Ch. 2: The Sea Floor

2.1 The Water Planet The oceans cover 71% of the planet and regulate its climate and atmosphere

The Geography of the Ocean Basins There are four ocean basins Pacific – the deepest and largest Atlantic Indian Arctic – smallest and shallowest Connected to the main ocean basins are shallow seas e.g. Mediterranean Sea, Gulf of Mexico, South China Sea They all connect to form a world ocean where seawater, materials, and organisms can move about

Largest percent of world ocean is found in the southern hemisphere

Big Bang The giant cosmic explosion that occurred 13.7 billion years ago and created the Universe. There is evidence that the earth and the rest of the solar system formed about billions years ago

The Structure of the Earth In the early molten Earth, lighter materials floated toward the surface They cooled to form the crust The atmosphere and oceans then formed Earth is the right distance from the sun for liquid water, and life, to exist

Internal Structure The dense core is mostly iron Solid inner core and liquid outer core The swirling motions produce the Earth’s magnetic field The mantle is outside the core and under the crust Near molten rock slowly flows like a liquid The crust is the outer layer, comparatively thin like a skin floating on the mantle

Continental and Oceanic Crusts There are differences in the crust that make up sea floors and continents Ocean crust Made of basalt – a dark mineral More dense Thinner Younger rock; 200 mil years Below sea level Continental crust Made of granite – lighter color Less dense Thicker Older rock; 3.8 bil years

So continental crust floats high on the mantle and ocean crust floats lower That’s why ocean crust is covered by water

2.2 The Origin and Structure of the Ocean Basins The Earth is a world of constant transformation, where even the continents move

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 and suggested that all the continents had once been a supercontinent, named Pangea Started breaking up ~180 mil years ago

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

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

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

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 (called sea- floor spreading) New sea floor is created rocks are older and sediment is thicker as you move away from the ridge This also explains the magnetic stripes (anomalies) found in the sea floor

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. – 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 – Called subduction Trenches are also called subduction zones

Ocean to ContinentOcean to OceanContinent to Continent Ocean plates always sinks below Earthquakes and volcanic island arcs; e.g. Aleutian Islands Neither plate sinks, instead they buckle Produces earthquakes and volcanic mountain ranges; e.g. Sierra Nevada Producing huge mountain ranges; e.g. Himalayas

3 types of plate boundaries (shear) (Tension) (Compression)

A third boundary type is shear boundary or transform fault The plates slide past each other Causes earthquakes; e.g. San Andreas Fault

Two forces move the plates Slab-Pull theory - the sinking plate pulls the rest behind it Convection theory – the swirling mantle movesthe plate

Review Types of plate boundaries – Shear – Divergent – Convergent – Mid-ocean ridge – trench

Geologic History of the Earth 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 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

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

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

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

Continental Margins Boundaries between the continental and ocean crust Consists of shelf, slope and rise 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

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

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

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

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 350 o C (660 o F) 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