The rock type formed from the slow cooling of magma beneath Earth’s surface is intrusive igneous rock , often called plutonic rock.

Quick Scoop

  • When magma cools slowly underground, it forms intrusive igneous rocks with large, visible mineral crystals.
  • Granite is a classic example of this kind of rock, formed deep in the crust from very slow cooling.

In short: slow cooling under the surface → intrusive igneous (plutonic) rock.

TL;DR: The answer to “what rock type is formed from the slow cooling of magma under the earth's surface?” is intrusive igneous rock (plutonic rock).

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