Deformation & Mountain Building Flashcards
On Exam 2
Stress
Forces acting on the surface of a material
Deformation
Change in a rock’s position, shape, and/or volume from the application of stress
Strain
The measure of deformation (the change in shape or volume that resulted from applied stress)
APPLYING STRESS = STRAIN
What are the 3 main types of stress?
Tensional (aka Extensional), Compressional, and Shear
Tensional Stress
A pulling motion that acts outward from the rock, causing it to become longer and thinner
Occurs at divergent plate boundaries and often leads to normal faults
Compressional Stress
A pushing or squishing motion that acts inward on a rock, causing it to become shorter and thicker
Occurs at convergent plate boundaries and often leads to reverse faults
Shear Stress
Forces acting parallel but in opposite directions, causing the rock to become tilted
Occurs at transform plate boundaries
What are the different types of Deformation (or results of stress)? What determines which type a rock will go through?
Elastic, Plastic/Ductile, and Brittle
The type of deformation is controlled by the rock type, composition, temperature, pressure, and time
Elastic Deformation
A temporary change in position, shape, or volume
The rock will return to its original shape once the stress is removed
Like Elastic Rebound in earthquakes
Plastic/Ductile Deformation
Bending, flowing, and/or smearing of rock in a way that does not return to its original shape once stress is removed
Brittle Deformation
A rock that breaks, cracks, or fractures under stress and does not return to its original shape once stress is removed
Are there some materials that can undergo all types of deformation?
Yes, metal is a good example of this
How does intensity of pressure affect deformation?
Some rocks are brittle at low pressure, but ductile at high pressure
What is Strike?
The compass direction that the rock is facing (based on an imaginary horizontal line/plane that the geologic structure falls on)
What is Dip?
The angle of the geologic feature or rock relative to the Strike (or horizontal plane)
How do we classify rock fractures?
Based on the amount of movement along the fracture
Joints vs Faults
Joints
Fractures with no appreciable movement
Can occur in columns due to weathering (this is not the same as cleavage) or onion-like cracks due to the expansion or uplifting of igneous rocks
Faults
Fractures with significant movement
Hanging Wall vs Foot Wall
What is Slip?
The direction of motion along a fault, divided into Dip-Slip and Strike-Slip
Dip-Slip faults
Movement occurs vertically in line with the dip of the fault (i.e. there will be an angle)
There are two types: Normal (hanging go down) and Reverse (hanging go up)
Rarely occur in isolated instances, usually happen in a series
Strike-Slip faults
Movement occurs horizontally parallel to the strike
There are two types: Right-Lateral (appears to move right) and Left-Lateral (appears to move left)
Usually occurs at transform boundaries
What if we bend a rock instead of breaking it (ductile instead of brittle)?
We get folds, which have two main parts: the hinge and the axial plane
Fold Hinge
The point or line that joins the two areas of greatest curvature (the very top or bottom of the curve)
Think door hinge - where the bend occurs
Axial Plane
The imaginary surface or line that divides the fold into two halves along the hinge
What are the different kinds of fold geometry?
- Symmetric: same angle on both sides (stress applied equally)
- Asymmetric: different angles (stress applied more strongly in one direction over the other)
- Horizontal: the hinge has no slope
- Plunging: the hinge in dipping
What are the types of Folds?
Anticline and Syncline (can be either horizontal or plunging)
We also see Domes and Basins
Anticline Folds
Arches upward, with the oldest rock at the center of the fold (when looking at it from the surface)
Makes an angry face
Syncline Folds
Arches downward, with the youngest rocks at the center
Makes a happy face
What shape does plunging anticlines and synclines make?
A zigzag
What are Domes?
Broadly circular bulge caused by the rock being pushed upwards from below
Rocks dip away from the summit like a hill
Youngest layer will be the top one, unless the bulge has been weathered away, in which case, the oldest layer will be the one at the very center of the summit
What are Basins?
Broadly circular bowl-shaped structure
Rocks dip towards the center
The youngest layer will be the one at the center of the bowl
What are the 4 main ways to create a mountain?
Hot Spots, Mid-Ocean Ridges, Subduction, and Collision
Hot Spot Mountains
Will be made of dark mafic rock
Ex: Hawaii
Mid-Ocean Ridge Mountains
Taller at center and gets deeper further away (the center is younger and less dense, further = older and more dense)
Subduction Mountains
Magma is generated at subduction zones, which may build mountains and volcanoes (though they are usually smaller)
Since the subducting crust will be oceanic, the lava will be mafic
Ex: the Andes
Collision Mountains
Categorized based on size and material colliding: continents crashing into either small crustal block or other continents
Continent & Crustal Block Collisions
Produces small mountains like terranes or accreted terranes (which are just multiple crustal blocks plastered together)
Ex: Western North America
Continent & Continent Collisions
Produces the tallest mountains, such as the Himalayas (which are the youngest + highest and are still colliding) and the Appalachians (which used to be bigger and were originally formed during the creation of Pangea)
Why did India remain relatively unchanged when the plate collided with Asia, while the Asian interior deformed significantly?
Rocks in the Asian interior were composed of younger crustal sediments, which are weaker and deform/fault much easier than the igneous rock of India
