Features of Plate Tectonics

Section 12.2 Notes: Features of Plate Tectonics Layers of the Earth 1. Crust (Outermost; 0-100km thick) • Consists of a series of interlocking plates • Thinnest layers found under oceans, thickest layers found under continents (land) • Continental crust tends to be made of granite, oceanic crust tends to be made of basalt (slightly denser than granite) • Makes up ~ 1% of Earth’s mass 2. Mantle (~ 2900km thick) • Part solid, part liquid, Denser than the crust layer • Most of the mantle is under high pressure and is very hot • Accounts for ~ 70% of the Earth’s mass • Behaves like a thick liquid (similar consistency to tar) • Convection currents cause plates to move Inner ( ~1250km thick) and Outer Core (~2200km thick) • Thought to be mainly composed of iron and nickel with extremely high temperature and pressure • Outer core
Liquid (molten = melted) • Inner Core
Solid • Lithosphere
lower part of the crust; upper part of the mantle • Asthenosphere
upper part of the mantle; has a tar-like consistency o Lithosphere plates move around in the mantle by floating on top of the asthenosphere o This causes earthquakes and volcanoes Theory of Plate Tectonics • The Earth’s lithosphere is divided into a number of moving (~2cm/year) rigid pieces called plates. The study of formation and movement of these plates is known as Plate Tectonics Page 1 of 7 • The Theory of Plate Tectonics states that lithospheric plates move around slowly on the fluid asthenosphere, with the plates interacting when they collide • If the plate is in an ocean, it is called an Oceanic Plate • If the plate carries a continent, it is called a Continental Plate Plate Movement • The lithospheric plates are believed to move due to convection currents in the asthenosphere (Convection is the movement of heat) • The asthenosphere has a similar composition to the lithosphere, however, it is partially melted and therefore has the ability to flow • As the heated asthenosphere rises to the Earth’s surface, it pushes lithospheric plates apart and begins to cool and solidify, creating new lithosphere at areas called RIDGES (spreading centres) • As the asthenosphere cools, the material becomes denser and sinks back into the mantle. Here, the lithospheric plates move together and form a valley called a TRENCH • The latest tectonic theory states that a subducting plate is actually pulling the rest of the plate into the asthenosphere along with it. This process is referred to as a “SLAB PULL” Types of Plate Boundaries The areas in which plates meet are known as Plate Boundaries. There are three types of plate boundaries: 1) Diverging 2) Converging 3) Transform Page 2 of 7 Type of Plate Boundary DIVERGING Description I m a g e • Where two plates are moving apart from one another • Volcanoes can occur here • a.k.a. spreading centres Example: MidAtlantic Ridge CONVERGING • When two plates come together (collide) • Often form subduction zones, where a denser plate (typically oceanic) is forced underneath a less dense plate (trench formed) • Or…..Formation of mountains are found at these zones two plates move upwards Page 3 of 7 Three Types of Convergent Plate Boundaries 1 & 2 – Volcanoes • One plate is pushed down under the other 3 – Mountains • Both plates are forced upward TRANSFORM • Where two plates are sliding past one another • a.k.a. Sliding Boundaries • Where earthquakes occur Fault • Your Data Booklet provides information about where the various types of plate boundaries are located throughout the World Earthquakes • Earthquakes and volcanoes generally occur along plate boundaries • Earthquakes detected by Seismographs • Occur when plates move suddenly along faults • Occur in waves called SEISMIC WAVES (P, S, & L). Can be BODY WAVES (TRAVEL UNDERGROUND) or a SURFACE WAVE (roll of earth’s surface like ripples on a pond) • Part of the fault where there is the most movement is the FOCUS Page 4 of 7 Type of Seismic Wave Description BODY WAVE (GOES THROUGH ALL LAYERS OF EARTH INCLUDING CORE) P Wave (Primary Wave) First to arrive and FASTEST Travels through solids, liquids and gases Travel through earth’s crust at a rate of 6km/s BODY WAVE (CRUST AND MANTLE ONLY) S Wave (Secondary Wave) Second to arrive (slower than P waves, travel at about 3.5 km/s) Waves are “larger” so greater damage to structures during earthquake Travels through solids but not liquids SURFACE WAVE (CRUST ONLY) L Wave Travels along Earth’s surface ONLY (Surface waves) Last to arrive (slowest) Roll along earth’s surface like ripples in a pond Page 5 of 7 • Point on the surface of the crust directly above the focus is called the Epicenter (it is the location where the earthquake feels the strongest) • Earthquakes result when stress begins to build up between two plates. • This initial stress does not usually cause any movement of the plate, because friction between the plates hold them together at the plate boundary • Eventually, this stress can cause deformation of the plates • If the stress gets large enough it overcomes the force of friction, resulting in the sudden movement of the plates • Once this occurs, the plates snap back into shape, but in new locations relative to one another • This particular explanation for the occurrence of earthquakes is known as the ElasticRebound Theory • Anytime an earthquake occurs under the ocean, a tsunami (giant wave) may occur, travelling at speeds up to 800km/hour. Volcanoes • When a volcano erupts, an earthquake always occurs • There are 3 types of volcanoes: composite, shield, and rift eruptions • Most of the Earth’s earthquakes and volcanoes occur along the boundaries between lithospheric plates • Some volcanoes, however, occur in the center of lithospheric plates • These areas are called Hot Spots. The Hawaiian Islands formed due to one such hot spot • A hot spot is believed to occur due to concentrations of heat from radioactive sources in the asthenosphere Homework: Page 6 of 7