Volcanic Landforms – Extrusive and Intrusive
The landforms formed due to the solidification of lava either inside or outside the earth surface are known as volcanic landforms.
Extrusive and Intrusive Landforms
- The landforms formed by the action of volcanoes and volcanic eruptions are known as volcanic landforms. The geological processes control the characteristics of various volcanic landforms. On the basis of cooling of magma, the volcanic landforms are divided into extrusive igneous rocks landforms and intrusive igneous landforms.
- Plutonic rocks are formed when the magma cools within the earth's crust. The extrusive igneous rocks are formed when the cooling of Lava occurs above the Earth's surface.
Extrusive igneous rocks landforms
When the Lava and other volcanic materials are thrown out to the Earth's surface during volcanic eruptions, the extrusive igneous landforms are formed. It includes volcanic Lava, pyroclastic debris, ash, volcanic bombs, and gases such as Sulphur dioxide, nitrogen compounds and other gases.
The conical vent and fissure vent
- The narrow cylindrical vent through which the lava flows out to the earth's crust during a volcanic activity is known as a conical vent. Conical vents are more common in the composite (or strato volcanic) volcanic features.
- The fissure is a narrow linear vent through which the lava comes out to the earth's crust during a volcanic eruption. The fissure vents are more commonly found in the areas of basaltic volcanism.The fissure vents are often few meters wide, which can be several kilometres long.
- Shield volcanoes are characterized by gentle upper slopes and a little steeper lower slopes. They are composed of relatively fluid lava flows which have been built over a central vent. Mostly, the low viscosity basaltic lava which is high in fluidity form Shield volcanoes. It leads to the formation of the extrusive igneous rocks.
- Shield volcanoes are mostly non-explosive, but they can become explosive if water gets inside the vent.
- Shield volcanoes are the largest volcanoes in the world. They extend to greater heights and distances. Examples of Shield volcanic landforms include Mauna Loa volcanoes of Hawaii.
Cinder cone volcanoes
- A Cinder cone has the features of a steep conical hill with loose pyroclastic fragments which include volcanic clinkers, cinder, volcanic ash (scoria) around the vent.
- Cinder cone volcanoes are made entirely of the loose grainy cinders, and they lack lava. Cinder cone usually has very steep sides along with a small crater on its top. They are small volcanoes.
- Composite volcanoes (strato-volcanoes) are mainly cone shaped with moderate steep Sites. The andesitic lava, along with the pyroclastic materials and ashes which find their way to the ground gets accumulated in the vicinity of vent openings. This leads to the formation of layers, which makes the volcanic mounts appear as composite volcanoes.
- Composite volcanoes are also known as stratovolcanoes. Most common and highest volcanoes have the features of composite cones. For example, Stromboli, the Lighthouse of Mediterranean, mount Fuji etc.
- Composite volcanoes are associated with the eruption of a cooler and more viscous lava than the basaltic lavas. The composite volcanoes often cause explosive volcanic eruptions.
Flood basalt provinces (Lava plateaus)
- When a very thin and fluid lava comes out to the Earth's surface, and flow after intervals for long periods of time, spreading to a large area; it produces a layered, undulating- wave-like flat surfaces.
- These types of extrusive igneous rocks and landforms are known as flood basalt landforms or Lava plateaus. The Deccan traps of India, Snake basin of USA, Canadian Shield etc. are the examples of Flood basalt provinces.
Mid-oceanic ridge volcanoes
- The mid-oceanic ridges occur in the underwater oceans. There is a system of 70000 km long mid-oceanic ridges that stretch along all the major ocean basins. The central portion of the mid-oceanic ridges is associated with frequent volcanic eruptions.
- The lava which comes out through these eruptions are Basaltic and have less silica content, so they are less viscous. Due to less viscosity, they flow through longer distances and cool slowly. This outpour of lava through volcanic eruptions is responsible for the phenomenon of seafloor spreading.
- When the Lava ceases to flow after the volcanic eruption, the creator of volcanoes turns into a lake, which is known as Caldera lake. The rainwater and snowmelt often get accumulated in these enclosed depressions leading to the formation of lakes.
- Lonar in Maharashtra, Krakatoa in Indonesia, and Lake Caldera in southern Oregon etc. are the examples of Caldera lakes.
Intrusive volcanic landforms
The intrusive igneous rocks or plutonic rocks are formed when the Magma cools within the earth's crust and does not erupt to the surface. Various forms of intrusive igneous rocks are formed due to the intrusive activity of volcanoes.
- Batholiths are the intrusive igneous rocks masses formed due to the cooling and the solidification of Magma inside the earth. These intrusive igneous rocks appear on the surface after the erosional process erode the materials lying above these rocks.
- The batholiths form the core of large mountains, and they get exposed to the surface after the erosional activities. Batholiths are granitic intrusive igneous rocks.
- Laccoliths are the large dome-shaped intrusive igneous rocks which are connected by a pipe-like conduit with the magma.
- These intrusive igneous rocks resemble like a composite volcano structure, but they are found below the earth s surface. Example Karnataka plateau.
Lopolith is formed when the Magma moves upwards, and a portion of this magma moves in a horizontal direction where it finds a weak plane. When it develops into saucer shape, it is known as Lopolith.
When a wavy mass of intrusive igneous rocks are formed at the base of synclines or on the top of anticline having a definite conduit with the magma chambers below, they are called the laccoliths.
Sills are the intrusive igneous rocks which are formed by the solidified and near horizontal lava layers inside the earth. The thinner deposits of these rocks are called sheets, while the thicker horizontal deposits are known as sills.
When the Magma moves upwards through the cracks and fissures, and solidifies almost perpendicularly to the earth s surface, developing a wall like structure, they are known as dykes. Dykes are the most common intrusive igneous rocks in Western Maharashtra and other parts of Deccan traps.