Dike (geology)

A dike is a sheet of rock.

It forms in a crack in older rock. Hot, melted rock flows into the crack. Then the melted rock cools and gets hard. This creates a long wall of rock.

These walls can be very long. Some can stretch for many miles. Often, these walls stand out from the ground. This happens because they are strong. They do not wear away as fast as other rocks.

They can even form in large groups called swarms. These rock walls tell us how the Earth works.

A dike is a sheet of rock.

It forms inside a crack in older rock. Most dikes are magmatic. This means they form from magma. Magma is hot, melted rock. It flows into a crack and then cools. As it cools, it turns into solid rock.

These rock sheets can be very thin. Some are only a few millimeters wide. Other dikes are hundreds of meters thick. They are usually very long. Some stretch for over 100 kilometers.

Most dikes stand up straight. They look like tall, natural walls. This happens because the dike rock is strong. It does not wear away as fast as the rock around it. Geologists study dikes to learn about volcanoes. They also look at dike swarms. A swarm is a large group of dikes. They often form when the Earth's crust pulls apart. Some swarms are very big. The Mackenzie dike swarm in Canada is the largest.

Some dikes are clastic. This means they form when sediment fills a crack.

A dike is a sheet of rock that forms inside a crack in older rock.

Most dikes are magmatic, which means they come from magma. Magma is hot, melted rock found deep underground. When this liquid rock flows into a fracture, it eventually cools and turns into a solid sheet. Because dike rock is often harder than the rock around it, it resists weathering. This means wind and water do not wear it away as fast. Over time, erosion leaves the dike standing out as a natural wall or ridge.

The way a dike forms depends on how deep it is. At shallow depths, the pressurized magma acts like a wedge. It uses a process called hydraulic fracture to pry the brittle rock apart. This creates a path for the magma to move upward. At much greater depths, the rock is hotter and less brittle. There, the magma acts like a bulldozer to force the rock aside. Some very large dikes, like the 500-meter-thick Gardar dike in Greenland, might form through stoping. This happens when the magma breaks and destroys the rock at its tip.

Geologists study these rock sheets to learn about the Earth. Dikes act like a record of the cracks that let magma reach the surface. They help scientists understand volcanic plumbing systems. Dikes also show when the Earth's crust is being pulled apart by tectonic forces. When many dikes form at once, they are called a dike swarm. These swarms often show the direction the crust is stretching. This is because dikes form at right angles to the direction of the pull.

Dikes come in many different sizes and shapes. A dike can be as thin as a few millimeters. However, they are most often between one meter and a few tens of meters thick. Some dikes are incredibly long, stretching over 100 kilometers. In some places, dikes form in a pattern called en echelon. This means they appear as short, overlapping segments in a line. You can see this pattern in the Higganum dike set in New England. These segments are typically four kilometers long and 60 meters wide.

Some dikes are even more special because they contain different types of rock. A composite dike forms when more than one injection of magma enters the same crack. If the new magma is different from the first, it creates a mix of rocks. You can see this in dikes in Scotland and northern Ireland. There are also massive groups called dike swarms. The Mackenzie dike swarm in Canada is the largest in the world. In Iceland, dike swarms are still forming today along moving plates.

In geology, a dike is a sheet of rock that forms within a fracture of a pre-existing rock body.

These structures are vital for understanding how the Earth's crust behaves. Dikes can be magmatic or sedimentary in origin. Magmatic dikes occur when magma—molten rock found beneath the surface—flows into a crack and solidifies as a sheet intrusion. This intrusion can cut across existing layers of rock or move through a single solid mass. Clastic dikes are different because they form when sediment, rather than magma, fills a pre-existing crack.

Magmatic dikes are often made of igneous rock that cuts across older rock beds. Because the rock in a dike is usually more resistant to weathering than the surrounding material, erosion often leaves it standing out. This process exposes the dike as a natural wall or ridge, which is how these structures earned their name.

Geologists study these walls to understand volcanic plumbing systems. Dikes preserve a record of the fissures that allow mafic magma to reach the surface. Mafic magma is a fluid type of magma that is low in silica. By studying dikes, scientists can also track ancient episodes of crustal extension. When tectonic forces pull the Earth's crust apart, large numbers of dikes called dike swarms are formed. These swarms show the direction of extension because dikes form at right angles to the direction of maximum extension.

A dike's shape is defined by its dimensions. The thickness of a dike is much smaller than its other two dimensions. The opposite walls are roughly parallel, meaning the thickness stays fairly constant. While dikes can range from a few millimeters to hundreds of meters thick, they are most typically between one meter and a few tens of meters. Their lateral extent can reach tens of kilometers. Dikes that are tens of meters thick often extend for over 100 kilometers. Most dikes are steeply dipping, meaning they are oriented nearly vertically. However, later tectonic deformation can rotate the rock layers so that a dike appears horizontal.

The mechanism of formation changes based on the depth at which the magma resides. At shallow depths, the rock is brittle. Here, pressurized magma uses a process called hydraulic fracture to wedge the rock apart. This happens because tremendous stress concentrates at the tip of the moving fracture. At greater depths, the rock becomes hotter and less brittle. In these areas, the magma acts like a bulldozer, forcing the rock aside along shear planes. These planes are oriented 35 degrees to the sides of the dike. At the greatest depths, where the rock is completely plastic, a rising plug of magma called a diapir forms instead of a dike.

Some dikes form through unique processes like stoping. In stoping, the magma fractures and disintegrates the rock at its advancing tip rather than prying it apart. This can create very large dikes, such as the 500-meter-thick Gardar dike in Greenland. Other dikes may form through metasomatism. This occurs when fluids moving along a narrow fissure change the chemical composition of the rock closest to the crack. The width and length of a dike are also related by a mathematical formula. This formula predicts that dikes will be longer and narrower at greater depths. For example, a 10-meter-thick dike near the surface might extend 3 km, but at depth, it could extend 30 km.

Dikes can also appear in complex patterns or compositions. An en echelon dike set consists of short, overlapping segments that form a line.

This pattern is seen in the Higganum dike set in New England, where segments are typically four kilometers long and 60 meters wide. These segments form a group extending around 10 km, though the entire set can extend for 250 km. Additionally, a composite dike forms when multiple injections of magma occur in the same fissure. If the second injection has a different composition than the first, it creates a mix of rocks.

Some dikes range in composition from basaltic to rhyolitic, but most are basaltic. These often form a rock type called diabase, which has a slightly coarser texture than basalt erupted at the surface.

Finally, dikes can appear in massive groups known as dike swarms. These swarms consist of several to hundreds of dikes placed during a single intrusive event. They are often associated with large igneous provinces and divergent plate boundaries.

The Mackenzie dike swarm in Canada is the largest in the world. In Iceland, dike swarms are forming today along moving plates. In some areas, the density of dikes is very high. In the center of a rift zone, there may be 50 to 100 dikes per kilometer. At the ocean floor, this density can reach 300 to 350 dikes per kilometer.