Day 3: by 10/14 (A) & 10/15 (B)
Faults and Mountain Building
When you think about enormous plates of lithosphere traveling around on the planet's surface, you can probably imagine that the process is not smooth. Most geological activity takes place where two plates meet, at plate boundaries. When plates are pushed or pulled, the rock is subjected to stress. Stress can cause a rock to change shape or to break. When a rock bends without breaking, it folds. When the rock breaks, it fractures. Mountain building and earthquakes are some of the responses rocks have to stress.
What Causes Faults?
Faults are classified by how they move, and there are three main types of stress that cause movement along faults. The stress occurs because as mentioned before, the plates fit together really well, but also float around on the mantle and rub against each other.
Tensional stress is when slabs of rock are pulled apart. Imagine stretching a rope out all the way and then continuing to pull on it from both ends. You're putting tension on the rope because it's being pulled in opposite directions. The same is true for tensional stress in rocks.
When you think about enormous plates of lithosphere traveling around on the planet's surface, you can probably imagine that the process is not smooth. Most geological activity takes place where two plates meet, at plate boundaries. When plates are pushed or pulled, the rock is subjected to stress. Stress can cause a rock to change shape or to break. When a rock bends without breaking, it folds. When the rock breaks, it fractures. Mountain building and earthquakes are some of the responses rocks have to stress.
What Causes Faults?
Faults are classified by how they move, and there are three main types of stress that cause movement along faults. The stress occurs because as mentioned before, the plates fit together really well, but also float around on the mantle and rub against each other.
Tensional stress is when slabs of rock are pulled apart. Imagine stretching a rope out all the way and then continuing to pull on it from both ends. You're putting tension on the rope because it's being pulled in opposite directions. The same is true for tensional stress in rocks.
Compressional stress is when slabs of rock are pushed together. They are literally being compressed into one another. This is like when two cars crash into each other - they're compressed into smaller, crumpled versions of what they were before.
Shear stress is when slabs of rock slide past each other horizontally in opposite directions. The rocks are not smashed into each other or pulled apart, but their edges slide along each other with a lot of friction. This is like when you rub your hands together to warm them up. One hand goes forward and the other goes backward, rubbing against each other.
Types Of Faults
The type of fault we get depends on the type of stress that caused it, which also tells us about how the fault moves. Faults have two sides: the hanging wall and the footwall. These terms were coined by miners because you can stand with your feet on the footwall and hang a lamp on the hanging wall on the opposite side. And the reverse situation would be impossible! In the diagram below, A is the hanging wall and B is the footwall.
The type of fault we get depends on the type of stress that caused it, which also tells us about how the fault moves. Faults have two sides: the hanging wall and the footwall. These terms were coined by miners because you can stand with your feet on the footwall and hang a lamp on the hanging wall on the opposite side. And the reverse situation would be impossible! In the diagram below, A is the hanging wall and B is the footwall.
Tensional stress, meaning rocks pulling apart from each other, creates a normal fault. With normal faults, the hanging wall and footwall are pulled apart from each other, and the hanging wall drops down relative to the footwall. Most of the area just west of the Rocky Mountains is affected by normal faults: places like southern Oregon, southwestern Idaho, eastern California, western Utah and the entire state of Nevada.
Strike-slip faults result from shear stresses. If you stand with one foot on one side and one foot on the other side of a strike-slip fault, the block on one side will be moving toward you and the block on the other side will be moving away from you. If the block moving toward you is the block that your right foot is on, the fault is known as a right-lateral strike-slip fault. If the block moving toward you is the one your left foot is on, the fault is a left-lateral strike-slip fault. The world's most famous strike-slip fault is the San Andreas Fault in California, which is a right-lateral strike slip fault. Because the San Andreas is a plate boundary, it is also called a transform fault. People sometimes say that California will fall into the ocean someday, but this is a joke. The portion west of the San Andreas Fault is moving northeastward and someday Los Angeles will be a suburb of San Francisco, but the land west of the San Andreas Fault is a solid piece of continental crust that will not disappear entirely.
Mountain Building
Mountains are usually formed by volcanic activity, folding or faulting.
Converging plates create the world's largest mountain ranges. Each combination of plate types — continent-continent, continent-ocean, and ocean-ocean — creates mountains. Two converging continental plates smash upwards to create gigantic mountain ranges Stresses from this uplift cause folds, reverse faults, and thrust faults, which allow the crust to rise upwards.
Subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges. This happens on continental crust, as in the Andes Mountains ( Figure below ), or on oceanic crust, as with the Aleutian Islands, which we visited earlier. The Cascades Mountains of the western U.S. are also created this way.
Mountains are usually formed by volcanic activity, folding or faulting.
Converging plates create the world's largest mountain ranges. Each combination of plate types — continent-continent, continent-ocean, and ocean-ocean — creates mountains. Two converging continental plates smash upwards to create gigantic mountain ranges Stresses from this uplift cause folds, reverse faults, and thrust faults, which allow the crust to rise upwards.
Subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges. This happens on continental crust, as in the Andes Mountains ( Figure below ), or on oceanic crust, as with the Aleutian Islands, which we visited earlier. The Cascades Mountains of the western U.S. are also created this way.
Amazingly, even divergence can create mountain ranges. When tensional stresses pull crust apart, it breaks into blocks that slide up and drop down along normal faults.
The result is alternating mountains and valleys, known as a basin-and-range ( Figure below). In basin-and-range, some blocks are uplifted to form ranges, known as horsts, and some are down-dropped to form basins, known as grabens.
Folds
Rocks deforming plastically under compressive stresses crumple into folds . They do not return to their original shape. If the rocks experience more stress, they may undergo more folding or even fracture.
You can see three types of folds.
Monocline
A monocline is a simple bend in the rock layers so that they are no longer horizontal (see Figure below).
Rocks deforming plastically under compressive stresses crumple into folds . They do not return to their original shape. If the rocks experience more stress, they may undergo more folding or even fracture.
You can see three types of folds.
Monocline
A monocline is a simple bend in the rock layers so that they are no longer horizontal (see Figure below).
What you see in the image appears to be a monocline. Are you certain it is a monocline? What else might it be? What would you have to do to figure it out?
Anticline
Anticline: An anticline is a fold that arches upward. The rocks dip away from the center of the fold ( Figure below). The oldest rocks are at the center of an anticline and the youngest are draped over them.
Anticline
Anticline: An anticline is a fold that arches upward. The rocks dip away from the center of the fold ( Figure below). The oldest rocks are at the center of an anticline and the youngest are draped over them.
Syncline
A syncline is a fold that bends downward. The youngest rocks are at the center and the oldest are at the outside ( Figure below).
A syncline is a fold that bends downward. The youngest rocks are at the center and the oldest are at the outside ( Figure below).
Questions
1. Describe the 3 types of faults (include stress, movement of hanging wall/footwall, diagrams)
2. What are 3 ways mountains are formed?
3. At what type of plate boundaries do we usually find mountains?
4. What are horsts and grabens?
5. Describe and draw the three types of folds.
6. What folds can you find in this photo of Monument Valley in Arizona? Notice the rock layers at the top of the ridge. What is the geologic history of this region?
1. Describe the 3 types of faults (include stress, movement of hanging wall/footwall, diagrams)
2. What are 3 ways mountains are formed?
3. At what type of plate boundaries do we usually find mountains?
4. What are horsts and grabens?
5. Describe and draw the three types of folds.
6. What folds can you find in this photo of Monument Valley in Arizona? Notice the rock layers at the top of the ridge. What is the geologic history of this region?
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