Ring dike · Lablypedia

Caldera collapse and ring dike formation

Collapse calderas form due to the emptying of a magma chamber. Effusive eruptions that take place on the flanks of the associated volcano and a fissure system that direct magma 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 volcano. 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 caldera.

Once a tension threshold is approached, the roof of the magma chamber collapses in on itself, and is known as cauldron subsidence. 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 faults and when they are outward dipping they are known as reverse faults. Ring faults then allow magma to rise through the fractures, 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.

3 more chapters are free with a Lably account.

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

Examples

Ossipee ring-dike complex

About 36 ring dikes have been found in the Ossipee Mountains in New Hampshire. Caldera 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 to quartz syenite.

Loch Bà ring dike

The Loch Bà ring dike, found on the Isle of Mull in Scotland, 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 to felsite, with phenocrysts of alkali feldspar and mafic minerals.

Pilanesberg Complex

The Pilanesberg Complex in South Africa is commonly described as an alkaline ring complex because of its circular outline and the concentric arrangement of its units. It is a Mesoproterozoic composite complex containing coeval volcanic rocks together with intrusive syenitic and foyaitic components. Uranium–lead dating of titanite gives an age of 1,395 ⁺¹⁰
₋₁₁ million years.

Although ring dike and cone sheet emplacement models have historically been applied to the complex, Cawthorn argued that its principal coarse-grained intrusive bodies, more than 1 km (0.62 mi) wide, are too large to be conventional ring dikes or cone sheets. The detailed three-dimensional geometry remains uncertain because of poor exposure.