Structural Geology - Lesson 2 - Stress and Strain

Summary

This lecture on structural geology covers the fundamental concepts of stress and strain, vital for understanding geological structures. It explains the nature of stress as a force per unit area, the difference between body and surface forces, and the importance of resolving stress into vector components (normal and shear). The instructor introduces the stress ellipse and ellipsoid, as well as the Mohr diagram for visualizing stress in different dimensions. The lecture transitions into strain, discussing its effects on rock deformation, behavior types (elastic, brittle, ductile), and how factors like pressure, temperature, and strain rate influence these processes.

Takeaways

• 💪 Stress is a force, defined as force per unit area, and is crucial in structural geology for determining the appearance of geological structures.
• 🌍 The four fundamental forces of the universe include electromagnetic, nuclear (strong and weak), and gravitational forces, with gravitational force being the weakest in magnitude.
• 📐 Stress can be resolved into vector components: normal stress (perpendicular to a plane) and shear stress (parallel to a plane). These components help analyze forces acting on various planes in the Earth.
• 🔵 Stress ellipses and ellipsoids visually represent stress in two and three dimensions, respectively. These tools simplify understanding the complex directions and magnitudes of stress.
• ⚙️ Isotropic stress results in a circular or spherical representation, while anisotropic stress (unequal in different directions) leads to ellipses and ellipsoids.
• 📊 The Mohr diagram is a graphical tool used to visualize stress, providing a way to understand normal and shear stresses on fracture planes.
• 🛠️ Stress fields and trajectories visually represent the orientation and magnitude of stress, helping predict strain and deformation patterns in geological materials.
• ⛏️ Strain, or deformation of rock, can be homogeneous (uniform throughout) or heterogeneous (varies across the material). It often reflects incremental stress over time.
• 🧱 Rock behavior under stress can be brittle (breaking or fracturing), ductile (flowing), elastic (temporary deformation), or viscous (permanent flow). Each is influenced by temperature, pressure, and strain rates.
• 🔄 High pressure increases material strength and ductility, while high temperatures decrease elastic behavior, reduce material strength, and encourage ductile flow.

Q & A

What is stress in the context of structural geology?

Stress in structural geology is defined as force per unit area. It determines how geological structures deform and change, impacting their shape and orientation. Stress can be caused by various forces acting on the Earth's crust.

What are the four fundamental forces mentioned, and which is most relevant in structural geology?

The four fundamental forces are electromagnetic force, strong and weak nuclear forces, and gravitational force. Gravitational force, though the smallest in magnitude, is most relevant to structural geology because it influences stress due to the weight of the Earth's materials.

What are body forces and surface forces, and how do they differ?

Body forces are proportional to the mass of an object, such as gravitational force acting on a rock. Surface forces act on the surface of an object and can cause acceleration or deformation, like the force of the ground acting on a rock when it falls.

How are stress components resolved in structural geology?

Stress components are resolved into vector components: normal stress and shear stress. Normal stress acts perpendicular to a plane, while shear stress acts parallel to it. These components help geologists analyze complex stress patterns in the Earth.

What is a stress ellipse and how does it help in visualizing stress?

A stress ellipse is a 2D representation of stress, used to show how stress acts on different planes. In 3D, it becomes a stress ellipsoid. It allows for the orientation of various planes around an axis, giving a better understanding of the direction and magnitude of stress in different dimensions.

What is the difference between isotropic and anisotropic stress?

Isotropic stress means stress is the same in all directions, creating a circular or spherical shape. Anisotropic stress occurs when stress magnitudes vary in different directions, which is more common in geology and results in an elliptical or ellipsoidal stress representation.

What is the purpose of a Mohr diagram in structural geology?

A Mohr diagram is used to visualize stress in an XY coordinate system. It helps geologists determine the normal and shear stress acting on a plane by plotting the magnitude of stress components. The diagram simplifies stress analysis by providing a graphical representation.

What are stress trajectories, and why are they important?

Stress trajectories are lines that represent the orientation of stress in a material, showing how stress varies across a body. They are important for predicting strain (deformation) in geological structures, helping geologists understand how stress influences rock behavior.

What is strain in structural geology?

Strain refers to the deformation a material undergoes due to stress. It can be elastic (temporary), plastic (permanent), brittle (fracture), or ductile (flow). Strain helps geologists understand the history of a rock's deformation and the forces that shaped it.

What factors influence whether a material behaves in a brittle or ductile manner?

Factors influencing whether a material behaves in a brittle or ductile manner include temperature, pressure, strain rate, and the composition of the material. Higher temperatures and pressures generally increase ductility, while lower temperatures and faster strain rates tend to cause brittle behavior.