Dike (geology)

It is formed when magma fills a fracture in the older beds and then cools and solidifies.

The texture is typically slightly coarser than basalt erupted at the surface, forming a rock type called diabase.

Shallow dikes also typically show columnar jointing perpendicular to the margins.

When vesicles are present, they tend to form bands parallel to walls and are elongated in direction of flow.

[13] Likewise, phenocrysts (larger crystals) on the margins of the dike show an alignment in the direction of flow.

[4] At the shallowest depths, dikes form when magma rises into an existing fissure.

[13][3] In the young, shallow dikes of the Hawaiian Islands, there is no indication of forceful intrusion of magma.

Magma deeper in the crust must force its way through the rock, always opening a path along a plane normal to the minimum principal stress.

Even if the magma is only slightly pressurized compared with the surrounding rock, tremendous stress is concentrated on the tip of the propagating fracture.

In effect, the magma wedges apart the brittle rock in a process called hydraulic fracture.

At the greatest depths, the shear planes become ductile faults, angled 45 degree from the sides of the dike.

At depths where the rock is completely plastic, a diapir (a rising plug of magma) forms instead of a dike.

This tendency of intruding magma to form shorter fissures at shallower depths has been put forward as an explanation of en echelon dikes.

[11] However, en echelon dikes have also been explained as a consequence of the direction of minimum principal stress changing as the magma ascends from deep to shallow levels in the crust.

In ancient dikes in deformed rock, the bridges and horns are used by geologists to determine the direction of magma flow.

However, the length of erupting fissure diminishes over time, becoming focused on a short segment of less than half a kilometer.

[16] The minimum possible width of a dike is determined by the balance between magma movement and cooling.

Dike swarms are almost always composed of diabase and most often are associated with flood basalts of large igneous provinces.

Dike swarms are forming in the present day along the divergent plate boundary running through Iceland.

The dike swarms extend radially out from volcano summits and parallel to the long axis of the volcanic shield.

It is likely that the number of dikes must increase with depth, reaching a typical value of 300 to 350 per kilometer at the level of the ocean floor.

In some respects, these dike swarms resemble those of western Scotland associated with the flood eruptions that preceded the opening of the Atlantic Ocean.

[7] In rock of the oceanic crust, pillow lava erupted onto the sea floor is underlain by sheeted dike complexes that preserve the conduits through which magma reached the ocean floor at mid-ocean ridges.

[21] The fractures take the form of a set of concentric cones dipping at a relatively shallow angle into the magma chamber.

[3][13] When the caldera is subsequently emptied by explosive volcanic activity, the roof of the magma chamber collapses as a plug of rock surrounded by a ring fracture.

[21][22] Good examples of ring dikes and cone sheets are found in the Ardnamurchan peninsula of Scotland.

For example, the Muskox intrusion in arctic Canada was fed by a large dike, with a thickness of 150 meters.