Mountains and Faults

Geology & Plate Tectonics Fundamentals

Course 1 · Chapter 5 · Mountains, Faults and Folds

In 1860s and 1880s geologists at the Geological Survey puzzled over the north-west Highlands, where older rocks seemed to lie on top of younger ones, which should not happen. Ben Peach and John Horne, working for the Survey, showed that a huge sheet of rock had been shoved for many kilometres over its neighbours along a low-angle fault, now known as the Moine Thrust (check). Their 1907 memoir became a classic of the science (check). This chapter looks at what bends and breaks rock, the kinds of folds and faults, how mountain belts rise and are worn down, and how to read the structures on a geological map. Dates and figures are from general knowledge and flagged "check".

Stress and Strain

Stress

A force acting on rock. There are three kinds: compression (squeezing), tension (pulling apart) and shear (sliding past). Plate boundaries produce each of them (Chapter 3).

Strain

The change in shape or volume that results. Rock can deform elastically (and spring back), by flowing like putty (ductile), or by breaking (brittle).

What decides the response

Temperature, pressure, rock type and the speed of stress. Rocks deep in the crust, hot and under pressure, tend to bend. Shallow, cold rocks tend to break. Given enough time, even hard rock folds (check).

Folds

Term	Meaning
Anticline	An arch-shaped fold, with the oldest rocks normally in the core
Syncline	A trough-shaped fold, with the youngest rocks normally in the core
Limb	The sloping side of a fold
Axial plane	The imaginary surface that divides a fold into two halves
Overturned or recumbent fold	A fold pushed so far that one limb is upside down or lies flat

Folds range from crinkles that fit in a hand to structures many kilometres across. A useful check is that the rocks in an anticline get older toward the centre, which helps a geologist read a landscape that has been cut across by erosion (check the exceptions with overturned folds).

Faults

Fault type	Stress	Movement (check)
Normal	Tension	The block above the fault surface moves down; the crust is stretched, as in rift valleys
Reverse (thrust, if low angle)	Compression	The block above moves up; the crust is shortened and thickened, as in mountain belts
Strike-slip	Shear	Blocks slide sideways past each other, as on the San Andreas Fault

Fault plane. The surface along which movement occurs; it may be steep or nearly flat.
Evidence of faulting. Offset layers, crushed rock (fault breccia), polished and grooved surfaces (slickensides) and abrupt changes of rock type along a line.
Activity. A fault may be active, with movements still happening, or ancient and inactive. Only the first kind is an earthquake risk (Chapter 4).

Mountain Building

Up, then down Mountains rise where plates collide, because crust is shortened and thickened by folding and thrusting, and because thick crust floats higher, like an iceberg. But they do not rise forever. Rain, rivers, ice and gravity wear them down, and as the weight is removed the roots rebound upward, which slows the loss (check). The result is a balance of uplift and erosion, and old mountain belts like the Appalachians and the Scottish Highlands are low and rounded because the uplift stopped long ago.

Young belts such as the Himalaya and Alps are still rising, with sharp peaks, steep valleys and many earthquakes (check).
Old belts such as the Caledonian mountains (Chapter 3) are worn to their roots, and their former heights are known mainly from the folded and metamorphosed rocks left behind (check).
Rift mountains and volcanic ranges follow different routes: stretching in the first case, and volcanic building in the second.

Reading a Geological Map

Colours and letters show the age and type of rock at the surface, according to a key (check the key for each map).
Lines mark boundaries between rock units and faults, and usually show which side moved.
Dip and strike symbols show the angle of the layers. A small T-shaped symbol and a number give the direction and steepness of the dip.
Cross-sections are interpretations of what lies underground, built by connecting surface information. Good geologists show where the interpretation is uncertain.

A British comparison The Highland Boundary Fault runs from Arran in the south-west across Loch Lomond to Stonehaven on the east coast, and divides the Highlands from the Lowlands (check). The Moine Thrust runs from Loch Eriboll in the north to Skye in the south-west (check). Siccar Point, where Hutton saw tilted layers capped by flatter ones, is also a structural lesson: it is an unconformity, a gap in time where older rocks were folded, worn down and covered by new ones. The British Geological Survey publishes maps of all three on its website and in the Edinburgh office.

Check the details Names of faults, lengths and dates vary across sources, and structures once thought simple have often been revised. Use the British Geological Survey maps and the key supplied with them. When visiting outcrops, stay safe, follow the Geological Conservation Review guidance and the Scottish Outdoor Access Code, and never hammer protected exposures.

Exercises

Exercise 1

Sketch (in words) an anticline and a syncline and explain how you could tell them apart on a map from the ages of the rocks.

→ Solution

Exercise 2

Match each fault type (normal, reverse, strike-slip) to a plate boundary type from Chapter 3, and say what happens to the width of the crust in each case.

→ Solution

Exercise 3

Older rocks lie on top of younger rocks along a low-angle fault. Explain what that suggests, and how Peach and Horne's explanation of the Moine Thrust solved a similar puzzle.

→ Solution

Quick Reference

Stress: compression, tension, shear; strain is the response
Folds: anticline (arch) and syncline (trough)
Faults: normal (tension), reverse or thrust (compression), strike-slip (shear)
Mountains: rise by collision and thickening, worn down by erosion
Scotland: Highland Boundary Fault, Moine Thrust, Siccar Point unconformity (check)