Structural Geology - Lesson 4 - Joints & Veins - Part 1 of 2

Summary

In this lesson on structural geology, the focus is on joints and veins, key features in brittle deformation. Joints are natural fractures with no shear displacement, while veins are mineral-filled fractures. The video explores the morphology and terminology of these features, such as plumo structures, which form due to stress propagation along fracture planes. It also introduces joint sets, systems, and classifications, highlighting systematic and nonsystematic joints, along with examples like columnar jointing in intrusions and exfoliation in plutons. The lesson encourages open-mindedness about the formation and interpretation of these geological structures.

Takeaways

• 😀 Joints are natural fractures in rocks where no shear displacement occurs along the fracture plane.
• 😀 Veins are mineral deposits that fill fractures as a result of water solution traveling through the fractures.
• 😀 Plumo structures represent the propagation of stress along joints and are key in understanding how fractures form in heterogeneous materials.
• 😀 The origin of plumo structures is marked by a small dimple, indicating where the joint begins to propagate.
• 😀 The mirror zone near the origin is smooth due to high stress, creating a near-planar surface.
• 😀 The mist zone shows roughness as stress moves along a rock's imperfections and varying composition.
• 😀 The Hackle zone is rough and characterized by feather-shaped barbs pointing back toward the fracture origin, representing the propagation of stress.
• 😀 Joint sets consist of parallel or subparallel joints that behave similarly, while joint systems consist of multiple sets with a quantifiable geometric relationship.
• 😀 Master joints are continuous fractures that extend across the entire bed, while cross joints terminate within master joints.
• 😀 Systematic joints are evenly spaced and subparallel, while nonsystematic joints are irregular and often terminate in larger joints.
• 😀 Columnar jointing forms in intrusive rocks, particularly sills and dikes, due to tensile stress from cooling and contraction of the rock.
• 😀 Exfoliation jointing occurs in plutonic intrusions exposed to lower pressure environments, creating onion-like patterns in the rock.

Q & A

What are joints in the context of structural geology?

Joints are natural fractures in rocks along which no shear displacement has occurred. The rock breaks, but the two sides of the fracture do not move relative to each other.

How do veins differ from joints in structural geology?

Veins are fractures filled with mineral precipitates that form as water travels through the fracture areas, while joints are fractures without mineral deposition or movement.

What is the significance of plumo structures in jointing?

Plumo structures help interpret the stress propagation along fracture planes. They represent the propagation of stress as the joint forms and reflect the heterogeneity and anisotropy of the rock.

What are the main zones in a plumo structure?

The main zones in a plumo structure include the origin (where the fracture begins), the mirror zone (smooth area close to the origin), the mist zone (less smooth, showing roughness), and the Hackle zone (feather-shaped, rough area with barbs pointing back toward the origin).

What is the purpose of AR rest lines in plumo structures?

AR rest lines mark areas where stress is relieved temporarily, and then stress accumulates again as the fracture propagates further. These lines help identify cycles of stress in the fracture's development.

What is a joint set in structural geology?

A joint set consists of joints that behave similarly, often parallel or subparallel to each other, and have a quantifiable relationship in their arrangement.

How do joint systems differ from joint sets?

A joint system consists of two or more joint sets that have a geometrical relationship to each other, whereas a joint set refers to a single set of joints with similar characteristics.

What is an orthogonal joint system?

An orthogonal joint system consists of two sets of joints that intersect at approximately right angles, such as one vertical set and one horizontal set.

What causes columnar jointing in rocks?

Columnar jointing develops in cooling intrusions like sills and dykes. As the rock cools and contracts, tensile stresses form, creating a pattern of regular hexagonal columns.

What is exfoliation in magmatic intrusions, and where is it commonly observed?

Exfoliation is a type of jointing that occurs in magmatic intrusions like plutons, where erosion exposes the rock to lower pressure environments. This causes joints to form parallel to the contact between the intrusion and surrounding country rock, resembling layers peeling off, often seen in domes like those in East Texas.