Plate Tectonics: Rock Response to Stress

Factors Controlling Rock Response to Stress

• Rocks react to stress by deforming elastically, with granite and quartzite tending to fracture in a brittle manner, while shale, limestone, and marble tend to deform plastically
• Temperature affects rock response, with higher temperatures favoring plastic deformation
• Pressure also influences rock behavior, with high non-directed pressure favoring plastic behavior
• Time is another factor, with slow application of stress favoring plastic deformation

Geologic Structure

• Any feature produced by rock deformation, including folds and faults
• For deformation to occur, rock material must be able to deform under pressure and heat, with higher temperatures making rocks more elastic
• Deformation requires slow application of stress, without exceeding the internal strength of the rock

Folds

• A bend in a rock, indicating plastic deformation with little or no fracturing
• Characteristics of folds include:
  • Results from compression
  • Shortens horizontal distances in rocks
  • Occurs as part of a group of many similar folds
• Types of folds:
  • Anticline: a fold arching upward, commonly caused by compressive stress
  • Syncline: a fold arching downward, often forming basins
  • Monocline: an open, step-like structure with layers inclined in the same direction

Faults

• A fracture where one side of the rock has moved relative to the other side
• Types of faults:
  • Normal fault: formed due to tensional stress, often resulting in stretching of the Earth's crust
  • Reverse fault: formed due to compressive forces, often resulting in fracturing of rocks
  • Thrust fault: a special type of reverse fault that is nearly horizontal
  • Strike-slip fault: a fracture where rocks on opposite sides of the fault move past each other horizontally

Joints

• Fractures in rocks where the rocks on either side of the fracture have not moved
• Formed by tectonic forces, becoming less abundant with depth

Plate Boundaries

• Different types of plate boundaries produce different tectonic stresses and resulting folds and faults
• Types of convergent boundaries:
  • Oceanic-oceanic: one crust sinks underneath, triggering melting and volcanic activity
  • Oceanic-continental: denser oceanic crust sinks beneath the continental crust, forming volcanic arcs and trenches
  • Continental-continental: folding occurs, forming mountain ranges
• Divergent boundaries: plates drift apart, producing normal faults and grabens, with little folding
• Transform fault boundaries: plates slide past each other, with no creation or destruction of land

Continental Drift Theory

• Proposed by Alfred Wegener, suggesting horizontal movement of continents
• Evidences include:
  • Fit of continental shorelines
  • Similar rock types and fossils across continents
  • Fossil evidence of similar plants and animals across continents
  • Climatic evidence, including glacial striations

Seafloor Spreading Hypothesis

• Proposed by Harry Hess, suggesting that the oceanic crust is moving, with magma rising to form new ocean floor at mid-ocean ridges

Plate Tectonics Theory

• Confirmation of seafloor spreading hypothesis, showing that continents are not moving above a fixed mantle
• The asthenosphere behaves like a fluid, allowing the lithosphere to float and move above it
• Convection currents in the asthenosphere drive plate movement

Crust Types

• Continental crust: less dense, made of older material, and comprises silica and aluminum-based granite
• Oceanic crust: denser, formed at the bottom of the oceans, and comprises silica and magnesia-based basalt content

Plate Tectonics

• The theory that the Earth's crust is divided into several plates that glide over the mantle
• Major tectonic plates include African, Antarctic, Arabian, Australian, Caribbean, Cocos, Eurasian, Indian, Juan de Fuca, Nazca, North American, Pacific, Philippine, Scotia, and South America
• Plate boundaries are regions where tectonic plates are moving relative to each other