Igneous rocks are formed when molten material solidifies. This molten rock is known as magma beneath the surface and lava when it erupts. Geologists categorize these rocks into two primary groups based on where solidification occurs: intrusive and extrusive. Distinguishing between them is straightforward, relying primarily on observable physical characteristics influenced by the location and speed of cooling.
The Fundamental Difference Cooling Time and Location
Intrusive igneous rocks, also known as plutonic rocks, crystallize from magma deep within the Earth’s crust. This depth provides a high-pressure, insulated environment, allowing the material to cool very slowly over thousands to millions of years. This prolonged cooling causes the coarse-grained appearance of these rocks.
Extrusive igneous rocks, or volcanic rocks, form when lava solidifies on or very near the Earth’s surface. Exposure to the atmosphere or water causes the lava to cool rapidly, sometimes almost instantly. This rapid heat loss prevents atoms from organizing into large structures, resulting in a fine-grained texture.
Crystal Size The Key to Identification
Identification involves examining the size of the mineral crystals, known as texture. Intrusive rocks, due to their slow cooling, develop a phaneritic texture. This means their mineral grains are large enough to be seen easily with the unaided eye. The slow cooling provides time for significant crystal growth. Granite, which displays interlocking, visible crystals of quartz, feldspar, and mica, is a common example of a phaneritic, intrusive rock.
Extrusive rocks exhibit an aphanitic texture, where individual mineral crystals are microscopically small and cannot be distinguished without magnification. This fine-grained appearance results directly from rapid cooling at the surface. The short cooling time prevents crystals from growing larger than about half a millimeter. Basalt, a dark, dense rock forming most oceanic crust, is the most common example of an aphanitic extrusive rock.
Distinct Features of Extrusive Rocks
Beyond general fine-grained texture, several unique features mark a rock as extrusive.
Glassy Texture
Glassy texture forms when lava cools so rapidly that no crystals have time to form. The atoms are instantly frozen in a disordered state, resulting in volcanic glass like obsidian.
Vesicular Texture
Vesicular texture refers to a rock pitted with numerous small holes, called vesicles. These cavities are created by gas bubbles dissolved in the magma that expand as the lava reaches the low-pressure surface. If the lava solidifies before the gas escapes, the bubbles are trapped, creating porous rocks such as scoria or the lightweight pumice.
Flow Banding
In some extrusive rocks, a layered pattern known as flow banding may be visible. This structure forms as the lava moves and deforms, aligning crystals and glass into parallel bands near the surface of the flow.