Trap rock is not a rock, but it is a broad term used to define dark-colored igneous rocks used to make crushed stone for construction. A general term makes it easy to work with rocks. Moreover, rock names are difficult. A layman or a homeowner cannot remember rock names. Hence, the term trap rock comes in handy. Trap rock is also used when the exact mineral composition or identity of the rock is unimportant or unknown.

Trap rock is also known as trapp or trap. The rocks that come under the umbrella of trap rock are non-granitic, dark-colored, fine-grained, intrusive, or extrusive igneous rocks.

Trap Rock – Name Origin

The word Trap Rock came from a Swedish word, “trappa,” meaning stair step. It refers to the stair-like landscape in areas with basaltic flows and shallow intrusions outcrop to form steep cliffs and narrow ledges.

This landscape is found in parts of New York, New Jersey, Washington, Oregon, and Idaho.

Types of Trap Rock

Basalt

Basalt is a volcanic igneous and the most commonly found rock on earth. It is a dark-colored rock with a fine-grained outlook. Basalt is usually black, dark green, or brown. Basalt and gabbro have similar compositions, but gabbro is a coarse-grained rock.

Peridotite

Peridotite is referred to as a group of dense and coarse-grained ultramafic rocks. The group contains kimberlite, chromite, or diamonds. Peridotite is the dominant rock of the upper part of the earth’s mantle, representing the earth’s mantle than the crust.

Peridotite is subdivided into 5 major categories based on mineral content. Most peridotite rocks are green in color due to their high olivine content.

Diabase

Diabase is a dark-colored fine-grained, intrusive igneous rock. Diabase is also known as dolerite in the United Kingdom and other parts of the world. Diabase’s mineral composition is similar to basalt and gabbro. However, all three rocks have a different grain sizes owing to the difference in cooling rates.

Gabbro

Gabbro is a black or dark-green intrusive igneous rock. The term Gabbro was first used in the 1760s to name a newly found rock in the ophiolites of the Apennine Mountains in Italy. It was named after a Gabbro, a hamlet near Rosignano Marittimo in Tuscany, Italy.

Gabbro and basalt have similar compositions but different grain sizes.

Where is Trap Rock found?

Trap rock is found all over the world. There are huge deposits in Europe, Asia, China, and Russia.

In the US, trap rock is only produced in areas where rocks are present on the surface. Almost 85% of trap rock produced in the US comes from 10 states.

Connecticut, New Jersey, North Carolina, Pennsylvania, Massachusetts, and Virginia are the biggest contributors to track rock production. Trap rock in these states is mostly produced from Triassic Basin flood basalts.

Columbia River basalts in Washington and Oregon account for 50% of the crushed stone. Most of the crushed stone used in New Jersey comes from Palisades Sill.

Trap Rock Uses

• Crushed basalt is used for road and housing construction.
• Trap rock is used as railroad ballast because it is insensitive to chemicals and mechanical stress. Trap rock has a high-dry relative density, frost resistance, and seawater resistance.
• It is used as a hydraulic engineering rock (riprap) in coast protection.
• Trap rock is used to produce cast rock for corrosion and abrasion protection.
• It is also used to make sewage pipes and acid-resistant rocks.
• Trap rock is frequently used in gardening and landscaping.
• Trap rock is best for making millstones, mineral fibers (basalt wool), flux in ceramic masses and gazes, making glass ceramics, and filter beds for water treatment.
• It has been used to build churches. A famous church, Trinity Church on the Green, in New Haven, Connecticut, is a colorful example of red, orange, and brown colored natural-faced trap rock.
• Trap rock was also used for foundations and railroad beds in the New Heaven area.