BATHOLITHS: THE QUEENS CROWNING RANCHI

Despite sounding like something out of Harry Potter, a batholith is a type of igneous rock that forms when magma rises into the earth’s crust but does not erupt onto the surface. The magma cools beneath the earth’s surface, forming a rock structure that extends at least one hundred square kilometres across, and extends to an unknown depth. The term “bath” comes from the Greek for “deep,” and “lith”, also from Greek, means “rock.”

If you come across an outcrop or exposed form of coarse-grained igneous rock, chances are you are standing on a batholith that crystallized several kilometres below the Earth’s surface. It may represent the magma chamber of an extinct volcano or a magma body that never produced any eruptions. It is a large body of igneous rock formed beneath the Earth’s surface by the intrusion and solidification of magma. It is commonly composed of coarse-grained rocks (e.g., granite or granodiorite) with a surface exposure of several hundred square kilometres.

A batholith has an irregular shape with sidewalls that incline steeply against the host rock. Most batholiths intrude across mountain folds and are elongated along the dominant axis of the range; faulting and contact metamorphism of the enveloping rock near the batholith is also observed. Although batholiths were once believed to extend to unknown depths, recent studies have shown that many of them have a thickness of perhaps 10 to 15 km (6 to 9 miles). A globally well-known batholith is located in the Chotanagpur Plateau near the edge of the historical Deccan lava that flows close to Ranchi.