Structural geology & mountain building

Summary

This video explores the fundamentals of structural geology and mountain building, focusing on the distinction between elastic and plastic deformation. It delves into the major types of faults and folds, such as synclines, anticlines, and recumbent folds, and explains their formation through block diagrams and maps. The video also evaluates the processes behind mountain building, outlining four primary mountain types: fault block, folded, volcanic, and island arc volcanoes, with examples like the Sierra Nevada and the Appalachians.

Takeaways

• 📚 Deformation in rocks is similar to bending a stick until it breaks, leading to fractures and faults, similar to what happens during an earthquake.
• 📏 The effects of directional stress on rock units include compressional stress causing horizontal shortening and vertical thickening, tensional stress leading to horizontal lengthening and vertical thinning, and shear stress causing horizontal displacement.
• 🔍 There are three main types of folds: synclines, which are down-warping of strata with the youngest rocks at the center; anticlines, which are up-warping with the oldest rocks at the center; and recumbent folds, where the folds are folded over themselves.
• 🏞️ Faults are fractures with displacement and include terms like fault scarp, hanging wall, and footwall. The hanging wall drops relative to the footwall, and the footwall is the block with the pointed toe.
• 📉 Normal faults occur when the hanging wall moves down relative to the footwall, often associated with extensional forces.
• 📈 Reverse faults are characterized by the hanging wall moving up relative to the footwall, typically forming in compressional environments.
• 🔄 Strike-slip faults involve horizontal motion, with examples including left-lateral and right-lateral faults, depending on the direction of the horizontal offset.
• 🌋 Geological structures like domes and basins are formed by up-warping and down-warping of rock layers, respectively, with different patterns of rock age distribution.
• 🏞️ Orogenesis, the process of mountain formation, includes four main types of mountains: fault block, folded, volcanic, and island arc volcanoes.
• 🏔️ Fault block mountains are associated with high angles of normal faulting and are common in regions with extensional forces, like the Sierra Nevada and the Grand Tetons.
• ⛰️ Folded mountains result from large-scale compressional forces, often found at convergent plate boundaries, with examples including the Appalachians and the Valley and Ridge province.
• 🌋 Volcanic mountains are formed from volcanic activity and can be seen in structures like Mount St. Helens and Mount Mayon.
• 🌴 Island arc volcanoes form at the convergence of two oceanic plates, with one being subducted beneath the other, as seen in the Tonga and Marianas islands.

Q & A

What is the basic concept of deformation in structural geology?

Deformation in structural geology refers to the process where rocks or materials are subjected to stress until they rupture, fracture, and break, similar to bending a stick until it snaps. This is what happens during an earthquake or when a fault occurs.

What happens to rock units under compressional stress?

Under compressional stress, rock units become shortened horizontally and thicken vertically, causing the layers to bulge up vertically as they shorten.

How does tensional or extensional stress affect rock units?

Tensional or extensional stress causes rock units to lengthen horizontally and thin vertically, sometimes even rupturing to form faults.

What is a syncline and how does it form?

A syncline is a fold where strata downwarp, with the oldest layers at the bottom and the youngest in the center. It forms when rocks are compressed and folded, creating a trough-like shape.

How can one identify the age pattern of rocks in a syncline?

In a syncline, if you walk from the axial region (center of the fold) out towards the limbs, you would walk from younger rock into progressively older rock.

What is an anticline and how does it differ from a syncline?

An anticline is an up warping of strata, with the oldest rock exposed at the center of the fold. It differs from a syncline in that the age pattern is opposite, with younger strata found towards the center and older ones on the limbs.

What is a recumbent fold and how does it appear?

A recumbent fold occurs when folds are folded over on top of themselves, creating a structure where the limbs of the fold are nearly flat and the axial region is compressed.

What are the key components of a fault?

A fault consists of a fracture in rock along which there has been displacement. Key components include the fault scarp (the exposed fault surface), the hanging wall (the block that drops down relative to the footwall), and the footwall.

How does the movement in a normal fault differ from that in a reverse fault?

In a normal fault, the hanging wall moves down relative to the footwall, often associated with extensional forces. In a reverse fault, the hanging wall moves up relative to the footwall, typically forming in a compressional environment.

What is a strike-slip fault and how is it identified?

A strike-slip fault is characterized by horizontal motion along the fault zone. It can be identified by the offset of features, such as roads or layers, with one side moving horizontally in one direction and the other side in the opposite direction.

What are the four main types of mountains and an example of each?

The four main types of mountains are fault block mountains (e.g., Sierra Nevada), folded mountains (e.g., Appalachians), volcanic mountains (e.g., Mount Etna), and island arc volcanoes (e.g., Tonga).

What is the process called that creates mountain belts?

Orogenesis is the process by which mountain belts are created, involving various geological forces and events that lead to the uplift and deformation of the Earth's crust.

How are domes and basins different in terms of rock age distribution?

In a dome, the youngest rocks are found on the flanks with the oldest rocks in the center, whereas in a basin, the pattern is reversed with the youngest rocks in the center and the older rocks on the flanks.