Geography · Ch 9 — Interior of the Earth
Intrusive Forms
Intrusive Forms
The diagram is a cutaway block, slicing through the crust so you see both the surface landscape and what lies beneath it. On top sits a full volcanic scene: a composite volcano, a cinder cone, and an ash cone crowned by a volcanic dome. Nearby, a lava plateau or mesa stands as a flat-topped remnant of old flows, and a volcanic neck with radiating dykes shows the exposed throat of a long-dead volcano. A caldera — the collapsed crater of an explosive eruption — has a fresh cinder cone growing on its floor, and ash flows spread across the ground. Every surface feature is labelled with a leader line pointing to it.
Below the surface, the same block reveals the intrusive bodies that feed and accompany these eruptions. A dyke cuts vertically through the country rock, while sills lie as near-horizontal sheets between layers. A laccolith bulges upward like a dome with a flat base, and an eroded laccolith shows what happens when denudation strips the cover. A stock and a much larger batholith occupy the deeper crust, the batholith representing the cooled magma chamber itself. A diatreme — the pipe-like conduit of an explosive vent — connects the depths to the surface. All of these are labelled and set against layered country rock, so you can see exactly how each intrusive form sits relative to the strata it intrudes. …
Intrusive Forms
When lava erupts onto the surface and cools, it hardens into igneous rock. But cooling does not always happen at the surface — a good part of the lava may solidify while still trapped inside the crust. This single difference in location gives us the two broad classes of igneous rocks: volcanic rocks, which cool at the surface, and plutonic rocks, which cool within the crust. The lava that cools inside the crustal portions does not form one uniform mass; it takes on a variety of shapes depending on how it moves and where it gets trapped. These underground solidified bodies are called intrusive forms.
The book describes several distinct intrusive forms, each with its own shape, origin, and typical location.
Batholiths
A batholith is a large body of magmatic material that cools at considerable depth within the crust. As it cools, it develops in the form of a large dome. Because it forms deep underground, it does not appear on the surface on its own — it is exposed only after denudational processes (erosion and weathering) strip away the overlying rock layers. Batholiths cover large areas and can extend to depths of several kilometres. They are granitic bodies, and in essence, a batholith is the cooled remnant of a magma chamber itself.
Lacoliths
A lacolith is a large, dome-shaped intrusive body with a level base. It is connected from below by a pipe-like conduit. In shape it resembles the surface volcanic domes of a composite volcano, but it sits at much greater depth. A lacolith can be thought of as a localised source of lava that eventually finds its way to the surface. The book gives a concrete Indian example: the Karnataka plateau is dotted with domal hills of granite rock, most of them now exfoliated, which are examples of lacoliths or batholiths.
Lapolith, Phacolith and Sills
As lava moves upward, a portion of it may shift in a horizontal direction wherever it encounters a weak plane in the rock. Depending on the shape it settles into, it takes different names.
- Lapolith — when the lava develops into a saucer shape, concave to the sky, it is called a lapolith.
- Phacolith — a wavy mass of intrusive rock found at the base of synclines or at the top of anticlines in folded igneous country. These wavy bodies have a definite conduit to a magma source beneath, which subsequently develops into a batholith.
- Sill and sheet — near-horizontal bodies of intrusive igneous rock. The distinction is purely one of thickness: thinner horizontal deposits are called sheets, while thick horizontal deposits are called sills.