The Earth is covered by 71% ocean.
Most of what we know
about the ocean floor was discovered after 1950, when advances in technology
permitted its exploration.
We study the ocean floor
Gravity and magnetic surveys
The ocean floor consists
of sediment lying on top of basaltic crust
Therefore oceanic crust
(basaltic) is compositionally and structurally simpler than continental crust
Structure of the Ocean Floor
– An underwater platform of continental crust at the edge of a continent.
It is inclined very gently seaward at an angle of less than 1°.
On the Atlantic coast of
the US, the shelf is 500 km wide. On the Pacific coast, it is only a few kilometers wide.
The shelf is covered with
young, loose, sediment derived from the land via rivers.
– A relatively steep (~4-5°)
slope extending from a depth of 100-200 meters at the edge of the continental
shelf down to the deep ocean floor.
This is where the
continental crust transitions into oceanic crust.
– The very flat region of the deep-ocean floor, consisting of oceanic (basaltic)
crust and overlying sediments.
The plain starts at the
base of the continental slope. The water depth is about 5 km.
flattest feature on the Earth (overlying sediment “fills in” the rugged
volcanic oceanic crust).
V-shaped erosional canyons incised in the continental shelf and slope, and end
at the abyssal plain.
Sediment transported within these canyons is deposited in
fan-shaped features called abyssal fans (analogous to alluvial fans on land).
may have initially been carved by rivers during the most recent period of glaciation when sea level was lower.
Currents related to tides
move up and down the canyons aiding in the transport of sediment and the erosion
of the canyon.
Also, turbidity currents
(underwater landslides triggered by earthquakes or strong storms) contribute to
the formation of these canyons.
Passive Continental Margins (East Coast
Passive continental margin- A margin that connects continental crust to oceanic crust without any tectonic
This is a geologically "quite" boundary
without volcanoes, earthquakes, or young mountain belts. The main activity is
Passive margins include a large
continental shelf, a continental slope, an abyssal plain, and a “continental
Continental rise- A wedge of
sediment that lies at the base of the continental slope on passive margins. It
connects the continental slope to the abyssal plain, and has a gentler slope than the continental
Active Continental Margins (West Coast
Active continental margins
are characterized by tectonic boundaries, volcanoes, earthquakes, and young mountain belts.
They include a continental shelf and
The continental rise is typically absent.
Instead, oceanic trenches are present.
are the deepest (8-10 km) parts of the ocean. They parallel the edge of a
continent and are related to a subduction zone.
Trenches are characterized by earthquakes associated with the subducting slab of oceanic crust (the Benioff zone). Volcanoes are produced above the
subduction zone on the continent.
The continental slope occurs on the
landward side of the trench. The continental slope angle
changes from 4-5°
on the upper part to 10-15°
or more near the bottom of the trench.
The Mid-Oceanic Ridge
are giant undersea mountain ranges. There are 49,700 miles of mid-ocean ridges
on earth. They are 930-1550 miles wide and 1.2-1.8 miles high.
The crests of the mid-ocean ridges are
rift valleys: normal fault-bounded, down-dropped
areas where the crust is undergoing extension. They are about the size of the
Mid-ocean ridges are characterized by:
1. Basalt eruption (pillow basalts)
2. Shallow earthquakes
2. High heat flow
4. Black smokers (sulfide minerals) and
associated exotic organisms (that survive toxic chemicals, high temperatures,
high hydrostatic pressure, and total darkness). These organisms may give some evidence for
how life first evolved on earth.
Sediments of the Sea Floor
Sea floor sediment varies
in thickness but can be up to thousands of meters thick in spots.
– sediment derived from land that finds its way to the sea floor (via turbidity
– sediment that settles slowly from ocean water. It is made of:
Fine grained clay-
washed to deep sea.
Skeletons of microscopic organisms (foraminifera
Sediment thickness increases away from
the mid-oceanic ridge. Why?
Oceanic Crust and Ophiolites
Oceanic crust is about 7 km thick on
average and has three distinct layers:
– marine sediment (~0.5 km thick)
– pillow basalts overlying vertical, sheeted, basaltic dikes. (~ 1.5 km thick).
– sills of gabbro (~5 km thick)
are slices of oceanic crust that have been “obducted” onto the continent
during subduction events. Ophiolites show a specific stratigraphy:
-Sheeted basaltic dike complex
-Gabbro intrusions and sills
Age of the Seafloor
The age of oceanic crust and seafloor
sediments do not exceed 200 million years in age (~ Jurassic). In
contrast, the oldest crustal rocks are 3.7 - 4.3 billion years old.
The young ages reflect recycling of
dense oceanic crust at subduction zones.
The seafloor's age gets older as you go
away from the mid-oceanic ridge. Why?