Bellwork What causes the continents to drift? Fact: The tallest mountain on the planet is not Mt. Everest, which is only ft. The tallest mountain in the world is actually Mauna Kea, in Hawaii, which is closer to ft. You just can't see the two-thirds of the mountain that is under water.

Sea-Floor Spreading

Earth: Compositional Layers

crust mantle core lithosphere asthenosphere mesosphere outer core inner core CompositionPhysical Characteristics Note: Lithosphere contains both crust and uppermost (brittle) layer of mantle Solid, but behaves plastically Brittle, solid Solid but flows slowly over long periods of time Liquid Solid Mainly iron and nickel Mostly silica, magnesium, iron, with some other stuff Similar to mantle but with lighter metals

Effects of Density: A block of continental lithosphere (containing continental crust plus uppermost ) stands higher than a block of oceanic lithosphere (containing oceanic crust) of the same dimensions Oceanic crust: 3.0 g/cm 3 Continental crust: 2.7 g/cm 3 Asthenosphere (near-liquid part of mantle) Lithospheric mantle ~ 3.3 g/cm 3 Lithospheric mantle ~ 3.3 g/cm 3 Oceanic lithosphere floats low on asthenosphere (forms basins) Continental lithosphere floats high on asthenosphere (forms continents)

Effect of Lithospheric Thickness: For a buoyant material, a thick block stands higher than a thin block Example: top surface of thick block of wood stands higher above water level than that of thin block of wood. However, the proportion of material standing above and below water mark is the same for all blocks. So, low density of continents combined with great crustal thickness make continents stand higher than ocean floors

Focus your attention to interaction of brittle lithosphere and near-liquid asthenosphere

Volcanoes Earthquakes Location,Location,Location

More recent evidence of seafloor spreading 1. Symmetry of magnetic stripes (defined by polarity of magnetic minerals in basaltic rock of seafloor) Symmetrical pattern of normal and reverse polarities on either side of a divergent boundary can only be explained if new crust was being formed and repeatedly split apart as magnetic field reversed

More recent evidence of seafloor spreading 2. Linear arrangement and relative heights of hotspot volcanoes (volcanoes produced by movement of plate over stationary magma plume from mantle) Note: heights of hotspot volcanoes are decreased with increasing distance from point of active volcanism (due to cooling and sinking of lithospheric material)

But how does seafloor spreading (divergence) start ? Hot plume in mantle upwarps lithosphere of continent Cracks develop (generally in a triple junction), forming rift valleys Zones where adjacent fractures connect allow further spreading to produce an ocean Failed arm ceases to spread (Bay of Fundy basin represents a “failed arm” of the Atlantic Ocean basin)

Mid-Ocean Ridge Longest chain of mountains in the world Longest chain of mountains in the world Mostly under water Mostly under water Iceland is where the ridge rises above the water Iceland is where the ridge rises above the water Sonar- device that scientists use to map the ocean floor Sonar- device that scientists use to map the ocean floor

Red Sea-Gulf of Aden: An ocean basin in the making Future ocean basin East African Rift will probably stop spreading and become a “failed arm”

Topographic map of the ocean floor

Sea Floor Spreading Molten material rises from the mantle and erupts Molten material rises from the mantle and erupts Molten material spreads out, pushing older rock to both sides of the ridge Molten material spreads out, pushing older rock to both sides of the ridge New crust is being created New crust is being created

Evidence for Sea-Floor Spreading 1) Molten Material 2) Magnetic Stripes 3) Drilling Samples

Molten Material Pillow lava at the Mid-Ocean ridge Pillow lava at the Mid-Ocean ridge - pillow lava are rocks that form when molten material cools and hardens quickly in water - pillow lava are rocks that form when molten material cools and hardens quickly in water

Magnetic Stripes rock on the ocean floor is in a pattern of magnetic stripes rock on the ocean floor is in a pattern of magnetic stripes Stripes show when the magnetic poles on Earth have reversed Stripes show when the magnetic poles on Earth have reversed Stripes match on each side of the ridge Stripes match on each side of the ridge

Drilling Samples drilled samples of ocean floor and dated the rocks drilled samples of ocean floor and dated the rocks Found rocks near the ridge were youngest and rocks farthest from ridge were oldest Found rocks near the ridge were youngest and rocks farthest from ridge were oldest

Subduction Process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary Process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary Deep-Ocean Trench- a deep valley along the ocean floor Deep-Ocean Trench- a deep valley along the ocean floor Destroys crust Destroys crust Pacific- shrinking- destroy more than create Pacific- shrinking- destroy more than create Atlantic- expanding- create more than destroy Atlantic- expanding- create more than destroy

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Continental Drift: 750 million years ago to present Implications of Plate Tectonics While ocean basins are created and destroyed continents are too light to be subducted, so simply break apart and collide

The End