Trap rock is a dark, fine-grained, and exceptionally durable material that serves as a foundation for much of modern infrastructure. It is a highly valued crushed stone commodity in the construction industry, prized for its strength and resistance to wear. The name originates from the Swedish word trappa, which translates to “staircase” or “step”. This geological naming convention refers to the characteristic step-like landscape formations created when layers of the rock erode at different rates.
Defining Trap Rock: Geological Classification and Composition
The term “trap rock” is a commercial or field name, rather than a precise geological classification. It is an umbrella term describing several types of dark-colored, fine-grained igneous rocks mechanically suitable for use as crushed stone. The most common rock types included are basalt, diabase, and sometimes gabbro or peridotite. These rocks are classified as mafic, meaning they are rich in magnesium and iron, which accounts for their typically dark color, ranging from black to dark gray or greenish-black.
The composition is dominated by silicate minerals, primarily plagioclase feldspar and pyroxene. Diabase, a common type of trap rock, often features a specific texture where tiny, lath-shaped plagioclase crystals are embedded within a pyroxene matrix. Geologists often refer to these fine-grained rocks as aphanites because the individual mineral crystals are too small to be seen without a microscope.
How Trap Rock Forms
Trap rock is formed through the cooling and solidification of molten magma or lava. The specific texture and mineral grain size depend on the rate at which the magma cools. When the molten material erupts onto the Earth’s surface as lava, it cools very rapidly, forming extrusive trap rock like basalt, which has an extremely fine-grained, or aphanitic, texture. Conversely, when the magma cools slowly beneath the surface, it forms intrusive trap rock like diabase or gabbro.
These intrusive formations occur when magma forces its way into cracks and layers of pre-existing rock. When the magma cuts across rock layers in a sheet-like body, it forms a dike. When it intrudes parallel to the existing layers, it forms a sill. The Palisades Sill in New Jersey is a famous example of this intrusive diabase formation, showcasing the columnar jointing that often occurs during slow cooling.
Essential Physical Characteristics
Trap rock possesses several exceptional physical properties that make it commercially valuable. Its high density is a direct result of its mafic composition, including heavy elements like iron and magnesium, making it significantly heavier than many common construction aggregates. The crystalline structure consists of tightly interlocking mineral grains, which minimizes pore space and contributes to low porosity and high internal strength.
This low porosity makes trap rock highly resistant to freeze-thaw cycles and chemical weathering, as water and corrosive agents have difficulty penetrating the material. Mechanically, trap rock exhibits high compressive strength, meaning it can withstand tremendous pressure without fracturing. It also possesses superior resistance to abrasion and wear, necessary for materials used in high-traffic applications.
Common Industrial Applications
Trap rock is a primary choice for various civil engineering and construction applications. Its most widespread use is as crushed stone aggregate, incorporated into concrete and asphalt mixtures. When used in high-strength concrete, the rock’s hardness allows the concrete to achieve strengths exceeding 15,000 pounds per square inch, necessary for massive structures like commercial parking decks.
The rock’s resistance to abrasion and high stability also makes it the preferred material for railway ballast. Laid beneath railroad ties, the sharp, angular fragments interlock to provide stable support, ensure efficient water drainage, and prevent vegetation growth. It is also used extensively as a road base material beneath pavement layers. Furthermore, large, irregular pieces are used as riprap, a form of erosion control placed along shorelines, riverbanks, and embankments to resist the scouring forces of moving water.