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Covers the deformation of the Earth's crust through stress and strain, leading to earthquakes and faults.
What if the 'solid' rock beneath your feet is actually flowing like slow-moving liquid or snapping like a dry twig? Discover how the Earth's crust reshapes itself into massive mountain ranges and deep valleys through the invisible power of tectonic stress.
Diastrophism refers to the large-scale deformation of the Earth’s crust. This process is driven by stress (the force applied per unit area, ) and results in strain (the change in shape or volume). Rock behaves differently depending on temperature, pressure, and the rate of stress. Elastic deformation is temporary; the rock returns to its original shape once stress is removed. However, once the elastic limit is exceeded, the rock undergoes permanent change. This can be plastic (ductile) deformation, where the rock flows and bends without breaking, or brittle deformation, where the rock fractures and faults. Generally, high temperatures and high confining pressures favor plastic behavior, while cold, shallow crustal conditions favor brittle failure.
Quick Check
If a rock layer returns to its original shape after a tectonic force stops, what type of deformation has occurred?
Answer
Elastic deformation.
When rocks are subjected to compressional stress in a ductile environment, they bend into waves called folds. The three primary types are: 1. Anticlines: Arch-like folds where the oldest rock layers are in the core (center). 2. Synclines: Trough-like folds where the youngest rock layers are in the center. 3. Monoclines: Step-like folds in otherwise horizontal sedimentary strata. The 'limbs' are the sides of the fold, and the 'axial plane' is an imaginary surface that divides the fold symmetrically. In advanced geomorphology, we also look at overturned folds, where the axial plane is tilted so far that the limbs dip in the same direction.
Imagine a cross-section of a fold shaped like an 'A'. 1. Observe the shape: It is an arch, meaning it is an anticline. 2. Locate the center: Because it is an anticline, the layers at the very bottom/center of the arch were deposited first. 3. Conclusion: The oldest rocks are found at the core of the anticline.
Quick Check
In a syncline, where would you find the youngest rock layers?
Answer
In the center (core) of the fold.
When stress exceeds the rock's strength in a brittle environment, it fractures, and displacement occurs along a fault. We classify faults based on the relative movement of the hanging wall (the block above the fault plane) and the footwall (the block below). Normal faults occur under tensional stress (pulling apart), causing the hanging wall to move downward relative to the footwall. Reverse faults occur under compressional stress (pushing together), forcing the hanging wall upward. If the fault angle is very low (), it is called a thrust fault. Finally, strike-slip faults occur under shear stress, where blocks slide horizontally past each other, like the San Andreas Fault.
A geologist measures a vertical offset in a coal seam across a fault line. 1. The fault plane dips at . 2. The hanging wall has moved 10 meters upward relative to the footwall. 3. Identify the fault: Since the hanging wall moved up, this is a reverse fault. 4. Identify the stress: This indicates the region is under compressional stress.
Consider the Himalayan mountain range, formed by the collision of the Indian and Eurasian plates. 1. The primary stress is compression. 2. Because the crust is thick and hot at depth but cold at the surface, you see a combination of folding (at depth) and thrust faulting (near the surface). 3. This creates a 'fold-and-thrust belt' where crustal shortening is calculated by .
Which type of fault is most likely to be found at a divergent plate boundary where the crust is being pulled apart?
A fold where the rock layers dip away from the axial center and the oldest rocks are in the middle is called a(n):
Brittle deformation is more likely to occur deep within the Earth's mantle than near the surface.
Review Tomorrow
In 24 hours, try to sketch the difference between a normal and reverse fault, specifically labeling the hanging wall and footwall.
Practice Activity
Find a photo of a mountain range (like the Alps or Appalachians) and try to identify if the visible 'wavy' layers are anticlines or synclines.