1. An Introduction to the Marine Environment
~71% of the Earth is covered by salt water
The average depth of the ocean is 3,800 meters (12,500 feet) and at its deepest depth is 36,163 feet (7,000 feet deeper than Mt. Everest is high)
That’s a total volume of 1,349,929,000 km3!
The oceans constitute the largest repository of organisms on the planet

2. The Geography of the Ocean Basins
The oceans (Atlantic, Pacific, Indian, Arctic, and Southern) are not distributed evenly
Two-thirds of land found in Northern Hemisphere
80% of Southern Hemisphere is ocean
A number of shallow seas are connected to the main ocean basins
All ocean basins are inter-connected, and together constitute 1 world ocean

3. One World Ocean Arctic Ocean Atlantic Ocean Pacific Ocean Indian Ocean
Southern Ocean

4. The Ocean’s Origins The Earth is believed to be ~4.6 billion years old
Originated from a tremendous debris cloud, the result of the “Big Bang”, a massive cosmic explosion which occurred 9 billion years prior
Tremendous amounts of heat were released from decaying radioactive elements deep within the newly-assembled planet, and from gravitational compression and nearly constant asteroid impacts

5. The Ocean’s Origins
So much heat was generated as the early Earth formed that the planet was partially molten
Gravity pulled most of the heavy metals such as iron and nickel towards the hot center
At the same time, lighter elements such as aluminum and silicon rose towards the surface, cooling into a thin crust

6. The Ocean’s Origins
The cool, rigid crust rests upon a hot molten layer of mantle that is denser still than the crust above it

7. The Ocean’s Origins
Due to physical and chemical differences in the rock that makes up the crust, it is thinner and thicker in some places
Thinner = oceans
Thicker = continents
Ocean floor (oceanic crust) is geologically distinct from continental crust

8. Geologic differences between oceans and continents
Oceanic crust consists of a thin, dense, dark mineral called basalt
Rich in iron and magnesium
Continental crust consists of a thick, light material known as granite
Rich in sodium, potassium, calcium and aluminum
Dense oceanic crust sinks deeper into the molten mantle, and sits even lower still since it is so thin

9. e.g., light continental crust
e.g., heavy oceanic crust

10. The Ocean’s Origins
Eventually, the young Earth cooled enough such that water vapor (perhaps originating from impacts by icy comets) condensed and collected in the deep ocean basins
As water collected, it dissolved minerals and salts within the rocks forming sea water
After only 20 million years of rain, the oceans were born!

11. The Ocean’s Origins Oceanic and continental crust also differ in age
The oldest oceanic rocks are less than 200 million years old, quite young by geological standards
Conversely, continental rocks can be as much 3.8 billion years old
Why???

12. The Origin and Structure of the Ocean Basins
Although the Earth appears stagnant in time, evidence for geologic change is all around us
From catastrophic earthquakes and volcanic eruptions to the slow erosion of river valleys and oceanic cliffs
Even a glimpse at a world map provides clues for the ocean’s origins…

13. From the time accurate charts became available in the 1700’s, observers noticed the remarkable coincidence of shape of the Atlantic coasts of South America and Africa

14. Early Evidence of Continental drift
In 1912, Alfred Wegener proposed the theory of continental drift, the idea that all Earth’s land was in fact once joined into a single supercontinent, called Pangaea
Discovery of coal (the fossilized remains of tropical plants) in Antarctica, and similarities in fossils across separate continents supported his theory

15. Pangaea Supercontinent
Continental Drift
190 million years ago
Future Arctic Ocean
Future Pacific Ocean
Future Indian Ocean
Pangaea Supercontinent

16. How did our current oceans form? Atlantic Ocean formed first
Continental Drift
How did our current oceans form?
Atlantic Ocean formed first
In fact, the Atlantic is expanding, while the
Pacific is shrinking!

17. Say What?!!?
By the 1950’s and 1960’s, scientists were able to piece all of the evidence together and proposed a mechanism for continental drift, known as plate tectonics
Plate tectonics describes the large-scale movements of the Earth’s upper layer, known as the lithosphere (includes oceanic and continental crust) over the molten, plastic layer below, the aesthenosphere
The lithosphere is broken up into pieces, or plates

18. The lithosphere is broken into plates: some major, some minor
Plate Tectonics
The lithosphere is broken into plates: some major, some minor

19. Seafloor Spreading
The theory of plate tectonics is supported by a process known as seafloor spreading
Helps explain large-scale movement of continents
Seafloor spreading occurs as new oceanic crust rises to the surface from the aesthenosphere below
This new crust is very hot and so less dense than older oceanic (basaltic) crust; as a result it rises high as a ridge system in the center of our oceans

20. The Mid-Atlantic Ridge

21. Plate Tectonics/Seafloor spreading
As seafloor spreading continues, new oceanic crust is laid down and the ocean steadily grows
The ridge is highest in its center as the new, (warm) less-dense oceanic crust rises higher than the (cool) more-dense, older crust that surrounds it
Takes millions of years to cool and sink; the center of the oceans are often the shallowest!

