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Batholiths and Plutons


Q: I am getting really confused about Batholiths and Plutons as different sources seem to suggest different things. Is Dartmoor part of the Cornubian Batholith or is it the actual Cornubian Batholith? Is this then part of a larger Pluton or is the same as a Pluton?

From Thomas Sturges-Allard (August 2010)

Reply by Dr Ted Nield

A “pluton” is any large igneous body that has congealed from magma underground. There are many sorts of pluton – including the lens-llike and subhorizontal laccoliths and lopoliths, and the vertical or near-vertical sided stocks and batholiths. All these categories of pluton are defined on their overall apparent shape and relationship to the country rock. A batholith is the largest of the pluton types and by definition cover at least 100 square kilometres. A stock is a small discordant pluton, shaped like a batholith but falling below the necessary 100 square km in extent.

The Dartmoor Batholith originated in the Variscan orogeny 280-270 million years ago. Various parts of it crop out today according to the erosion pattern – on Dartmoor itself, but also on Bodmin Moor, and Land’s End and the Scilly Isles. All these bodies join up at depth into one giant batholith. This appears to have taken the form of a thick, sheet-like structure with an uneven surface, intruded towards the north. Its thicker regions, projecting nearer to the surface, are exposed at the present day land surface by erosion, so giving us the typical moorland topographies of the South West of England.