Lecture 3 Plate Tectonics The Unifying Theory TakeAway

Lecture 3 Plate Tectonics: The Unifying Theory

Take-Away Points 1. How we know plate tectonics happens 2. Most earthquakes and volcanoes occur along plate boundaries 3. New ocean crust is created along mid-ocean ridges 4. Old ocean crust is recycled at subduction zones 5. Subduction zones are where mountain-building (orogeny) occurs

About Plate Tectonics • It is the movement of plates and the forces acting on them. • It explains the distribution of volcanoes, earthquakes, mountain chains, rock assemblages, and seafloor structures. • The forces that drive plate motions arise from the mantle convection system.

Discovery of Plate Tectonics Continental drift: “jig-saw puzzle” fit of continents

Discovery of Plate Tectonics Continental drift: similarity of rock assemblages and ages across oceans

Discovery of Plate Tectonics Continental drift: distribution of certain fossils

Continental Drift theory was proposed by Alfred Wegener in 1912. He used continental fit, distribution of fossils, ancient climates, and the wandering of Earth's polar regions to support his Theory. He stated the present continents were part of a super continent – Pangaea.

Pangaea

Discovery of Plate Tectonics Seafloor spreading: geological activity in midocean ridges

Discovery of Plate Tectonics Seafloor spreading: new crust formed there

Sea-Floor Spreading Harry Hess proposed that new ocean floor is formed at the rift of mid-ocean ridges. The ocean floor, and the rock beneath it, are produced by magma that rises from deeper levels. Hess suggested that the ocean floor moved laterally away from the ridge and plunged into an oceanic trench along the continental margin.

Sea-Floor Spreading Mid oceanic ridge = construction of lithosphere Subduction zone = destruction of lithosphere

The Plates and Their Boundaries ● mosaic of rigid plates

Types of Plates • Ocean plates - plates below the oceans • Continental plates - plates below the continents

What are tectonic plates made of? • Plates are made of rigid lithosphere. The lithosphere is made up of the crust and the upper part of the mantle.

The Plates and Their Boundaries ● three types of boundaries

The Plates and Their Boundaries ● divergent, convergent, transform

The Plates and Their Boundaries ● next: a detailed look at the above

Three Basic Types of Plate Boundaries Divergent Using hands to show relative motion Transform Convergent USGS Graphics

Three Basic Types of Plate Boundaries Divergent Transform Convergent USGS Graphics

Divergent Boundaries (a) Oceanic plate separation rifting, volcanoes, and earthquakes an c i r e m A th r o e N Plat Mid. Atlantic Ridge Eur asia n Pla te

Divergent Boundaries (b) Continental plate separation rift valleys, volcanoes, and earthquakes East African Rift Valley ate l P an c i r f A Soma li Sub plate

Divergent Boundaries • Boundary between two plates that are moving apart or rifting • RIFTING causes SEAFLOOR SPREADING

Features of Divergent Boundaries • Mid-ocean ridges • rift valleys • fissure volcanoes

Iceland: An example of continental rifting • Iceland has a divergent plate boundary running through its middle

Convergent Boundaries • Oceanic-Continental Convergence • Continent-Continent Convergence • Oceanic-Oceanic Convergence

Convergent Boundaries (a) Ocean-ocean convergence oceanic trench, volcanic island arc, and deep earthquakes Mariana Islands e n i p ip Phil e Plat Marianas Trench Pacif ic Pla te

Convergent Boundaries (b) Ocean-continent convergence volcanic mountain chain, folded mountains, and deep earthquakes Andes Mountains Peru-Chile Trench te a l P a c Naz South American Plate

Convergent Boundaries (c) Continent-continent convergence Himalaya Mountains crustal thickening, folded mountains, and earthquakes late P n a i l a r t s Indian-Au Tibetan Plateau subduction Eurasian Plate

Convergent Boundaries • Boundaries between two plates that are colliding • There are 3 types…

Type 1 • Ocean plate colliding with a less dense continental plate • Subduction Zone: where the less dense plate slides under the more dense plate • VOLCANOES occur at subduction zones

Subduction As new ocean floor is created by sea-floor spreading at the midoceanic ridges it is consumed at the subduction zones where the lithosphere sinks under the asthenosphere.

Andes Mountains, South America

Type 2 • Ocean plate colliding with another ocean plate • The less dense plate slides under the more dense plate creating a subduction zone called a TRENCH

Type 3 • A continental plate colliding with another continental plate • Have Collision Zones: –a place where folded and thrust faulted mountains form.

Himalaya

Transform-Fault Boundaries (a) Continental transform fault lateral (transform) fault and earthquakes ate l P c i f Paci North Ameri can Pl ate

Transform-Fault Boundaries (b) Mid-ocean ridge transform fault lateral (transform) faults and earthquakes Euras ian Pl ate late P n a c i er m A h t r No

Transform Fault Boundaries • Boundary between two plates that are sliding past each other • EARTHQUAKES along faults

San Andreas Fault, CA