2.2 The Origin and Structure of the Ocean Basins

2. 2.2 The Origin and Structure of the Ocean Basins • The Earth is a world of constant transformation, where even the continents move • A. Early Evidence of Continental Drift • 400 years ago Sir Francis Bacon noted the continental coasts of the Atlantic fit • together like pieces of a puzzle • Later suggested the Americas might have been once joined to Europe and Africa • Geologic formations and fossils matched from opposing sides • Alfred Wegner gave hypothesis of Continental Drift in 1912 • Suggested that all the continents had once been a supercontinent, named • Pangea • Started breaking up ~180 mil years ago

3. B. The Theory of Plate Tectonics • Could not explain how the continents moved • The Theory of Plate Tectonics explains it all • Continents do drift slowly around the world

4. Discovery of the Mid-Ocean Ridge • After WWII sonar allowed detailed maps of the sea floor • They discovered the mid-ocean ridge system • A chain of submarine volcanic mountains that encircle the globe, like seams on a baseball • The largest geological feature on Earth • Some of the mountains rise above sea level to form islands, e.g. Iceland • The mid-Atlantic ridge runs down the center of the Atlantic Ocean and follows the curve of the opposing coastlines • Sonar also discovered deep trenches

6. Significance of the Mid-Ocean Ridge • Why are they there? How were they formed? • Lots of seismic and volcanic activity around • the ridges and trenches • Rock near the ridge is young and gets older • moving away from the ridge • There is little sediment near the ridge, but it • gets thicker moving away

8. Found symmetric magnetic bands on either side of the ridge which alternate normal and reversed magnetism

9. Creation of the Sea Floor • Huge pieces of oceanic crust are separating at the • mid-ocean ridges • Creating cracks called rifts • Magma from the mantle rises through the rift • forming the ridge • The sea floor moves away from the ridge

10. This continuous process is called sea-floor spreading • New sea floor is created • This explains why rocks are older and sediment is thicker as you move away from • the ridge • This also explains the magnetic stripes found in the sea floor

11. Sea-Floor Spreading and Plate Tectonics • The crust and part of the upper mantle form the • lithosphere • 100 km (60 mi) thick, rigid • It’s broken into plates • May be ocean crust, continent crust, or both • The plates float on a fluid layer of the upper mantle called the asthenosphere.

12. At mid-ocean ridges the plates move apart • If the plate has continental crust it carries the continent with it • Spread 2-18 cm/year • This explains continental drift • As new lithosphere is created, old lithosphere is destroyed somewhere else • Some plate boundaries are trenches where one plate sinks • below the other back down into the mantle and melts

13. At mid-ocean ridges the plates move apart • If the plate has continental crust it carries the continent with it • Spread 2-18 cm/year • This explains continental drift • As new lithosphere is created, old lithosphere is destroyed somewhere else

14. Some plate boundaries are trenches where one plate sinks below the other back down into the mantle and melts • Called subduction • Trenches are also called subduction zones • The plates colliding can be ocean  continent • ¨ Ocean plates always sinks below • ¨ Produces earthquakes and volcanic mountain ranges; e.g. Sierra Nevada

16. The plates colliding can be ocean  ocean • ¨ Earthquakes and volcanic island arcs; e.g. Aleutian • Islands • The plates colliding can be cont  cont • ¨ Neither plate sinks, instead they buckle • ¨ Producing huge mountain ranges; e.g. Himalayas • A third boundary type is shear boundary or transform fault • The plates slide past each other • Causes earthquakes; e.g. San Andreas Fault

17. Two forces move the plates • Slab-Pull theory - the sinking plate pulls the rest • behind it • Convection theory – the swirling mantle moves • the plate

18. Geologic History of the Earth • 1. Continental Drift and the Changing Oceans • 200 mil years ago all the continents were joined in Pangea • It was surrounded by a single ocean called Panthalassa • 180 mil years ago a rift formed splitting it into two large continents

19. Laurasia – North America and Eurasia • Gondwana – South America, Africa, Antarctica, India, and Australia • The plates are still moving today • Atlantic ocean is growing, Pacific is shrinking

20. The Record in the Sediments • Two types of marine sediments: • Lithogenous – from the weathering of rock on land • Biogenous – from skeletons and shells of marine organisms • ¨ Mostly composed of calcium carbonate or silica • Microfossils tell what organisms lived and past ocean temperatures

21. Climate and Changes in Sea Level • The Earth alternates between interglacial (warm) period and ice age (cold) periods • Sea level falls during ice ages because water is trapped in glaciers on the • continents

22. 2.3 The Geological Provinces of the Ocean • Two main regions of the sea floor • Continental margins – the submerged edge of the continents • Deep-sea floor

23. Continental Margins • Boundaries between the continental and ocean crust • Consists of shelf, slope and rise • 1 The Continental Shelf • The shallowest part • Only 8% of the sea floor, but biologically rich and diverse • Large submarine canyons can be found here • Ends at the shelf break, where it steeply slopes down

24. 2 The Continental Slope • The edge of the continent • Slopes down from the shelf break to the deep-sea floor • 3 The Continental Rise • Sediment accumulates on the sea floor at the base of • the slope

25. Active and Passive Margins • Active margin – the subducting plate creates a trench • Narrow shelf, steep slope, and little or no rise • Steep, rocky shorelines • Passive margin – no plate boundary • Wide shelf, gradual slope, and thick rise

26. Deep-Ocean Basins • 10,000-16,000 ft • Abyssal plain - flat region of the sea floor • Seamounts – submarine volcanoes • Guyots – flat-topped seamounts • Both were once islands, but now covered with water • Trenches – the deepest part of the ocean • Mariana Trench is 36,163 ft deep

27. The Mid-Ocean Ridge and Hydrothermal Vents • At the center of the ridge, where the plates pull apart, is a central rift valley • Water seeps down through cracks, gets heated by the mantle, then emerges through hydrothermal vents 350oC (660oF)

28. Dissolved minerals from the mantle, like sulfides, are • brought up • Black smokers form when minerals solidify • around a vent • Marine life, including chemosynthesizers, exist • around hydrothermal vents