In geological terms, what does 'intrusive' refer to?

'Intrusive' refers to igneous rock that forms below the Earth’s surface. This type of rock, also known as plutonic rock, forms when magma cools and solidifies slowly beneath the Earth's crust. The slow cooling process allows for the development of large mineral crystals, which is a characteristic feature of intrusive igneous rocks. Common examples include granite and diorite.

In contrast, igneous rock that forms on the Earth’s surface is termed 'extrusive.' While metamorphic rocks are formed from existing rocks that undergo a transformation due to heat, pressure, or chemically active fluids, they do not fall under the definition of intrusive igneous rocks. Volcanic ash, also not classified as intrusive, is a product of explosive volcanic eruptions that can accumulate on the surface. Thus, the correct definition of 'intrusive' is specifically linked to the formation process and location of igneous rocks beneath the surface.