Overview of Geologic Structures Part 1: Rock Deformation, Stress and Strain

Summary

The video explores the four eons of geologic time—Hadean, Archean, Proterozoic, and Phanerozoic—and delves into the deformation of rocks, which creates dramatic landforms over millions of years. It explains the mechanical properties of geologic materials, including stress and strain, and how pressure and temperature affect rock behavior at different depths. The video also covers various deformation mechanisms like brittle, elastic, plastic, and viscous deformation, highlighting how these processes shape Earth's geological structures and influence events like earthquakes, particularly in subduction zones.

Takeaways

• 🌍 Geologic time is divided into four eons: Hadean, Archean, Proterozoic, and Phanerozoic.
• 🪨 Rocks change shape under different conditions, creating landforms that geologists study.
• 🔥 Pressure and temperature increase with depth, which causes rocks to deform differently at shallow vs. deep levels.
• 🧊 Rocks in the mantle behave similarly to glaciers, flowing slowly due to high temperature and pressure.
• 📏 Stress refers to the forces causing deformation in rocks, measured in Pascals, and strain is the resulting change in shape.
• 🔧 Young’s modulus measures how rocks respond to stress, varying with mineral composition.
• 🌋 Rocks can deform in brittle, elastic, plastic, or viscous ways, depending on conditions.
• 💥 Brittle deformation, common at shallow depths, leads to fractures and earthquakes.
• 🧱 Plastic deformation happens under high pressure and temperature, especially in the mantle and lower crust.
• 🏔 Subduction zones experience large earthquakes due to the brittle behavior of oceanic crust at great depths.

Q & A

What are the four eons of geologic time mentioned in the script?

The four eons of geologic time mentioned are the Hadean, Archean, Proterozoic, and Phanerozoic.

How do rocks behave under extreme conditions of temperature and pressure in the Earth's mantle?

Under extreme conditions of temperature and pressure, such as 1400°C and 100,000 atmospheres, rocks behave like silly putty or glacial ice, meaning they become viscous and capable of slow flow.

What is the difference between compressive stress and tensile stress?

Compressive stress occurs when a force pushes inward on an object, causing it to squash, while tensile stress occurs when a force pulls outward on an object, causing it to stretch.

What is Young’s modulus and why is it important in structural geology?

Young’s modulus represents the relationship between stress and strain in a material. It measures how a material responds to stress and is important for determining how rocks deform under different conditions.

What is the difference between competent and incompetent rocks?

Competent rocks are composed of strong, rigid minerals like garnet or feldspar, making them more resistant to deformation. Incompetent rocks, on the other hand, are composed of soft, weak minerals like clay and are more easily deformed.

How do pressure and temperature affect the strength of rocks?

As pressure and temperature increase, the strength or viscosity of rocks decreases, making them more susceptible to deformation.

What is the brittle regime in geology?

The brittle regime refers to the upper 10 kilometers or so of the Earth’s crust, where rocks tend to break or fracture under stress rather than bend. This is where brittle deformation is most common.

What is elastic deformation, and how does it differ from plastic deformation?

Elastic deformation occurs when a material’s atomic bonds stretch and then revert back to their original shape after the stress is removed. Plastic deformation occurs when the atomic bonds break under stress, and the material does not return to its original shape.

What happens when a rock fails under stress?

When a rock fails under stress, it fractures, and the accumulated stress is released, causing an earthquake. This is similar to the snapping of a stretched rubber band.

Why do subduction zones tend to produce massive earthquakes?

Subduction zones produce massive earthquakes because they transport competent materials, such as oceanic crust, to great depths and pressures where rocks are more likely to fracture rather than bend.