Built by a geologist

Deformation & Mountain Building

Deformation & Mountain Building Rocks may seem permanent and rigid, but under the right conditions, they bend, break, and flow — creating folds, faults, and the great mountain ranges of the world. Stress and Strain - Stress : Force applied to rock per unit area - Strain : The resulting deformation change in shape or volume Three types of stress: 1. Compressional : Squeezing convergent boundaries 2. Tensional : Pulling apart divergent boundaries 3. Shear : Sliding past transform boundaries How Rocks Respond Rocks respond differently depending on temperature, pressure, and rate of stress: - Brittle deformation : Rock breaks faults, fractures — happens at shallow depths, low temperature, rapid stress - Ductile deformation : Rock bends without breaking folds — happens at depth, high temperature, slow stress Folds When ductile rocks are compressed, they buckle into folds: - Anticline : Arch-shaped fold oldest rocks in center - Syncline : Trough-shaped fold youngest rocks in center - Monocline : A step-like bend in otherwise flat-lying layers - Overturned fold : One limb has been pushed past vertical - Recumbent fold : Folded nearly horizontal — common in mountain belts Faults When brittle rocks are stressed beyond their strength, they fracture and move: - Normal fault : Hanging wall moves down tensional stress — creates rift valleys and grabens - Reverse/Thrust fault : Hanging wall moves up compressional stress — common in mountain belts - Strike-slip fault : Horizontal movement shear stress — e.g., San Andreas Fault Orogenesis Mountain Building Mountains form through several mechanisms: - Collision orogenesis : Two continental plates collide → massive folding and thrusting Himalayas, Alps - Subduction orogenesis : Oceanic plate subducts → volcanic mountain chain Andes, Cascades - Fault-block mountains : Normal faulting uplifts blocks of crust Basin and Range, Tetons - Volcanic mountains : Built from accumulated volcanic material Mount Fuji, Mount Kilimanjaro Isostasy Earth's crust "floats" on the denser mantle like an iceberg in water. When mountains erode, the crust slowly rises rebounds . When glaciers melt, the land rises — Scandinavia is still rising at 1 cm/year from the last ice age.