Intrusive igneous rocks form when magma, which is molten rock, cools and solidifies beneath the Earth’s surface. Igneous rocks are separated into two categories: extrusive, which cool quickly on the surface, and intrusive, which cool slowly deep within the crust. This slow crystallization process under unique subsurface conditions determines the rock’s overall structure and visible texture.
Defining the Subsurface Environment
Intrusive igneous rocks form deep within the Earth’s crust, away from surface conditions. They are commonly referred to as plutonic rocks, named after Pluto, the Roman god of the underworld. The magma originates from various sources, such as the melting of existing crustal material or the rising of molten material from the Earth’s mantle.
The formation environment is characterized by immense pressure and high surrounding temperatures. Deep burial places the magma under lithostatic pressure, which keeps volatile components dissolved in the melt. The pre-existing rock surrounding the magma body, known as the country rock, acts as an effective insulator. This insulation dictates the subsequent cooling process and the final rock properties.
The Process of Slow Crystallization
The insulating country rock prevents the magma from losing heat rapidly, resulting in a slow cooling process that spans thousands to millions of years. This prolonged cooling time is the defining characteristic of intrusive rock formation. The slow rate allows atoms and ions within the molten material ample opportunity to move and bond together.
Over geologic timescales, mineral components crystallize in a specific order based on their chemical composition and melting points. This process results in the growth of large, interconnected mineral crystals visible to the unaided eye. This coarse-grained appearance is known as a phaneritic texture, which is the hallmark of intrusive rocks. Common examples include granite, gabbro, and diorite.
Classifying Intrusive Rock Structures
The solidified bodies of intrusive igneous rock are collectively known as plutons. These structures are classified based on their size, shape, and relationship to the surrounding country rock. Plutons that lie parallel to the existing layers of the country rock are termed concordant, while those that cut across the existing layers are considered discordant.
Major Pluton Types
One of the largest discordant plutons is the batholith, an enormous, irregularly shaped body with a surface exposure area greater than 100 square kilometers. Stocks are smaller versions, covering less than 100 square kilometers. Tabular, sheet-like plutons are also common. Sills are concordant bodies that form when magma intrudes between existing layers. Dikes are discordant, vertical, or steeply angled sheets of rock that solidify after cutting across the country rock layers.