Dolerite is a widespread, dark-colored igneous rock that forms a significant component of the Earth’s crust. It is often encountered in large, sheet-like formations that cut across existing geology. This medium-grained material is a valuable resource that has been used in construction for centuries, making it relevant to both geologists and engineers.
Defining Dolerite: Classification and Identity
Dolerite is an igneous rock classified as mafic, meaning it is rich in magnesium and iron, which results in its characteristic dark coloration. Chemically, it is similar to basalt and gabbro, but its classification is based on its formation environment.
The rock is further classified as hypabyssal or subvolcanic, meaning it formed beneath the Earth’s surface at a relatively shallow depth. This intermediate depth of formation determines the rock’s final texture and grain size. The term dolerite is the preferred name in much of the world, including the United Kingdom and Australia.
In North America, this same rock is commonly referred to as diabase, and the two names are considered synonyms for the identical material. Some geologists occasionally use the term microgabbro to avoid regional differences in nomenclature.
Physical and Mineral Composition
Dolerite is characterized by its medium-grained texture, where mineral crystals are generally visible without a microscope. The individual grains typically measure between 1 and 5 millimeters, a size intermediate between the fine grains of basalt and the coarse grains found in gabbro.
The rock is dark gray to black, a shade imparted by its high concentration of iron and magnesium-rich minerals. Its most defining textural feature is its ophitic or subophitic structure. This structure involves slender, lath-shaped crystals of plagioclase feldspar being partially or completely enclosed by larger, irregularly shaped crystals of pyroxene. This interlocking arrangement contributes to the rock’s toughness and density.
The primary mineral constituents are calcic plagioclase feldspar (such as labradorite or bytownite) and clinopyroxene, specifically the mineral augite. Plagioclase laths can constitute 40 to 70 percent of the rock’s volume. Minor amounts of olivine, magnetite, and ilmenite are also often present, contributing to the rock’s dark appearance and density.
Geological Occurrence and Formation
Dolerite forms when basaltic magma is injected into pre-existing rock layers but fails to reach the surface as a volcanic eruption. The magma solidifies at shallow depths, typically less than two kilometers down, after being forced through deep fractures in the crust. This location allows the magma to cool more slowly than surface lava flows (which form fine-grained basalt), but faster than the deep-seated intrusions that create coarse-grained gabbro.
The surrounding country rock acts as an insulator, moderating the cooling process and allowing the mineral crystals sufficient time to grow to a medium size. Dolerite is most commonly found in two distinct intrusive structures. It forms dikes, which are vertical or near-vertical sheet-like bodies that cut across the layering of the surrounding rock.
It also forms sills, which are horizontal sheets of rock injected parallel to the existing sedimentary or metamorphic layers. These intrusions can span hundreds of square kilometers and often resist erosion better than the surrounding rock, leading to distinctive landforms like escarpments and prominent ridges. Notable examples include the Palisades Sill in New Jersey and the vast intrusions across Tasmania.
Common Uses of Dolerite
The inherent properties of dolerite—its hardness, density, and resistance to weathering—make it a valuable resource for various commercial and industrial applications. Its durability allows it to be heavily used in the construction industry as a crushed aggregate material.
- It serves as a primary material in the production of asphalt pavement and concrete, providing strength and stability.
- Dolerite is widely used as railway ballast, the crushed stone spread beneath railway tracks to hold the ties in place and facilitate drainage.
- It is commonly used as a road sub-base material and for macadam surfaces.
The rock is sometimes quarried and cut into blocks to be used as dimension stone for architectural purposes, such as paving stones, curbs, and building facades. Its dark color and polished finish mean it is sometimes marketed commercially under the non-geological name of “black granite.”