A ring dike or ring dyke is an intrusive

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

body that is circular, oval or arcuate in plan and has steep contacts. While the widths of ring dikes differ, they can be up to several thousand meters. The most commonly accepted method of ring dike formation is directly related to collapse

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...

s.Gudmundsson, Agust. "Formation of collapse calderas." ''Geology'' 16.9 (1988): 808–10.

Caldera collapse and ring dike formation

Collapse calderas form due to the emptying of a

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

. Effusive eruptions that take place on the flanks of the associated

volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

and a

fissure A fissure is a long, narrow crack opening along the surface of Earth. The term is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in Earth's crust, on ice sheets and glaciers, and on volcanoes. Ground fissure A ...

system that direct

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

away from the chamber are both mechanisms that can empty a magma chamber. As pressure in the magma chamber changes, an increase in tensile stresses create tension fractures at the surface of the

. The geometry of the top of the magma chamber dictates the location and magnitude of the tension fractures. In addition, it was found that the higher the radius to depth ratio of the magma chamber, the higher the probability of forming a collapse

. Once a tension threshold is approached, the roof of the magma chamber collapses in on itself, and is known as cauldron subsidence.Blatt, Harvey, Robert Tracy, and Brent Owens. ''Petrology: igneous, sedimentary, and metamorphic''. Macmillan, 2006. Tension fractures extend deeper in the profile and shear fractures or dip-slip faults form in a circular pattern around the caldera and are known as ring faults. Ring faults can be either vertical or steeply dipping faults. When they are inward dipping, they are known as

normal fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

s and when they are outward dipping they are known as

reverse fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic f ...

s. Ring faults then allow magma to rise through the

fractures Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...

, forming a ring dike. These dikes can form as a direct result of collapse caldera formation, or through many injections around the ring fault over time. The magma of a ring dike is typically composed of acidic or intermediate composition due to the less dense melt that exists at the top of the magma chamber.

Another mechanism of ring dike formation

It has been hypothesized that ring dikes may form when inclined sheets are captured within a ring fault system, which cause them to act as feeder dikes. The deflection of the sheets may be caused by the difference in material properties between and within the fault zone.

Implications

Whether or not a caldera ring fault dips inward or outward from the center of subsidence is a highly contentious issue. Ring faults near the surface are subject to

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

and

mass wasting Mass wasting, also known as mass movement, is a general term for the movement of rock (geology), rock or soil down slopes under the force of gravity. It differs from other processes of erosion in that the debris transported by mass wasting is no ...

, changing the morphology of the caldera walls and making it difficult to tell the dip of the fault at formation.

Well known examples

Ossipee ring-dike complex

About 36 ring dikes have been found in the Ossipee Mountains in

New Hampshire New Hampshire ( ) is a U.S. state, state in the New England region of the Northeastern United States. It borders Massachusetts to the south, Vermont to the west, Maine and the Gulf of Maine to the east, and the Canadian province of Quebec t ...

Caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...

subsidence appears to be the mechanism leading to the formation of some of the ring dikes, but not all of them. The composition ranges from

monzonite Monzonite is an igneous intrusive rock, formed by slow cooling of underground magma that has a moderate silica content and is enriched in alkali metal oxides. Monzonite is composed mostly of plagioclase and alkali feldspar. Syenodiorite is an ...

to quartz

syenite Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). It is considered a Loch Bà ring dike, found on the

Isle of Mull The Isle of Mull or simply Mull ( ) is the second-largest island of the Inner Hebrides (after Skye) and lies off the west coast of Scotland in the Council areas of Scotland, council area of Argyll and Bute. Covering , Mull is the fourth-lar ...

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

, serves as a classic example of a well formed ring dike. This intrusive body forms an oval and its diameter can be measured at roughly 5.8 km by 8.5 km. The width of the dike varies throughout the profile, with a maximum width of approximately 300 meters. The composition varies from

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

felsite Felsite is a very fine-grained volcanic rock that may or may not contain larger crystals. Felsite is a field term for a light-colored rock that typically requires petrographic examination or chemical analysis for more precise definition. Color ...

, with

phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...

s of

alkali feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocla ...

and

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

minerals.

References

{{reflist Volcanology Igneous petrology he:מחדר פלוטוני#מחדר טבעתי