Ring Dikes: Formation, Mechanisms, and Geological Examples

In the study of igneous geology, a ring dike is a specialized intrusive igneous body characterized by a circular, oval, or arcuate (curved) plan with steep contacts. These structures can vary significantly in size, with some reaching widths of several thousand meters. While they appear as distinct rings of rock on the surface or in cross-section, their origin is deeply tied to the violent and complex processes of volcanic collapse.

Key Facts

Shape: Circular, oval, or arcuate in plan view.
Primary Cause: Most commonly formed through the process of collapse calderas.
Composition: Typically acidic or intermediate, as they originate from the less dense melt at the top of a magma chamber.
Structure: Formed when magma fills ring faults created during cauldron subsidence.
Scale: Widths can extend up to several thousand meters.

Caldera Collapse and Ring Dike Formation

The most widely accepted theory for the creation of ring dikes is the formation of a collapse caldera—a large cauldron-like hollow that forms shortly after the emptying of a magma chamber. This emptying can occur through effusive eruptions on the volcano's flanks or via a fissure system that diverts magma away from the central chamber.

As the magma chamber empties, the internal pressure changes, leading to increased tensile stresses. This creates tension fractures on the volcano's surface. The specific location and size of these fractures are determined by the geometry of the magma chamber's roof; specifically, a higher radius-to-depth ratio increases the likelihood of a caldera collapse.

When the tension reaches a critical threshold, the roof of the magma chamber collapses inward, a process known as cauldron subsidence. This collapse extends tension fractures deeper into the earth, creating circular shear fractures or dip-slip faults called ring faults. These faults can be vertical or steeply dipping. If they dip inward, they are classified as normal faults; if they dip outward, they are reverse faults.

These ring faults serve as conduits, allowing magma to rise and fill the fractures. A ring dike may form as a single result of the collapse or through multiple injections of magma into the ring fault over an extended period.

The Pilanesberg Ring Dike Complex in South Africa

Alternative Mechanisms of Formation

While caldera collapse is the primary driver, some geologists hypothesize an alternative mechanism involving inclined sheets. In this scenario, inclined sheets of magma may be captured within a ring fault system, causing them to function as feeder dikes. This deflection is believed to be caused by differences in material properties between the fault zone and the surrounding rock.

Geological Implications and Challenges

A point of significant debate among geologists is whether a caldera ring fault dips inward or outward from the center of subsidence. Determining the original dip is difficult because ring faults near the surface are subject to mass wasting (the downward movement of rock and soil) and erosion, which alter the morphology of the caldera walls over time.

Questa Caldera ring dike, exposed in the valley of the Red River, New Mexico, US

Notable Examples of Ring Dikes

Across the globe, several sites provide critical insight into how these structures form and evolve:

Loch Bà Ring Dike (Scotland): Located on the Isle of Mull, this is a classic example of a well-formed ring dike. It is oval-shaped, measuring roughly 5.8 km by 8.5 km, with a maximum width of 300 meters. Its composition ranges from rhyolite to felsite, containing phenocrysts of mafic minerals and alkali feldspar.
Pilanesberg Alkaline Ring Complex (South Africa): One of the largest alkaline ring complexes in southern Africa, this site features multiple concentric intrusive phases. It is a key study site for Proterozoic intraplate magmatism.
Ossipee Ring-Dike Complex (USA): Found in the Ossipee Mountains of New Hampshire, this complex contains about 36 ring dikes. While some were formed by caldera subsidence, others may have resulted from different mechanisms. The rock composition here ranges from monzonite to quartz syenite.

Location	Key Characteristics	Composition
Loch Bà, Scotland	Oval shape, 5.8 x 8.5 km diameter	Rhyolite to Felsite
Pilanesberg, South Africa	Multiple concentric intrusive phases	Alkaline igneous rocks
Ossipee, New Hampshire	Approximately 36 ring dikes	Monzonite to Quartz Syenite

Frequently Asked Questions

What is the difference between a ring fault and a ring dike?

A ring fault is the actual fracture or crack in the earth's crust created during the collapse of a magma chamber. A ring dike is the resulting body of igneous rock that forms when magma fills that ring fault and solidifies.

Why are ring dikes usually composed of acidic or intermediate magma?

This is because the magma that forms ring dikes typically comes from the top of the magma chamber, where the melt is less dense and more chemically evolved (acidic or intermediate) compared to the denser magma at the bottom.

What is cauldron subsidence?

Cauldron subsidence is the process where the roof of a magma chamber collapses inward after the chamber has been emptied by eruptions or magma diversion, leading to the formation of a caldera.

Can ring dikes form without a caldera collapse?

While most are related to calderas, some hypotheses suggest they can form when inclined sheets of magma are captured and deflected by a ring fault system due to differing material properties.

How do geologists determine the dip of a ring fault?

Geologists analyze the angle at which the fault slopes. However, this is often difficult because erosion and mass wasting change the shape of the caldera walls over millions of years.