The physical characteristic that is used to distinguish intrusive igneous rocks from extrusive igneous rocks is grain size
One of the three primary rock types, along with sedimentary and metamorphic rocks, is igneous rock, which is derived from the Latin word ignis, which means "fire." When lava or magma cools and solidifies, igneous rock is created.
The partial melting of pre-existing materials in the mantle or crust of a planet can produce the magma. Usually, one or more of the following three processes—an increase in temperature, a drop in pressure, or a change in composition—lead to the melting. Either underground as intrusive rocks or above the surface as extrusive rocks, solidification into rock happens. Natural glasses can arise from igneous rock without crystallization, or granular, crystalline rocks can form from it.
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