22. Plate Tectonics/Seafloor spreading
Since the Earth is not continuously growing, the creation of new oceanic crust must be balanced by the destruction of crust elsewhere!
In fact, while oceanic crust is being created in the center of an ocean, oceanic crust is being destroyed at the edges of the surrounding continents; a process known as subduction

23. Dense oceanic crust always sinks below the less-dense continental crust; explains why oceanic crust is so young, relative to the much older continental crust which floats along on the asthenosphere

25. Plates come together at convergent boundaries
What happens when two plates come together, or converge?
Plates may come together in one of the following ways
Oceanic to continental plate: oceanic plate subducts beneath continental
Oceanic to oceanic plate: the older (denser) oceanic crust subducts beneath the newer (less dense) oceanic crust
Continental to continental plate: the 2 plates share equal density (same age) & so they are compressed, folded & uplifted into each other forming mountain ranges

26. Oceanic to continental convergence

27. Oceanic to oceanic convergence

28. The Marianas Trench is ~11,033 meters (36,163 feet)
The Marianas Trench – the world’s deepest ocean trench was formed by 2 oceanic plates converging
The Marianas Trench is ~11,033 meters (36,163 feet)
Pacific plate
Mariana plate

29. Continental to continental convergence

30. Island Living
Volcanic island chains such as Hawaii, are not the result of plate convergence, but rather the result of an oceanic plate moving over a stationary plume of mantle, called a hot spot
Usually a hot spot is far from a plate boundary, but results from a rising mantle plume
As the plate moves over the plume, volcanic islands are created

31. The Hawaiian island chain was formed this way!
Functions as a giant assembly line; volcanoes become inactive as they move away from the hot spot

32. Seafloor spreading pushing plate westward

33. Geological Provinces of the Ocean
The structure of the ocean floor is determined largely by geological processes
The sea floor is divided into 2 main regions
Continental margins (submerged continental crust)
Deep sea floor (oceanic crust)

34. Continental Margins
Continental margins generally consist of a gently-sloping continental shelf, and a steeper continental slope, leading to the continental rise, the junction between continental and oceanic crust
Continental shelves make up ~8% of the ocean’s surface area, but are the biologically richest part of the ocean
Continental shelves is composed of continental crust that happens to be submerged underwater

35. You are here

36. Continental margin (examples)

37. (raised up because hot = buoyant)
Continental Margins
Continental margins are shallow water areas close to the continents; they literally are the submerged outer edges of a continent
(raised up because hot = buoyant)

38. (raised up because hot = buoyant)
Deep Ocean Basins
Ocean basins are deep-water areas farther from land, beyond the continental margin
(raised up because hot = buoyant)

39. Deep Ocean Basins
Deep sea floor is oceanic crust (basaltic) and so sinks deeply into the asthenosphere
As a result, most of the deep sea floor lies at a depth of 3,000 to 5,000 meters (10,000 to 16,500 feet)
Known as the abyssal plain, the deep sea floor rises at a very gentle slope (~1°) towards the mid-oceanic ridge
Blanketed with up to 3 miles of sediments

40. The Abyss?
Despite the misnomer, abyssal plains are not entirely flat, but are dotted with volcanic islands, submarine volcanoes (seamounts) and channels, rises, plateaus and other features
In fact, the abyssal plain and seamounts are home to a tremendous variety of marine life

41. Mid-Oceanic Ridge Systems
Mid-oceanic ridges form when material rising from the mantle pushes up new oceanic crust
At the center of the ridge, however, 2 plates are pulling apart, leaving a gap, or depression, known as the central rift valley
Seawater seeps through cracks and fissures along the sides of the valley, becomes super-heated by the very high temperatures of the hot mantle

42. Hydrothermal Vents
The heated water then forces its way back up through the crust and emerges in hydrothermal vents
As the hot water seeps through cracks, it dissolves a variety of minerals, e.g., sulfides
When the super-heated water emerges at the vent and mixes with the surrounding water, it cools and causes the minerals to solidify, forming chimney-like structures around the vent

43. The average temperature in the vicinity of hydrothermal vent activity is ~8-16°C (46-61°F), much warmer than usual for ocean-bottom water (~3-4°C)

44. Hydrothermal Vents
Despite the temperatures, hydrothermal vents host a unique assembly of organisms, fueled by the chemicals dissolved in the vent fluids
Chemosynthetic Archaea form the base of the food chain (we return to this later)
Hydrothermal vents dot the mid-oceanic ridge systems, but have also now been found near trench systems where oceanic plates converge

45. Our present knowledge of vent systems