TB Chapter 13 In order to study Plate Tectonics, we must first reorganize our layering system for the Earth: Old System – 4 layers Crust (rigid) (rigid) Mantle (putty like) (semi rigid) Outer core (liquid) Inner core (solid) New System – 5 layers Lithosphere Asthenosphere Mantle Outer Core Inner Core ESRT’s Page 10 Cross Section of the Layers of the Earth/ Temperature Graph/ Pressure Graph What is Plate Tectonics? • Earth’s surface is made up of moving, solid pieces called plates – (Reference Tables pg 5) • Plate tectonics is the study of the formation and movement of these plates • Earth’s plates are part of the lithosphere – The lithosphere is the crust and the upper part of the mantle Properties of the Crust • Oceanic crust is mainly made of the igneous rock basalt • Continental crust is mainly made of the igneous rock granite • Oceanic crust is denser than continental crust (b/c basalt is MAFIC…remember?...ROCKS!) How Plates Move… • Asthenosphere - middle part of the mantle • Similar composition to the rigid upper mantle, but it is partially melted • This makes the asthenosphere like a very thick “liquid” (viscous) – (think VERY THICK MUD or SILLY PUTTY) • The “liquid” property of the asthenosphere allows it to flow very slowly How Plates Move…cont’d • You can think of the lithospheric plates as “floating” on the asthenosphere • The heat from inside the Earth causes convection currents to form in the asthenosphere • Where convection currents rise, molten rock forces the plates apart • Where the convection currents are sinking, the plates are moving together DIVERGENT BOUNDARY NEW ROCK LIQUID HOT MAGMA CONVERGENT BOUNDARY MOUNTAIN BUIDLING LIQUID HOT MAGMA Alfred Wegener’s Theory of Continental Drift • In 1915, German geologist and meteorologist, Alfred Wegener, first proposed the theory of continental drift, which states that parts of the Earth's crust slowly drift atop a liquid core • Wegener hypothesized that there was a gigantic supercontinent 200 MYA, which he named Pangaea, meaning "All-earth“ • Wegener published this theory in his book, On the Origin of Continents & Oceans Wegener OK, so we’re saying that the lithospheric plates move around on top of the asthenosphere, but what evidence do we have of this Plate Movement? Evidence of Plate Movement 1) “Puzzle-like” fit • The shape of the west coast of Africa and the east coast of South America seem to fit together like a puzzle • It is believed that they were once together, but moved apart over time (Theory of Continental Drift) Evidence of Plate Movement 2) Geologic Evidence • Similar fossil remains in Africa and South America of a reptile that is found nowhere else in the world • Distinctive rocks were also found in Africa and South America in regions where the two continents were most likely joined together Evidence of Plate Movement 3) The majority of earthquakes and volcanoes are found in belts that occur along plate boundaries (where the plates are moving together or apart) – Ex. the Pacific Ring of Fire Pacific Plate RING OF FIRE – PACIFIC PLATE Evidence of Plate Movement 4) Magnetic polarity reversals are recorded in ocean floor rocks as the sea floor spreads apart • During certain periods in Earth’s history, the magnetic fields have reversed • Iron in the igneous rocks on the ocean floor would shift before the magma cooled and hardened • If the polarity changed, new magma would cool with minerals shifted in the opposite direction Evidence of Plate Movement 5) Heat flowing out of the rocks (temperature) is greatest where the rocks are spreading apart (rising convection current) • Heat (temperature) decreases where the rocks are moving together (sinking convection current) ESRT’s Page 5 Plate Tectonic Boundaries 3 Main Types of Plate Boundaries I Divergent Boundaries II Transform Boundaries III Convergent Boundaries A. Collision (C-C) B. Subduction (O-C) & (O-O) Plate Boundaries • Divergent Boundaries- plates are moving apart at spreading centers • These boundaries form mid-ocean ridges or rises (like underwater mountain ranges) • In between the 2 peaks of the ridge are valleys called rift valleys – Ex. mid-Atlantic Ridge, East Pacific Ridge (Overhead View) Mid-Atlantic Ridge Plate Boundaries • Transform (Sliding) Boundary - 2 plates slide past each other • The sliding movement often causes earthquakes to occur along faults • A fault is nothing more than a crack in the Earth’s crust where movement has occurred – Ex. North American Plate and the Pacific Plate are sliding past each other along the San Andreas Fault in California Someday, Los Angeles will be North of San Francisco!!!!! Convergent Boundaries - two plates are converging or coming together (2 main types) Type 1 Collision Boundary • (continental-continental) (C – C) - the two plates moving together are both continental crust plates • The collision causes the plates to form a single, larger continent and the crust is pushed upward into a mountain range – Ex. Himalayan Mountains (including Mt. Everest). The Indian Plate and the Eurasian Plate are still moving together, so Mt. Everest is still slowly getting taller! Continental crust Continental crust Type 2 Subduction Boundary - one of the plates plunges under (subducts) the other • Occurs between two oceanic plates (O-O), or an oceanic and continental (O-C) plate • Most common characteristic is a deep-sea trench VIF!!!! • • These trenches are the deepest spots in the ocean Ex - the Marianas Trench is approx. 35,000 feet deep! Mt. Everest could be put into the trench and the peak would still be about 1 mile below the ocean’s surface! Continental crust Oceanic crust Subduction Boundaries cont’d • Because oceanic crust is denser than continental crust, the oceanic crust will always subduct underneath the continental • Volcanoes always seem to form at subduction boundaries – If two oceanic plates converge, volcanic islands will form (ex - Aleutian Islands in Alaska) – If one plate is continental and the other oceanic, the volcanoes will form along the edge of the continental crust (ex - Cascade Mountain Range on Western US) Subduction Boundaries cont’d • When the oceanic crust gets buried deeper and deeper under the Earth, it begins to melt into “liquid hot” magma. • Since “liquid hot” magma is less dense than the solid rock surrounding it, it has to rise. • When it rises, it comes to the surface of the Earth and forms volcanoes.