Deformation of the Crust 5.1 How the crust is deformed 5.2 The results of stress 5.3 Mountain formation

Deformation Many features of the earth result from deforming of the the crust Bending Folding Faulting Plate tectonics These rocks on the coast of New Zealand have been deformed by stress

Isostasy The balance of two forces acting on the Earth’s crust The mass of the crust pushing done and the force of the mantle pushing up Isostasy occurs when the two forces are balanced This is when deformation occurs

Isostasy is the vertical movement of the crust to to bouyancy in the mantle. Just like blocks of wood in water float higher the thicker they are, the crust rises and sinks because it is lighter than the underlying mantle. Variations in elevation are due both to thickness and density. The continents stand high because continental crust is thick and light. The ocean basins are low because oceanic crust is thin and dense.

Isostasy This type of deformation is common in areas where mountain or glaciers exist For example the building up of the Himalayas pushes downwards on the crust Erosion washing away the Appalachian Mountains changes their mass and as the result the area is rising Hudson’s Bay continues to rebound since the mass of the last glacier (ice age) receded Sediments washing down a river into the ocean add to its mass causing it to sink, allowing more deposits to settle etc.

It's probably no accident that two of the largest epicontinental seas (seas extending deep into the continent) on Earth are Hudson Bay and the Baltic, both dead center in areas of active isostatic uplift. In all likelihood, the crust in these regions is still depressed and has not finished rising, and when uplift is complete both seas will mostly or entirely disappear. Gravity measurements suggest that the crust in the Hudson Bay region has another 100 meters still to rise.

Isostasy Up and down movements of the crust to reach a balance is called isostatic adjustment This causes stress in the rocks of the crust and includes: compression, tension and shearing All three forces contribute to mountain building Different kinds of rock respond differently to various types of stress.

Compression When forces are pushing on either side of the rock Squeezing makes layers thicker and shorter

Tension When forces pull rocks apart Rock layers tend to become thinner and longer, layers may fracture

Shearing Forces push rocks in opposite horizontal directions These rocks could bend, twist or break Common in earthquakes

Folding This occurs when rock becomes permanently deformed but does not break Layers may have cracks they they don’t break Fold layers look wavelike and have various thicknesses They can be small and localized or cover large areas There are three types of folds: Anticline, syncline and monocline

Isoclinal folds

Tight isoclinal folding, the upfold on the left is the anitcline and on the right is the syncline

Recumbent folds

Recumbent fold

Black Hills, South Dakota

Overturned fold

At a small scale (outcrop-sized), contorted folds are found in metamorphic rocks such as gneisses. The rocks actually soften (see below) as they are heated during deep burial. This type of folding, called ptygmatic, is illustrated here.

Open folding showing buckling and bending

Anticline Upwards fold where the lowest layer is in the centre The sides of the fold are called limbs The steepness of the limbs reflects the amount of force applied Generally forms a ridge                                

Anticline on a road cut near the Canadian Rockies

Syncline A downward fold where the youngest area is in the centre Generally forms a valley

Syncline on a road cut near the Canadian Rockies (same area as anticline shown earlier

Lulworth Cove Dorset

Synclines and anticlines commonly found next to each other (Canadian Rockies)

A small anticline next to a syncline

Folds can be extreme in their extent of crumpling Folds can be extreme in their extent of crumpling. This is especially common in weak, thin-bedded shales, as shown here.

Monocline A fold where both limbs remain horizontal

The House Rock Valley which runs between the reddish escarpment of the Vermilion Cliffs (right) and the green East Kaibab Monocline (left), is visible far to the north. (Grand Canyon)

Monocline in the Waurn Ponds Limestone Member of the Jan Juc Formation (Oligocene to Miocene) at Waurn Ponds. Victoria State. Australia

                                                                                    Island monocline, Colorado National Monument

Another view of the same monocline

Folding The Appalachians are good examples of folded mountains with many anticlines and synclines Folding stresses often begin deep in the Earth and the pressure increase heat making rocks more malleable