Folds, faults & deformation
Push on rock and it either breaks or bends. Faults and folds are the record of which one happened, and they decide where fluids, and the minerals they carry, can flow.
- Tell stress from strain, and name the three kinds of stress
- Explain why shallow rocks break while deep rocks flow
- Recognise normal, reverse and strike-slip faults, and anticlines and synclines
- Describe how a tilted layer is measured with strike and dip
- Explain why structures matter for ore deposits and groundwater
Stress and strain
Stress is the force acting on a piece of rock, spread over its area: how hard it is being pushed or pulled. Strain is the rock's response: how much it changes shape or size. Stress is the cause; strain is the effect. Geologists can never see past stresses directly, but they can measure strain in the rocks, from squashed fossils, stretched pebbles and bent layers, and work backwards.
Stress comes in three kinds, and each one is typical of a different kind of plate boundary:
Break or bend?
The same rock can behave in two completely different ways. Brittle rock breaks, like a dry biscuit. Ductile rock bends and flows without breaking, like modelling clay. Which one happens depends on four things:
- Temperature. Hot rock is softer and flows more easily.
- Pressure. The weight of rock above (confining pressure) squeezes cracks shut, so it's harder for rock to break apart.
- Strain rate. Pull silly putty sharply and it snaps; pull it slowly and it stretches. Rock is the same, except "slowly" means millimetres per year.
- Rock type. Salt and shale flow easily; quartz-rich sandstone and granite are stiff.
Temperature and pressure both rise with depth, so the result is simple: near the surface, cold rocks break; deep down, hot rocks flow. In continental crust the switch, the brittle–ductile transition, typically happens around 10–15 km down. That is also why most earthquakes are shallow: you need brittle rock to snap. How deep the switch sits depends mostly on how fast the crust warms with depth, its geothermal gradient.
At each depth, rock gives way by whichever mechanism needs less stress: breaking (which gets harder with depth, because pressure clamps cracks shut) or flowing (which gets easier, because heat softens the rock). Change how hot the crust is and watch the crossover move.
A simplified model. Brittle strength rises in a straight line with depth (friction on faults); ductile strength follows a laboratory flow law for wet quartz, deforming at a typical slow geological rate. Temperature is assumed to rise steadily with depth. Real crust varies with rock type, water and how it is being stressed, so treat the depths as rough guides, not predictions.
Faults: where rock broke and moved
A crack with no movement across it is a joint. A crack where the two sides have moved past each other is a fault. Faults are named by which way the blocks moved, which tells you what kind of stress caused them.
For a sloping fault, the block above the fault plane is the hanging wall and the block below is the footwall. The names come from old miners, who worked along mineral veins in faults: they hung their lanterns on the rock above their heads and stood on the rock at their feet. A low-angle reverse fault (dipping less than about 45°) is called a thrust.
Normal faultPulled apart: the hanging wall slides down.
Reverse faultSqueezed: the hanging wall rides up. Below about 45° it's called a thrust.
Strike-slip faultSheared: the blocks slide sideways past each other.
Pick a kind of stress, then drag the movement slider (or press Play) to slide the blocks. Matching coloured layers show how far they moved. For sloping faults, change the dip and watch the name change.
Real faults move a few centimetres to metres in each earthquake (or creep slowly), and the total offset builds up over thousands to millions of years. Here the movement is exaggerated so you can see it, and erosion, which would wear the step down, is left out.
| Fault | Stress | Movement | Example |
|---|---|---|---|
| Normal | Tension | Hanging wall moves down; crust gets longer | East African Rift; the Narmada–Son rift zone in central India is an ancient example |
| Reverse / thrust | Compression | Hanging wall moves up; crust gets shorter and thicker | The Himalaya |
| Strike-slip | Shear | Blocks slide sideways; little up or down | San Andreas Fault, California |
Folds: where rock bent
Under compression, especially deep and hot, layered rock buckles into folds, like a rug pushed against a wall. An upward arch is an anticline; a downward trough is a syncline. Folds range from crinkles in a hand specimen to arches tens of kilometres across.
Erosion usually planes off the top of folds, so what geologists see at the surface is a pattern of stripes. The trick for telling folds apart is age. Layers were deposited oldest at the bottom (see Stratigraphy), so when they are arched up and eroded, the oldest rocks end up in the middle of an anticline. In a syncline the youngest rocks are in the middle.
