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

Explore the factors that control how rocks respond to stress, including the nature of the rock, temperature, and pressure. Learn how different types of rocks react to stress, from elastic deformation to brittle fracture and plastic deformation.