Features of Plate Tectonics Chapter 12.2 BC Science 10
The Layers of Earth • Earth is over 1200 km thick and has four distinct layers: • Crust – outer solid rock layer (granite on land, basalt in oceans) • Mantle – thickest layer, mostly solid except for upper mantle being able to flow like “thick toothpaste” • Outer core – composed of liquid iron and nickel • Inner core – mostly solid iron, at tremendous temperature and pressure
The Upper Mantle.... • We know the “Lithosphere” contains the Earth’s crust and the uppermost part of the upper mantle. • Just below this is the “Asthenosphere” • A partly MOLTEN layer of the upper mantle
Plate Motion • The plates move because of convection currents arising in the hot magma of the upper mantle in the Asthenosphere.... 2 Q’s: What makes this layer HOT? What causes the “currents”?
What Makes the Layer HOT! • Partly the upper mantle is hot because of Earth’s hot core.... • The temp of the upper mantle varies throughout • Large amount of RADIOACTIVE elements (ex: Uranium) occur in certain spots • When these decay...they release HEAT!
What Causes the “Currents” Just like in air masses.....HEAT RISES! Hot (less dense) material rises…cools….sinks….reheats
The Driving Force... • This “mantle convection” is the driving force behind the movement of tectonic plates!
Tectonic Plates can PUSH or PULL “Spreading Centers” are areas of the Earth’s surface where magma rises up.... (common) (less common) IN OCEAN.... “Spreading Ridge” On LAND.... “Rift Valley”
“Out with the OLD, In with the NEW” • When magma rises and cools it forms new rock. • This new rock PUSHES old rock ASIDE “RIDGE PUSH”
SUBDUCTION • When plates “bump” into each other, they may subduct.... Consider a “heavy, dense” oceanic plate bumping into a lighter continental plate....
SUBDUCTION • The HEAVIER plate slides under the lighter plate. This is called “SUBDUCTION”
SLAB PULL • When a plate subducts deep into the mantle....it PULLS the rest of the plate too “SLAB PULL”
Action in the Subduction Zones! • SUBDUCTION ZONES experience more • EARTHQUAKES • VOLCANIC ERRUPTIONS!
Plate Interactions • A plate boundary an area where two plates are in contact. • Divergent plate boundaries – areas where plates are spreading apart • Convergent plate boundaries – areas where plates meet • Oceanic-continental • Oceanic-oceanic • Continental-continental • Transform plate boundaries – areas where plates move past each other
Divergent Plate Boundaries • Divergent plate boundaries are areas where plates are spreading apart. • Ocean ridges and continental rifts are examples. • The Mid-Atlantic Ridge is the longest mountain range on Earth.
Convergent Plate Boundaries • Convergent plate boundaries are areas where plates collide. • Oceanic-continental plate convergence • The oceanic plate subducts under the continental plate, forming a trench. • Cone-shaped volcanoes can form from magma seeping to the surface. • This is how the volcanic belt of the North America’s west coast has formed. • Mountain ranges like the Coast Mountain range also form from the collision. • Earthquakes can occur when subduction, ridge push, and slab pull stall.
Oceanic-Oceanic Convergence • Oceanic-oceanic plate convergence • The cooler, denser plate will subduct under the less dense plate. • Convergence may produce a volcanic island arc, such as those found in Japan, Indonesia, and Alaska’s Aleutian islands.
Continental-Continental Convergence • Continental-continental plate convergence • Since both are continental plates, their densities are similar. • As they collide, their edges fold and crumple, forming mountain ranges. • The Himalayas are the world’s youngest (and tallest) mountain range, formed as Asia and Africa plates collided 40 million years ago. They are still growing taller today.
Transform Plate Boundaries • Transform plate boundaries are where plates move past each other. • Usually are found near ocean ridges • Since rock slides past rock, no mountains or volcanoes form. • Earthquakes and faults are very common.
Can Earthquakes be Predicted? Earthquake Precursors • changes in elevation or tilting of land surface • fluctuations in groundwater levels • magnetic field • electrical resistance of the ground • Release of gases
Can Earthquakes be Predicted? Earthquake Prediction Programs • include laboratory and field studies of rocks before, during, and after earthquakes • monitor activity along major faults • produce risk assessments
Elastic Rebound Theory Rocks bend under stress while storing elastic energy. When the strain in the rocks exceeds their strength, breaking will occur along the fault. Stored elastic energy is released as the earthquake. Rocks“snap back”, or rebound to their original condition.
Types of Earthquake (Seismic) Waves • Body Waves • P-Waves (primary waves) • S-Waves (secondary waves) • Surface Waves - L – Love Waves - R – Raleigh Waves (surface, vertical)
Body Waves: P and S waves • Body waves • P or primary waves • fastest waves • travel through solids, liquids, or gases • compressional wave, material movement is in the same direction as wave movement • S or secondary waves • slower than P waves • travel through solids only • shear waves - move material perpendicular to wave movement
Surface Waves: R and L waves • Surface Waves • Travel just below or along the ground’s surface • Slower than body waves; rolling and side-to-side movement • Especially damaging to buildings
Distribution of Quakes A direct result of Continental Drift