CHAPTER 9 PLATE TECTONICS

Theory of PLATE TECTONICS Section 9.3

PLATE TECTONICS A FAR-REACHING THEORY A better understanding of geologic processes Mountain building Paleontology (Study of prehistoric life) Volcanism Earthquakes Etc.

PLATE TECTONICS MODEL Earth’s surface - composed of rigid plates Lithosphere - rigid outer shell includes: Crust – Continental & Oceanic crust Uppermost Mantle Consists of numerous plates Plates move slowly: 1 – 5 cm/yr. Plates are mostly beneath the oceans Pacific Plate is the largest Asthenosphere - Weaker, amorphous layer in the mantle Hotter & weaker than lithosphere Allows for motion of lithosphere (Soft & plastic)

LITHOSPHERE & ASTHENOSPHERE

LITHOSPHERIC PLATES & BOUNDARIES

N.A. & EURASIAN Plate boundary uncertain PLATE & BOUNDARIES 7 major plates & a dozen smaller plates N.A. & EURASIAN Plate boundary uncertain

Seismic activity (Earthquakes) Volcanism (Volcano eruptions) PLATE & BOUNDARIES The Outline of Plate Boundaries Looks at geologic activity associated with the plates Seismic activity (Earthquakes) Volcanism (Volcano eruptions) Mountain building

400,000+ Earthquake Events: 1963 - 2010 As early as the 1920s, scientists noted that earthquakes are concentrated in very specific narrow zones indicated by dots.

Actions at Plate boundaries

3 TYPES OF PLATE BOUNDARIES DIVERGENT CONVERGENT TRANSFORM

Types of Plate Boundaries Divergent Boundaries (constructive) Two plates move apart Molten mantle Rises to surface Creates new seafloor Convergent Boundaries (destructive) Two plates move together Ocean plate plunges into mantle beneath overriding plate Transform Fault Boundaries plates grind (slide) past each other Crust is not destroyed or created

DIVERGENT PLATE BOUNDARY Divergent/Constructive Plate Boundary (spreading apart) Most occur along ocean ridges Two plates move apart Causes: Seafloor spreading Rifts / Rift Valleys Molten mantle material Rises to surface Creates new seafloor

Mid-Atlantic Ridge

Oceanic spreading ridge E.g. Mid-atlantic ridge Close-up: mid-ocean ridge & sea-floor spreading Oceanic spreading ridge E.g. Mid-atlantic ridge

Ocean crust broken by “rift-type” faults SEAFLOOR SPREADING The Process where plate tectonics forms new oceanic lithosphere Ocean crust broken by “rift-type” faults Fast spreading “Pacific” type ridge Slow spreading “Atlantic” type ridge

FORMATION OF AN OCEAN Up-warping Magma pushes up the crust

Divergent: Atlantic Ridge LAVA FOUNTAINS KRAFLA VOLCANO ICELAND

RIFTS & RIFT VALLEYS

Ocean Rifts Rifts form in center of mid-ocean ridges Magma from mantle flows up and creates new crust older crust is pushed away from the rift

CONTINENTAL RIFTS Rift Valleys: (Divergent plate Boundary) Can occur on land Volcanoes often line rift valleys Magma from Mantle pushes up under continent Continent is stretched and broken Valley increases in size until water fills in to form a sea or ocean EAST AFRICAN RIFT (Triple Junction) The Continent breaks in 3 places At least 2 of these “arms” split apart and become an ocean.

Diagram of a rift valley E.g. West African rift

Ol Doinyo Lengai volcano erupts inside the Africa rift zone

A modern triple junction East African Rift The “Afar Triangle” A modern triple junction

Convergent Plate Boundaries

CONVERGENT PLATE BOUNDARY Covergent/Destructive Plate Boundary Two Plates come together Ocean plate plunges into mantle beneath overriding plate destruction (recycling) of crust takes place along convergent boundaries Overhead view Sometimes one plate sinks (is subducted) beneath the other plate SUBDUCTION ZONE

Example: Andes Mountains West margin of the South American continent oceanic Nazca Plate is pushed toward and beneath the continental of South America on the South American Plate typical example of a convergent plate boundary

Continental-Continental 3 Types of Convergent boundaries Oceanic-Continental Oceanic-Oceanic Continental-Continental

Continental volcanic arc 3 Types of Convergent plate boundaries Oceanic - Oceanic convergence E.g. Japan, New Zealand Oceanic - Continental convergence E.g. Andes & Sierra Nevada Mtns. Continental - Continental convergence E.g. Himalaya Mtns. Volcanic island arc Sea-floor & continental sediment Continental volcanic arc

Oceanic-Oceanic Convergent Boundary Subduction processes in oceanic-oceanic plate convergence form volcanic island arcs along side deep ocean trenches

Features formed by: Oceanic - Oceanic convergence Ocean - Ocean Boundary Features formed by: Oceanic - Oceanic convergence Deep-Ocean Trench Volcanic Island arc Piled up sedimentary rock

example of a “ Volcanic Island Arc” system Japan: example of a “ Volcanic Island Arc” system Deep-Ocean Trench Volcanic Island arc Examples: Aleutian Islands Mariana Islands Lesser Antilles Tonga Islands

Oceanic-Continental Convergent Boundary Subduction zone forms w/deep ocean trench (sediment from the continent fills the trench) Forms Continental Volcanic Mountain Arcs

Oceanic-Continental Feature formed by: convergence Continental volcanic arcs Examples: Andes mtns. Sierra Nevada mtns. Cascades mtns.

Continental-Continental Convergent Boundary Neither plate subducts completely, they just pile up like two colliding icebergs plates resist downward motion the continental rocks are relatively light Mountains formed Examples: Appalachians Himalayas Urals Alps

Features formed by: Continental - Continental convergence Old ocean sediments mountains Uplifted plateau

HIMALAYAS Collision between the Indian and Eurasian plates pushed up the Himalayas and the Tibetan Plateau

RING OF FIRE OCEANIC-CONTINENTAL COLLISONS SUSTAIN MOST VOLCANIC ACTIVITY AROUND THE PACIFIC OCEAN

Transform Boundaries

TRANSFORM BOUNDARY Two Plates grind (slide) past one another Two Plates grind (slide) past one another No new crust created nor is crust destroyed Transform Faults - (Strike-Slip Fault) Most are in oceanic crust Perpendicular to mid-ocean ridges Parallel to the direction of plate movement Aids movement of crustal material

North American plate Pacific plate

Example: San Andreas fault in California Pacific Plate slides past the North American Plate.

Volcanoes and Plate boundaries Continental Volcanoes Oceanic Volcanoes Continental Volcanoes Subduction Zone Subduction Zone Hot Spot Hot Spot Rift Rift

Tectonic Settings and Volcanic Activity