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Intrusive Forms
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- The lava that is released during volcanic eruptions on cooling develops into igneous rocks.
- The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion.
- Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust).
- The lava that cools within the crustal portions assumes different forms. These forms are called intrusive forms.
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Caldera
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- These are the most explosive of the earth’s volcanoes.
- They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure.
- The collapsed depressions are called calderas.
- Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity.
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Flood Basalt Provinces
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- These volcanoes outpour highly fluid lava that flows for long distances.
- Some parts of the world are covered by thousands of sq. km of thick basalt lava flows.
- There can be a series of flows with some flows attaining thickness
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Batholiths
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- A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes.
- They appear on the surface only after the denudational processes remove the overlying materials.
- They cover large areas, and at times, assume depth that may be several km. These are granitic bodies.
- Batholiths are the cooled portion of magma chambers.
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Lacoliths
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- These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below.
- It resembles the surface volcanic domes of composite volcano, only these are located at deeper depths.
- It can be regarded as the localised source of lava that finds its way to the surface.
- The Karnataka plateau is spotted with domal hills of granite rocks.
- Most of these, now exfoliated, are examples of Lacoliths or batholiths
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Lopolith, Phacolith and Sills
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- As and when the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane.
- It may get rested in different forms. In case it develops into a saucer shape, concave to the sky body, it is called Lopolith.
- A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of anticline in folded igneous country.
- Such wavy materials have a definite conduit to source beneath in the form of magma chambers (subsequently developed as batholiths). These are called the Phacolith.
- The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material.
- The thinner ones are called sheets while the thick horizontal deposits are called sills.
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Dykes
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- When the lava makes its way through cracks and the fissures developed in the land, it solidifies almost perpendicular to the ground.
- It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes.
- These are the most commonly found intrusive forms in the western Maharashtra area.
- These are considered the feeders for the eruptions that led to the development of the Deccan traps.
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Distribution of Earthquakes and Volcanoes
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- There is a line of dots in the central parts of the Atlantic Ocean almost parallel to the coastlines.
- It further extends into the Indian Ocean.
- It bifurcates a little south of the Indian subcontinent with one branch moving into East Africa and the other meeting a similar line from Myanmar to New Guiana.
- The shaded belt showing another area of concentration coincides with the Alpine- Himalayan system and the rim of the Pacific Ocean.
- In general, the foci of the earthquake in the areas of mid-oceanic ridges are at shallow depths whereas along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones.
- The rim of the Pacific is also called rim of fire due to the existence of active volcanoes in this area.
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