The Water Planet

A. The Geography of the Ocean Basins The oceans cover 71% of the planet and regulate its

climate and atmosphere

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 organismscan move about

B. The Structure of the EarthIn 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 exist1. 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

2. Continental and Oceanic Crusts There are differences in the crust that make up

sea floors and continentsa. Ocean crust Made of basalt – a dark mineral More dense Thinner Younger rock; 200 mil yearsb. Continental crust Made of granite – lighter color Less dense ThickerOlder 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

The Origin and Structure of the Ocean BasinsThe Earth is a world of constant transformation,

where even the continents moveA. Early Evidence of Continental Drift 400 years ago Sir Francis Bacon noted the

continental coasts of the Atlantic fittogether 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, namedPangea Started breaking up ~180 mil years ago

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

Discovery of the Mid-Ocean Ridge After WWII sonar allowed detailed maps of

thesea floor They discovered the mid-ocean ridge system A chain of submarine volcanic mountainsthat encircle the globe, like seams on abaseball 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 followsthe 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 aroundthe ridges and trenches Rock near the ridge is young and gets oldermoving away from the ridge There is little sediment near the ridge, but itgets thicker moving awayFound symmetric magnetic bands on either

side ofthe ridge which alternate normal and reversedmagnetism

Creation of the Sea Floor Huge pieces of oceanic crust are separating at

themid-ocean ridges Creating cracks called rifts Magma from the mantle rises through the riftforming the ridge The sea floor moves away from the ridge 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 fromthe ridge This also explains the magnetic stripes found in

the sea floor

Sea-Floor Spreading and Plate Tectonics The crust and part of the upper mantle form thelithosphere 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 The plates colliding can be ocean - continent¨ Ocean plates always sinks below¨ Produces earthquakes and volcanic mountainranges; e.g. Sierra Nevada The plates colliding can be ocean - ocean¨ Earthquakes and volcanic island arcs; e.g.

AleutianIslands 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

Two forces move the plates

Slab-Pull theory - the sinking plate pulls the rest behind it

Convection theory – the swirling mantle moves the plate

C. Geologic History of the EarthContinental 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, andAustralia The plates are still moving today Atlantic ocean is growing, Pacific is shrinking

The Record in the SedimentsTwo 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 temperaturesClimate 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 thecontinents

The Continental Slope The edge of the continent Slopes down from the shelf break to the deep-

sea floorThe Continental Rise Sediment accumulates on the sea floor at the

base ofthe slopeActive 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

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, isa central rift valley Water seeps down through cracks, gets heated by themantle, then emerges through hydrothermal vents 350oC (660oF) Dissolved minerals from the mantle, like sulfides, arebrought up Black smokers form when minerals solidifyaround a vent Marine life, including chemosynthesizers, existaround hydrothermal vents