- Materials that deform steadily under stress. Purely viscous materials like liquids deform under even the smallest stress. Rocks may behave like viscous materials under high temperature and pressure.
- Material does not flow until a threshold stress has been exceeded.
- Combines elastic and viscous behavior. Models of glacio-isostasy frequently assume a viscoelastic earth: the crust flexes elastically and the underlying mantle flows viscously.
It is fairly common for rocks to show both brittle and ductile deformation. Rocks may fold but also fracture as they fold. Faults may fracture but also show evidence of ductile behavior as well.
Brittle deformation results in fracturing of the rocks. There are two principal kinds of fractures:
A lot of different types of fractures are lumped together as joints but really are unrelated phenomena:
Faults are classified according to the kind of motion that occurs on them
|The opposing side of the fault moves to the observer's left. Note that this definition does not depend on which side the observer is on. A pebble in the fault zone will rotate counterclockwise.|
|The opposing side of the fault moves to the observer's right. Note again that this definition does not depend on which side the observer is on. A pebble in the fault zone will rotate clockwise.|
Important effects of ductile deformation in the crust include:
|Anticlines are folds where the rocks arch upward. On geologic maps they show up as structures with old rocks in the middle and younger rocks outside.|
|Synclines are folds where the rocks bow downward. On geologic maps they show up as structures with young rocks in the middle and older rocks outside.|
|Monoclines are steps in otherwise horizontal, or nearly horizontal, rocks. Very often they overlie faults, but the rocks merely sag over the fault offset instead of being broken.|
|Homoclines aren't really folds, but are places where the rocks slope, or dip, in the same direction over a large area. The homocline may or may not be part of a larger structure.|
Salt domes are the best known and most common diapirs. In the cross-section above, light blue represents salt and other shades represent other sedimentary rock layers. Salt is both very ductile and much less dense than most other rocks. If it finds a weak spot in the overlying rocks (left) it begins to flow upward. The pressure of adjacent rocks on the salt layer, plus the low density of the salt, cause the salt to continue rising (center). In advanced stages, the salt often takes on a mushroom shape (right) and can even become entirely disconnected from its roots. The upturning of layers adjacent to the salt creates numerous traps for petroleum. In the North Sea and Gulf of Mexico, the search for petroleum basically amounts to a search for salt domes.
The salt can and often does reach the surface. On the Gulf Coast, the salt is very vulnerable to solution, and collapse due to solution is common. In extremely arid regions, notably Iran and Morocco, salt that reaches the surface can flow downhill as a salt glacier.
Isostasy is vertical movement of the crust due to application of loads. The most significant examples at present are the vertical uplifts associated with removal of the Pleistocene ice sheets. The map below shows isostatic rebound in Canada since 6000 years ago.
Epeirogeny is gentle uplift or subsidence of the crust, sometimes by kilometers, but with little igneous activity, faulting, metamorphism, or intense deformation. The mechanisms of epeirogeny are not well understood. Epeirogenic regions are characterized by domes, arches, and basins. The Great Lakes region is one of the best illustrations of epeirogeny anywhere.
Jurassic (Blue-Gray, Michigan)
Permian (Dark Blue)
Devonian (Light Blue)
Silurian (Light Green)
Units within the Great Lakes are horizontally ruled.
Note how the Great Lakes correlate with the rock units. Lake Michigan, the main portion of Lake Huron, and Lake Erie are mostly underlain by Devonian rocks, mostly shales, which were easily excavated by the glaciers. The Green Bay lowland and Green Bay itself were scoured out of soft Ordovician shales (the Maquoketa Formation). So were the channels between the islands in Lake Huron and the mainland, Georgian Bay, the lowland across southern Ontario, and Lake Ontario. Four of the Great Lakes have their shapes and locations determined by structures in the Paleozoic rocks (and Lake Superior is excavated out of relatively soft Precambrian sedimentary rocks as well).
The diagram above gives a three-dimensional view of the structure of the rocks. The vertical scale is very much exaggerated. . It's fairly common for sedimentary basins to resume activity after long periods of inactivity, and subsidence seems to correlate with major periods of mountain-building, but the exact connection is still not well understood.
|The major uplifts and basins in the Great Lakes region. These structures actually grade smoothly into one another; we can think of the Silurian rocks of Wisconsin as either the eastern edge of the Wisconsin Arch or the western edge of the Michigan Basin.|