a notable example of this. The Hawaiian Islands ride atop the Pacific plate as it moves in a north-westerly direction over the hot spot that creates the volcanoes.
      • Therefore, the oldest volcanic island is found at the northwest end of the chain and the youngest to the southeast.
      • Volcanic activity ceases as the older islands move off the hot spot.

Types of volcanoes found due to Hotspots:

Shield Volcanoes or Lava domes:

    • These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. They are not steep.
    • They become explosive if somehow water gets into the vent; otherwise, they are less explosive.
    • Example: Mauna Loa (Hawaii).

Lava Plains and Basalt Plateaus:

    • Sometimes, a very thin magma escapes through cracks and fissures in the earth’s surface and flows after intervals for a long time, spreading over a vast area, finally producing a layered, undulating (wave like), flat surface.
    • Example: Deccan traps (peninsular India), Snake Basin, U.S.A, Icelandic Shield, Canadian Shield etc.