Stress and Strain in Geology: Deformation Basics

Squeeze, stretch, or shear a rock — and it responds. Learn the difference between stress and strain, and why the same rock can spring back, bend, or break depending on the conditions.

Uni Year 1Earth science
⏱️ About 16 min
Stress and Strain in Geology: Deformation Basics — illustration
Illustrative image (AI-generated).

Pick up a piece of modelling clay and squeeze it. Push gently and it springs back when you let go. Push harder and it stays squashed. Push sideways and it shears. Rock behaves the same way — just at much higher forces, higher temperatures, and over much longer times.

💡
The big idea: Stress is the force applied per unit area on a rock. Strain is the deformation that results. The same rock can respond elastically (springing back), plastically (permanently bent), or brittlely (breaking) depending on the stress magnitude, temperature, pressure, and how fast the force is applied.
🎯 By the end, you'll be able to
  • Define stress and strain and state their units
  • Distinguish compressional, tensional, and shear stress with simple sketches
  • Predict whether a given stress regime is more likely to fold or fracture rock
  • Explain the difference between elastic, plastic, and brittle deformation
📎 Helpful to know first

Stress — force concentrated on an area

When you stand on a floor, your weight is spread over the area of your feet. If you stand on one foot, the same force is concentrated on half the area — the pressure doubles. In geology, that pressure is called stress: the force applied divided by the area it acts on.

Stress is what actually deforms rock. A giant force spread over an enormous area may produce only tiny stress, while a modest force on a small crack tip can generate enough stress to split a boulder.

\[ \sigma = \frac{F}{A} \]
Stress (σ) equals force (F) divided by area (A). In SI units, stress is measured in pascals (Pa); geologists often use megapascals (MPa), where 1 MPa = 10⁶ Pa.

Three kinds of stress

The orientation of the force matters enormously. Geologists classify stress into three regimes based on the direction of the maximum and minimum stresses:

  • Compressional stress squeezes rock from two opposite directions, like a vice. It is the dominant stress at convergent plate boundaries and produces folds, reverse faults, and thrust faults.
  • Tensional stress pulls rock apart, stretching it. It dominates at divergent boundaries and produces normal faults and rift valleys.
  • Shear stress pushes two sides of a body in opposite directions parallel to a plane, like tearing a sheet of paper sideways. It dominates at transform boundaries and produces strike-slip faults.
Three types of geological stress: compression, tension, and shear Compression σ1 horizontal Tension σ1 vertical (unroofing) Shear σ1 and σ3 both oblique Stress regimes control whether rock folds, faults, or stays intact
The three stress regimes. Compression squeezes, tension stretches, and shear slides. Each produces a different style of deformation and a different fault type.
🔑 Stress is the cause; strain is the effect
Stress is what you do to the rock (push, pull, shear). Strain is what the rock does in response (shorten, stretch, bend, break). A strong rock under low stress may show almost no strain; the same rock under high stress may fracture. A weak rock under the same stress may fold instead.

Strain measures how much the rock changes shape

Strain is the ratio of the change in length (or volume, or angle) to the original value. Because it is a ratio of two lengths, strain has no units — it is just a number. A strain of +0.01 means the rock is 1 percent longer than it was before; a strain of −0.01 means it is 1 percent shorter. Shear strain is an angle change, also dimensionless.

Geologists measure three kinds of strain:

  • Linear strain — change in length divided by original length.
  • Shear strain — distortion of angles (originally perpendicular lines become tilted).
  • Volume strain — change in volume divided by original volume.
\[ \varepsilon = \frac{\Delta L}{L_0} \]
Linear strain (ε) equals the change in length (ΔL) divided by the original length (L₀). A positive value means extension; a negative value means compression.

Elastic, plastic, and brittle behaviour

Rocks do not all respond the same way to stress. Three regimes:

  • Elastic deformation — the rock bends or compresses but springs back completely when the stress is removed. Think of a bent diving board. This is what stores the energy that drives earthquakes.
  • Plastic (ductile) deformation — the rock bends permanently and stays bent after the stress is removed. This happens at high temperature and pressure deep in the crust, producing folds.
  • Brittle deformation — the rock breaks. This happens at low temperature and pressure near the surface, producing faults and fractures.

The same rock can behave elastically at low stress, plastically at high temperature, and brittlely at low temperature. Depth — and therefore temperature — is often the deciding factor.

📝 Worked example: A cylindrical rock core 5.0 cm in diameter is subjected to a compressive force of 7854 N. What is the stress on the sample? (Give the answer in MPa.)
  1. Radius r = 5.0 cm ÷ 2 = 2.5 cm = 0.025 m.
  2. Cross-sectional area A = πr² = π × (0.025 m)² ≈ 0.0019635 m².
  3. Stress σ = F ÷ A = 7854 N ÷ 0.0019635 m² ≈ 4,000,000 Pa = 4.00 MPa.
✓ About 4.0 MPa of compressive stress.
✏️ Practice: A rock sample with a circular cross-section 4.0 cm in diameter is compressed by a force of 5027 N. What is the stress in MPa? (Use π ≈ 3.1416.)
MPa
Solution
  1. r = 2.0 cm = 0.020 m.
  2. A = π × (0.020)² = 0.0012566 m².
  3. σ = 5027 ÷ 0.0012566 ≈ 4,000,000 Pa = 4.0 MPa.
✏️ Practice: A 2.00 m long rock pillar is compressed until its length is 1.97 m. What is the linear strain? (Strain is dimensionless.)
Solution
  1. ΔL = 2.00 − 1.97 = 0.03 m.
  2. ε = ΔL ÷ L₀ = 0.03 ÷ 2.00 = 0.015 (or 1.5 % compression).

Check your understanding

1. Which stress regime is most likely to produce folding in layered rock?
Compressional stress squeezes layered rock from the sides, causing it to buckle and fold rather than break — especially at depth where temperature is high enough for plastic deformation.
2. What is the difference between stress and strain?
Stress is the cause (force/area). Strain is the effect (change in shape or volume). A rock under stress may show little strain if it is strong, or large strain if it is weak.
3. At shallow crustal levels, rock tends to respond to high stress by:
Near the surface, rocks are relatively cold and under low confining pressure, so they break brittlely rather than bend plastically. Folding dominates at greater depth where temperature and pressure are higher.
🔎 Related lessons
Next: Folds: Anticlines, Synclines, Domes & Basins Plate Boundaries: Divergent, Convergent, Transform Earth's Layers: Crust, Mantle, Core
✅ Key takeaways
  • Stress is force per unit area (σ = F/A); strain is the resulting deformation (ε = ΔL/L₀).
  • Three stress regimes: compression (squeezing), tension (stretching), and shear (sliding parallel).
  • Compression produces folds and reverse/thrust faults; tension produces normal faults; shear produces strike-slip faults.
  • Rocks respond elastically (spring back), plastically (permanently bend), or brittlely (break) depending on stress, temperature, and pressure.
  • Brittle deformation dominates near the surface; plastic deformation dominates at depth.
➡️ Now that we know how stress deforms rock, we can look at the structures it creates. When compression acts on layered rock at depth, the layers bend into folds — the subject of the next lesson.
Want to test yourself on this? Try the Science practice tests →
🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of structural geology & earthquakes.

External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.