A pluton is a body of intrusive igneous rock that forms when molten material, known as magma, solidifies deep within the Earth’s crust. This mass of rock crystallizes beneath the surface, rather than erupting as lava. Plutons are distinct because they cooled slowly, allowing for the formation of large, visible mineral grains. These subterranean rock bodies represent the solidified remnants of ancient magma chambers and conduits.
Formation of Plutons
The formation of a pluton begins with magma rising from the mantle or lower crust into cooler overlying rock layers. Unlike lava, which has erupted onto the surface, magma remains trapped at depth beneath a thick cover of existing rock. This surrounding rock acts as an effective insulator, preventing rapid heat loss from the molten intrusion.
Because the magma cools over thousands to millions of years, its constituent atoms have sufficient time to organize themselves into large, well-defined crystal structures. This extended crystallization process results in a coarse-grained texture known as phaneritic, where individual mineral crystals are easily seen. Common plutonic rocks include granite, diorite, and gabbro, which are chemically equivalent to the fine-grained volcanic rocks rhyolite, andesite, and basalt, respectively.
Classification of Plutonic Structures
Plutons are classified based on their shape, size, and their geometric relationship to the surrounding host rock, referred to as country rock. This relationship is defined as either concordant (parallel to the host rock’s existing layers) or discordant (cutting across them). Tabular, sheet-like intrusions are the most common forms and include both dikes and sills.
A dike is a discordant intrusion that cuts vertically or obliquely across the bedding planes or foliation of the surrounding rock. These sheet-like bodies form when magma is injected under pressure into existing fractures or cracks. Conversely, a sill is a concordant intrusion that forms when magma spreads horizontally, parallel to the layers of the host rock.
Larger, irregularly shaped plutons are classified as either stocks or batholiths. A stock is a massive, discordant body of intrusive rock exposed at the surface over an area less than 100 square kilometers. If the exposed surface area is greater than 100 square kilometers, it is categorized as a batholith. Batholiths are complex, composite structures formed from the coalescence of multiple smaller plutons.
Another unique form is the laccolith, a mushroom-shaped, concordant intrusion with a flat base. Magma injected between rock layers can be viscous, forcing the overlying strata to bulge upward into a dome shape without cutting across the layers.
Exposure and Erosion of Plutons
Although plutons form kilometers beneath the surface, they eventually become exposed through a process combining tectonic uplift and denudation. Tectonic forces slowly raise deep-seated rock bodies toward the surface over geologic time. Denudation, which encompasses the effects of weathering and erosion, then strips away the overlying rock layers.
Plutonic rocks like granite are resistant to chemical and physical weathering compared to the surrounding sedimentary or metamorphic rocks. Once the pluton is exposed, the less-resistant country rock erodes more quickly around it. Consequently, the pluton often remains as a prominent feature on the landscape.
This differential erosion is responsible for many dramatic geological formations, such as the rocky domes and mountain cores seen in places like the Sierra Nevada Batholith. The slow removal of the overlying rock unearths the massive, crystalline body, transforming a deep-crustal feature into an observable surface landform.