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…
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)
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
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
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