Earth’s crust is a dynamic system, constantly shaped by processes on and beneath its surface. Among these geological formations are plutons, masses of rock that solidify deep within the Earth. Understanding these subterranean structures offers insight into the planet’s internal workings and the forces that build continents.
Defining a Pluton
A pluton is a body of intrusive igneous rock that forms when molten rock, known as magma, cools and crystallizes far beneath the Earth’s surface. Intrusive means magma pushed into pre-existing rock layers without reaching the surface; igneous refers to rocks formed from solidified molten material. Plutons vary in size, shape, and orientation, from small, irregular bodies to massive structures. Although they form deep underground, plutons can become visible at the surface over vast stretches of geological time as overlying rock layers are removed by erosion.
How Plutons Form
Pluton formation begins with magma originating deep within Earth’s mantle or lower crust. This molten material is less dense than surrounding solid rock, causing it to rise. As magma ascends, it intrudes into cracks and weaknesses in Earth’s crust, or it can push aside and melt surrounding “country rock.” This process occurs at depths several kilometers below the surface.
Once emplaced, magma within a pluton cools and solidifies slowly due to the insulating effect of surrounding rock. This cooling can take thousands to millions of years, depending on the size and depth of the intrusion. The slow cooling allows mineral crystals within the solidifying magma to grow large. This results in the coarse-grained texture of plutonic rocks, with individual mineral grains visible.
Common Types and Features of Plutons
Plutons exhibit various forms, classified by shape, size, and relationship to surrounding rock layers. One of the largest types is a batholith, an irregularly shaped intrusive body with an exposed surface area exceeding 100 square kilometers. These structures are composite, formed from the coalescence of multiple smaller intrusions. Smaller, irregularly shaped plutons with an exposed surface area less than 100 square kilometers are called stocks.
Other forms include tabular intrusions, which are sheet-like bodies. Dikes are discordant plutons that cut across existing rock layers. In contrast, sills are concordant, forming when magma intrudes parallel to the bedding planes of the surrounding rock. Laccoliths are similar to sills but are lens-shaped, causing the overlying rock layers to bulge upward due to the viscosity of the magma.
The rocks composing plutons are primarily coarse-grained igneous rocks such as granite, granodiorite, diorite, and gabbro. Granite, a common plutonic rock, consists mainly of quartz and feldspar, with smaller amounts of mica and amphibole. Granodiorite is similar but contains more plagioclase than alkali feldspar. Gabbro, formed from mafic magmas, is darker and composed of minerals like pyroxene, olivine, and plagioclase feldspar.
Geological Significance of Plutons
Plutons play a role in Earth’s geological history and landscape. They represent the solidified “roots” of ancient mountain ranges, exposed after uplift and erosion. For instance, many large mountain belts, such as the Sierra Nevada in California, are underlain by vast batholiths. The presence and study of plutons provide scientists with information about past tectonic activity and the evolution of continental crust.
These subsurface intrusions are associated with mineral deposits. As magma cools and crystallizes, certain elements can become concentrated, leading to the formation of ore bodies. Plutons can be sources of metals like gold, copper, tin, and gemstones. Fluids released during pluton emplacement can also carry and deposit minerals in surrounding rocks.