Volcanic Landforms

How Volcanoes Build the Land

A volcano does more than throw ash into the sky. The lava it releases, on cooling, turns into igneous rock. But that cooling can happen in two very different places — either on the surface, after the lava has erupted, or deep inside the crust while the lava is still underground. This single difference gives us the two great families of igneous rocks: volcanic rocks, which cool at the surface, and plutonic rocks, which cool within the crust.

When lava cools inside the crust, it does not form one uniform mass. It takes on a variety of shapes depending on how it moves, where it gets trapped, and the structure of the surrounding rock. These underground bodies are called intrusive forms. They are invisible at first, but erosion eventually exposes many of them, and they become some of the most striking landforms we see today.

Surface Volcanic Landforms

Not all eruptions build mountains. Some of the most dramatic volcanic landforms are actually holes in the ground.

Composite Volcanoes and Calderas. These are the most explosive volcanoes on Earth. They erupt with such violence that instead of building a tall cone, they tend to collapse in on themselves. The collapsed depression left behind is called a caldera. This explosiveness is a clue to what lies beneath: the magma chamber feeding such a volcano is not only enormous but also sits very close to the surface.

Flood Basalt Provinces. At the opposite extreme are volcanoes that outpour highly fluid lava. This lava does not pile up; it flows for long distances across the land. In some parts of the world, thousands of square kilometres are covered by thick basalt lava flows. These are not single eruptions but a series of flows, some of which attain great thickness. The result is a vast, flat, layered plateau of basalt.

Intrusive Forms

The book describes seven distinct intrusive forms, each with its own shape and origin.

Batholiths. A large body of magmatic material that cools at great depth develops in the form of a huge dome. It remains buried until denudational processes — weathering and erosion — strip away the overlying rock and bring it to the surface. Batholiths cover large areas and can extend to depths of several kilometres. They are granitic bodies, and they are essentially the cooled remnants of magma chambers themselves.

Laccoliths. These are large, dome-shaped intrusive bodies with a level base. A pipe-like conduit connects them from below. In shape they resemble the surface domes of a composite volcano, but they sit at much greater depths. A laccolith can be thought of as a localised source of lava that finds its way to the surface. The Karnataka plateau offers a fine natural example: it is dotted with domal hills of granite, most of them now exfoliated, which are examples of laccoliths or batholiths.

Lapolith, Phacolith and Sills. As lava moves upward, a portion of it may shift sideways wherever it finds a weak plane in the rock. The shape it takes depends on that plane. If it develops into a saucer shape — concave to the sky — it is called a lapolith. A wavy mass of intrusive rock found at the base of synclines or at the top of anticlines in folded igneous country is called a phacolith; such wavy bodies have a definite conduit to a magma source beneath, which subsequently develops into a batholith. Finally, near-horizontal bodies of intrusive igneous rock are called sills or sheets, depending on thickness — the thinner ones are sheets, the thicker ones are sills. …