Download The Sea Floor – Chapter 2

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
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
One World Ocean
Arctic
Ocean
Pacific Ocean
Atlantic
Ocean
Indian
Ocean
Southern Ocean
http://www.gebco.net/data_and_products/gebco_world_map/images/gda_world_map_large.jpg
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
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
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
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
http://www.lemontree.co.nz/photograph-gallery-kaikoura-city.html
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
e.g., light
continental crust
e.g., heavy oceanic crust
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!
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???
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…
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
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
Continental Drift
190 million years ago
Future Arctic Ocean
Future
Pacific
Ocean
Future
Indian
Ocean
Pangaea Supercontinent
Continental Drift
How did our current oceans form?
Atlantic Ocean formed
first
In fact, the Atlantic is
expanding, while the
Pacific is shrinking!
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
Plate Tectonics
The lithosphere is broken into plates: some major, some minor
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
The Mid-Atlantic Ridge
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!
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
Dense oceanic crust always sinks below the lessdense continental crust; explains why oceanic crust
is so young, relative to the much older continental
crust which floats along on the asthenosphere
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
Oceanic to continental convergence
Oceanic to oceanic convergence
• 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
Continental to continental convergence
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
• 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
Seafloor spreading
pushing plate
westward
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)
Continental Margins
• Continental margins generally consist of a gentlysloping 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
You are here
http://oceanexplorer.noaa.gov/explorations/02hudson/background/mapping/mapping.html
Continental margin (examples)
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)
Deep Ocean Basins
• Ocean basins are deep-water areas farther
from land, beyond the continental margin
(raised up because
hot = buoyant)
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
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
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 superheated by the very high temperatures of the
hot mantle
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
• The average temperature in the vicinity of
hydrothermal vent activity is ~8-16°C (4661°F), much warmer than usual for oceanbottom water (~3-4°C)
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 2
oceanic plates converge
Our present knowledge of vent systems