Types of Stress
Plate boundaries experience three primary types of stress:
* Tensional Stress: This occurs when plates move apart (divergent boundaries). The stress pulls the rocks in opposite directions, stretching and thinning them.
* Compressional Stress: This happens when plates collide (convergent boundaries). The stress pushes rocks together, causing them to fold, fracture, and thicken.
* Shear Stress: This happens when plates slide past each other (transform boundaries). The stress causes rocks to slide horizontally, leading to fracturing and displacement.
Rock Behavior under Stress
The behavior of rocks under stress depends on several factors, including:
* Rock Type: Different rock types have different strengths and abilities to deform. Igneous rocks (like granite) tend to be brittle, while sedimentary rocks (like sandstone) can be more ductile.
* Temperature and Pressure: Higher temperatures and pressures make rocks more ductile (able to bend and flow).
* Rate of Stress Application: Sudden stress leads to brittle behavior (fracturing), while slow, gradual stress allows for ductile deformation.
What Happens at Different Boundaries
* Divergent Boundaries: Rocks undergo tensional stress, leading to:
* Fracturing: The rocks break, forming faults.
* Thinning: The crust thins and can even split apart, creating rift valleys.
* Magmatism: The thinning crust allows magma to rise from the mantle, forming new oceanic crust.
* Convergent Boundaries: Rocks undergo compressional stress, leading to:
* Folding: Rocks bend and warp, forming mountains and mountain ranges.
* Faulting: Rocks break and slide past each other, creating thrust faults and reverse faults.
* Metamorphism: The intense pressure and heat can transform existing rocks into new, denser metamorphic rocks.
* Subduction: One plate may slide beneath another, melting the lower plate and creating magma that leads to volcanic activity.
* Transform Boundaries: Rocks undergo shear stress, leading to:
* Faulting: The rocks fracture and slide horizontally, creating transform faults (like the San Andreas Fault).
* Earthquakes: The sudden movement along these faults is the cause of earthquakes.
Key Concepts
* Elastic Deformation: Rocks can temporarily deform (stretch or compress) under stress, returning to their original shape when the stress is released.
* Ductile Deformation: Rocks can deform permanently by bending or flowing under stress, especially at high temperatures and pressures.
* Brittle Deformation: Rocks break or fracture under stress, especially at lower temperatures and pressures.
In summary, the stress at plate boundaries causes rocks to deform in various ways, leading to the formation of mountains, volcanoes, earthquakes, and the creation of new crust.
