What is a dike (dyke)?
A body of rock, either sedimentary or igneous, that cuts across the layers of its surroundings. They form in pre-existing fractures, meaning that dikes are always younger than the body of rock that they have intruded into.
Dikes are normally very easy to find when looking at an outcrop. For starters, they intrude the rock at a relatively vertical angle. They also have a completely different composition than the surrounding rock, giving them unique textures and colors.
What is a ring dike?
Ring dikes are intrusive igneous sheets that are circular, oval or arcuate in overall trend. They form most commonly from caldera collapse. When a shallow magma chamber empties its contents and releases pressure, its roof often collapses into the voided reservoir. Where the roof collapses, it forms dip-slip faults that are nearly vertical or steeply sloping. Magma can then rise up through these fractures, cooling as dikes that make up the outer edge of a collapsed caldera.
What Are Dikes and How Do They Form? Andre Alden, ThoughtCo., 2/11/2019
Examples of ring dikes
- One of the best exposed and studied ring dikes is the Loch Bà ring dike, Isle of Mull, Scotland
- The ring dikes in the Pawtuckaway State Park and the Ossipee Mountains of New Hampshire.
- The Pilanesberg Game Reserve in the North West Province of South Africa
- The Ring of Gullion in southern County Armagh, Northern Ireland
Ring Dikes in New Hampshire
Ossipee Mountains ring dike complex
Aerial photo of the Ossipee Mountains.
Geology of the Ossipee Mountains ring dike
Alex MacPhail writes “…This is a recent geologic diagram of the Ossipee Ring (Dike) Complex from a paper written and presented by G. Nelson Eby, University of Massachusetts, Lowell, MA, and Ben Kennedy, McGill University, Montreal, QC, Canada. In their introduction they state that “Ossippee has played a central role in models dealing with the origin of ring dikes (Billings, 143, 1945; Chapman, 1976) and has long been considered a classic example of a ring-dike complex.” They also state that the ring dike is part of the Younger White Mountain igneous province and between 130-100 million years old. In this province, in addition to the Ossipee Ring Complex, they write that “the largest intrusive complex is the White Mountain batholith which consists of multiple ring dikes intruded into by composite plutons.””
Belknap Dike in New Hampshire
This ring dike is harder to see, because part of it is under Lake Winnipesaukee.
Percy area ring dike
Franconia quadrangle ring dyke
How do calderas form?