Folds, Faults, and Mountain BuildingFolds, Faults, and Mountain Building
Chapter 9Chapter 9
Stress!!!
• Stress—a force exerted on an object• Rocks react to stress by – Deforming plastically– Deforming elastically– Breaking by brittle fracture
• Question of the hour (#1)– How will a particular rock react to a particular stress?
It’s all in the Nature…
• Rock composition– Ex: Hitting a chunk of
concrete with a sledgehammer VS. hitting a giant rubber tire with a sledgehammer
I Can’t Take the Pressure!!
• Pressure and Temp. increase with depth– Temp vs. pressure– Burial promotes plastic
deformation
If you can’t stand the heat…
• Higher temperatures favor plastic deformation– Ex. Bending a steel nail
• Need to be super strong• Or just heat the thing up…
It’s About Time…• Stress applied slowly favors plastic behavior• Stress applied suddenly favors brittle behavior
May the Tectonic Force be With You….
• Plate tectonic forces– Tensional• Stretch and pull formations apart
– Compressive• Squeeze and shorten rock formations
– Shearing• Two sides of a formation are pushed in opposite
directions (sliding past one another)
Geologic Structures
• Plate tectonics—primary stressor• 3 main types of structures– Folds—ductile deformation (primarily)– Faults—brittle deformation– Joints—brittle deformation
Folds
• A fold is a bend in the rock– Plastic deformation
• Three characteristics– Usually results from compression– Folding always shortens the horizontal distances
in rock– A fold usually occurs as part of a group of folds
Anticline Vs. Syncline
Plunging Folds
Overturned and Asymmetric
Circular Features
• Domes– Circular anticlinal
structure (both pics)– Beds dip away from
center
• Basin– Circular synclinal
structure– Beds dip towards center
All My Faults are Stress Related!!!• Brittle deformation• Faults
– Cracks in rock along which motion has occurred
– Movement may be gradual or sudden (earthquake)
– Usually associated with other faults in a fault zone
• Question of the hour (#2): Why do rocks move repeatedly along faults and fault zones?
Nature is Lazy
• Rock moves repeatedly along faults and fault zones because– Tectonic forces often
affect one location for a long time
– It’s easier
• Three types of faults– Depend on tectonic
forces
Normal Faulting
Reverse Fault (part 1)
Reverse Fault (part 2)
Strike-slip Fault
Ah, How My Joints Ache…
• A joint is a fracture in rock along which no motion has occurred– Planes of weakness—like
mineral cleavage– Good for mining
Styles of Continental Deformation
Relating Small Structures to Big Picture
Question of the Hour (#3)
• Where do mountains come from?– In a subducting zone• Magmas/lavas—stall/erupt• High temp—rocks expand when heated
– Continent/continent collision—underthrusting• Crust is shoved under other crust• Compression squeezes crust—thickness increases
Tensional Tectonics
• Plates can split apart– Rift valleys – long narrow troughs formed by a
block that has dropped down between two big normal faults
Rifting and Normal Faulting
Compressive Tectonics• Fold and thrust belts:– When two continental plates collide, crust
compressed– Crust thickened by underthrusting—thicken crust
2x
Transverse Margins