How to Build a Mountain and other Geologic Structures But first a short review
Where do we see deep earthquakes? What is happening there?
What can happen at a plate boundary? 1. Plates can move apart This makes new oceanic crust, but it doesn t make mountains 2. Plates can slide by one another This causes earthquakes, but it doesn t make mountains 3. Plates can collide This makes mountains
How to build a mountain We will talk about valleys (erosion and weathering later)
Ocean-continent plate convergence Oceanic plate of denser basaltic material subducted under less dense granite-type continental shelf Marked by oceanic trench, deep-seated earthquakes and volcanic mountains Example: convergence of South American Plate with Nazca Plate
The Aleutian Island Chain
Continent-continent plate convergence Less dense, granite-type materials resist subduction Colliding plates pile up, producing a deformed and thicker crust of lighter materials Example: Tibetan Plateau and Himalayan Mountains
The Himalayas
Another way to build a mountain
Glacier National Park
Northwest Montana Fold and Thrust Belt Chinese Wall
Northwest Montana Fold and Thrust Belt Flathead Lake
Northwest Montana Fold and Thrust Belt Chief Mountain
Chief Mountain
Principle of Superposition Youngest Strata Oldest Strata
Another way to build a mountain
Volcanic Eruptions Lava is produced when magma reaches Earth s surface Explosive eruptions can produce rapidly cooled rock fragments called pyroclasts Size range from dust (ash) to boulders (blocks and volcanic bombs) Calm oozing of magma out of the ground produces lava flows Pyroclastics and lava flows form extrusive igneous rocks Lava flows and pyroclasts pile up to form volcanoes
Mt Fuji: Stratovolcano
Elkhorn Mountains
Where do we see deep earthquakes? What is happening there?
Yet another way to build a mountain
Ocean-continent plate convergence Oceanic plate of denser basaltic material subducted under less dense granite-type continental shelf Marked by oceanic trench, deep-seated earthquakes and volcanic mountains Example: convergence of South American Plate with Nazca Plate
The Sierra Nevada Mountains Half Dome
The Sierra Nevada Mountains
The Boulder Batholith outside of Butte
The Boulder Batholith outside of Butte
One last way to build a mountain Although there are others
The Beartooths
The Beartooth Plateau Rocks that are billions of years old, pushed up about 70 million years ago, and sculpted by glaciers for the last 2 million years
The Beartooth Plateau SW
Geologic Structures Geologic structures are dynamically-produced patterns or arrangements of rock or sediment that result from, and give information about, forces within the Earth
Orientation of Geologic Structures Geologic structures are most obvious in deformed sedimentary rocks Tilted beds, joints, and faults are planar features whose orientation is described by their strike and dip Strike is the compass direction of a line formed by the intersection of an inclined plane with a horizontal plane Dip is the direction and angle from horizontal in which a plane is oriented
Figure 15.6
Figure 15.7
What do rocks do when you put a stress on them? They bend or They break If they bend we call it a fold. If they break we call it a fracture. It the fracture moves, we call it a fault
The three basic types of stress are compressive, tensional and shear
Stress Compressive The three basic types of stress are compressive, tensional and shear Tensional Shear
Geologic Structures: Folds Folds are wavelike bends in layered rock Represent rock strained in a ductile manner, usually under compression Anticlines are upward-arching folds Synclines are downward-arching folds
Types of Folds Plunging folds are folds in which the hinge line is not horizontal Where surfaces have been leveled by erosion, plunging folds form V- or horseshoe-shaped patterns of exposed rock layers (beds) Open folds have limbs that dip gently, whereas isoclinal folds have parallel limbs Overturned folds have limbs that dip in the same directions, and recumbent folds are overturned to the point of being horizontal
Folds.exe
Structural Domes and Basins Domes are structures in which the beds dip away from a central point Sometimes called doubly plunging anticlines Basins are structures in which the beds dip toward a central point Sometimes called doubly plunging synclines
Figure 15.19
Fractures in Rock Joints - fractures bedrock along which no movement has occurred Multiple parallel joints are called joint sets Faults - fractures in bedrock along which movement has occurred Considered active if movement has occurred along them within the last 11,000 years Categorized by type of movement as dip-slip, strike-slip, or oblique-slip
Types of Faults Normal faults have movement parallel to the dip of the fault plane In normal faults, the hanging-wall block has moved down relative to the footwall block In reverse faults, the hanging-wall block has moved up relative to the footwall block Insert revised Fig. 15.22 here Insert revised Fig. 15.26a here
Normal and Reverse Faults
Types of Faults Dip-slip (normal) faults have movement parallel to the dip of the fault plane Fault blocks, bounded by normal faults, that drop down or are uplifted are known as grabens and horsts, respectively Grabens associated with divergent plate boundaries are called rifts Thrust faults are reverse faults with dip angles less than 30 from horizontal
The Tetons
The Tetons
Basin and Range
Basin and Range
Basin and Range
Types of Faults Strike-slip faults have movement that is predominantly horizontal and parallel to the strike of the fault plane A viewer looking across to the other side of a right-lateral strike-slip fault would observe it to be offset to their right A viewer looking across to the other side of a left-lateral strike-slip fault would observe it to be offset to their left Oblique-slip faults have movement with both vertical and horizontal components Right-lateral San Andreas Fault
Juan de Fuca plate
San Andreas Fault
San Andreas Fault
Figure 15.21 Faulitng.exe