Plunging folds
Fold axes are rarely perfectly level. When the axis tilts, the fold is plunging. Seen from the front (the cross-section) it is still an arch or a trough, but on the eroded land surface on top its layers make U or V shapes. An anticline's curved "nose" points the way the axis sinks into the ground; a syncline's nose points the other way. Try both in the block below.
Joints
Almost every rock outcrop is cut by joints: fractures with no movement along them. They form as rock is unloaded by erosion, cools or is gently flexed. Cooling lava shrinks and cracks into six-sided columns, which you can see in the Deccan basalt at Gilbert Hill in Mumbai. Joints are where weathering starts, because they let water into the rock.
The block is a plunging fold. Its top is the land surface, which is what a geological map shows. Lower the land surface to erode deeper into the fold, slice the front back to see the cross-section further along it, and switch between an anticline and a syncline.
Foliation and cleavage. Under strong compression at depth, flat and elongated minerals such as mica rotate and regrow at right angles to the squeeze. The rock gains a planar fabric called foliation. In slate it shows up as cleavage, the planes along which it splits into roofing tiles. Because the planes form across the squeeze, foliation tells you which way the rock was squeezed.
Strike and dip: measuring a tilted layer
To map structures, geologists need to record the orientation of every tilted surface they find: a bed, a fault, a joint or a foliation. Two numbers do it. Picture a tilted roof. The ridge line along the top is level; that horizontal direction on the tilted surface is the strike. Rain runs straight down the roof at right angles to the ridge; the angle that line makes with the horizontal is the dip.
Strike is given as a compass bearing and dip as an angle from 0° (flat) to 90° (vertical), plus the direction it dips towards. On a geological map each measurement becomes a small symbol: a long line along the strike, a short tick pointing down the dip, and the dip angle written beside it.
The block shows a sandstone bed (yellow) in grey shale, with north towards the back. Turn and tilt the bed, and watch the map symbol and the written measurement change.
By convention the bed dips to the right as you face along the strike, so the dip direction is the strike bearing plus 90°.
Geologists measure strike and dip with a compass-clinometer, a compass with a built-in spirit level and a swinging pointer for reading angles. To get strike, hold the compass edge against the rock surface, level it with the bubble, and read the bearing. To get dip, turn the instrument on its edge, lay it straight down the slope at right angles to the strike, and read the angle from the clinometer. Today many geologists use smartphone apps that do the same job with the phone's sensors.
Why structures matter
Most rock is tight: water can barely move through it. Faults, fractures and the hinges of folds are the exceptions. They are the plumbing of the crust. Hot fluids rising from depth follow them, and where the fluids cool or react, they drop the metals they carry. That is why many gold, copper and other ore deposits sit along faults or in fractured zones (see Hydrothermal deposits). Exploration geologists spend a lot of time mapping structures for exactly this reason.
- Hot fluids from depth carry dissolved metals and silica.
- The fault zone is a pipe. Its broken rock is far more permeable than the solid rock around it, so fluids follow it upward. Little leaks into the tight rock beside it (dashed).
- A vein forms near the surface, where the fluid cools, boils or reacts with the wall rock: quartz fills the fault and metals drop out as ore.
The same applies to groundwater. Across much of peninsular India, including the Deccan basalts, the solid rock holds very little water. Groundwater sits in the weathered layer near the surface and in joints and fractures below it, so whether a well succeeds often depends on whether it hits a fracture zone.
- Weathered zone. Soft, broken-down rock a few metres to a few tens of metres thick. It soaks up rain and holds most of the water, like a sponge.
- Fractured zone. Joints and cracks below the weathered layer carry water, but hold far less of it.
- Massive basalt. Solid, unbroken rock: almost no water can move through it.
- Well A hits a fracture zone connected to the layers above, so water keeps flowing in.
- Well B misses. Below the weathered zone it finds only massive rock, so it runs dry once the water table falls in the dry season.
Structures also record history. The Himalaya are built from slices of crust stacked on top of each other along great thrust faults. Beneath them all runs the Main Himalayan Thrust, a gently dipping fault on which India is sliding under Asia. Its shallow part is locked, building stress that is released in earthquakes like the 2015 Gorkha earthquake (see Earthquakes & volcanoes).
Fractures control how water moves through rock and soil, and water is what drives weathering. A jointed, fractured basalt weathers much faster than a solid one, because water can reach more surface.
Structures matter just as much in mineral exploration. Faults and folds channel the hot fluids that deposit many ores, so the metal often sits where a fault bends, branches or meets a reactive layer. Mapping structures is often the first step in deciding where to drill.