The Earth is made up of four ocean basins - Pacific, Atlantic, Indian, Arctic - which cover 71% of the planet. The ocean floor is composed of tectonic plates that are constantly shifting and spreading at mid-ocean ridges. As new crust is formed at ridges, old crust is destroyed through subduction at trenches. This process of sea floor spreading and plate tectonics explains continental drift over millions of years, from the single supercontinent Pangaea to the current configuration of lands and oceans.

1. The Water Planet

2. 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 organisms
 can move about

4. B. 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
1. 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

6. 2. Continental and Oceanic Crusts
 There are differences in the crust that make up sea floors
and continents
a. Ocean crust
 Made of basalt – a dark mineral
 More dense
 Thinner
 Younger rock; 200 mil years
b. 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

7. 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

8. 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

9. 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

10. 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

12. 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
 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

14. 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

16.  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 mountain
 ranges; e.g. Sierra Nevada
 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

18. 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

20. C. 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

22. 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

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

25.  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
 Passive margin – no plate boundary
 Wide shelf, gradual slope, and thick rise

27.  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

28.  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)
 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