1. Where is the deepest known spot in the world oceans found? How deep is it?
The deepest part of the Ocean is the Challenger Deep, in the Pacific Ocean's Mariana Trench, located
just east of the Philippines. The Challenger Deep is 6,201 feet (11,034 meters) below sea level.
The deepest spot of the ocean is located in the Mariana trench situated just east of the fourteen
Mariana Islands in the Pacific Ocean, east of the Philippines.
The Geographic coordinates of the Mariana trench is 11"21'
North latitude and 142" 12' east longitude. The Marianas
Trench's depth is about 10,924 m, or almost 11 km (7 miles).
The Mariana trench is not only the deepest part of the
ocean but also the lowest part of the planet. This trench is
created by an ocean-ocean subduction zone, where an
oceanic crust plate subducts under another oceanic crust
plate. The deepest part of the Mariana trench is the
“Challenger Deep” named after the British Navy Boat named
Challenger II, which first surveyed the islands in 1951.
Challenger Deep is found at the extreme southwest part of
the Mariana trench and was reached by a manned
submersible in 1960. The Mariana trench is 2183 meters
taller than the highest point of the Earth, Mount Everest.
The trench is very cold and highly pressurized. The floor of
the trench is covered by hydro thermal vents formed by spreading plates.

2. Referring to Plate 2 at your text book (Fowler, 2005), why do you think that the oldest ocean floor is
only about 200 million years old, whereas the Earth is about 4.6 billion years old?
Plate 2 show that the youngest part of the world’s
oceans is in the spreading centers such as the mid-
oceanic ridges and that the sea floor is getting older
away from these centers. This due to the fact that
new oceanic crust is being formed at the spreading
centers and older oceanic crust is being pushed away
as plates move away from the spreading centers. The
oceanic crust is then sub-ducted below active plate
margins and is returned to the mantle. This forms a
continuous circle that constantly creates new
oceanic floor at mid-ocean ridges and consumes these oceanic crust at destructive plate margins. This
dynamic change of the oceanic crust is responsible for the relatively young oceanic crust compared to
the Earth’s age.
These are oceanic ridges where new oceanic lithosphere is created by upwelling mantle that melts,
resulting in basaltic magmas which intrude and erupt at the oceanic ridge to create new oceanic
lithosphere and crust. As new oceanic lithosphere is created, it is pushed aside in opposite directions.
Thus, the age of the oceanic crust becomes progressively older in both directions away from the ridge.
Because oceanic lithosphere may get subducted, the age of the ocean basins is relatively young.
The oldest oceanic crust occurs farthest away from a ridge. In the Atlantic Ocean, the oldest
oceanic crust occurs next to the North American and African continents and is about 180 million
years old (Jurassic) (see figure 20.5 in your
text). In the Pacific Ocean, the oldest crust is
also Jurassic in age, and occurs off the coast of
Japan. J Because the oceanic ridges are areas
of young crust, there is very little sediment
accumulation on the ridges. Sediment
thickness increases in both directions away of
the ridge, and is thickest where the oceanic
crust is the oldest. Knowing the age of the
crust and the distance from the ridge, the
relative velocity of the plates can be
determined. (Absolute velocity requires further
information to be discussed later). i Relative
plate velocities vary both for individual plates and for different plates. p Variations in individual
plate velocities occur because spreading of the sea floor takes place on a spherical surface rather
than on a flat surface. Velocities are greatest at large distances away from the spreading pole (see
figures 20.7 & 20.8 in your text). f Different plates have different velocities depending on the
amount of continental lithosphere within the plate. Plates with continental lithosphere have lower
relative velocities than plates with only oceanic lithosphere. Plate Tectonics 6 of 13 11/18/2003
r Sea floor topography is controlled by the age of the oceanic lithosphere and the rate of
preading. If the spreading rate (relative velocity) is high, magma must be rising rapidly and

3. the lithosphere is relatively hot beneath the ridge. Thus for fast spreading centers the ridge stands
at higher elevations than for slow spreading centers. The rift valley at fast spreading centers is
narrower than at slow spreading centers. As oceanic lithosphere moves away from the ridge, it
cools and sinks deeper into the asthenosphere. Thus, the depth to the sea floor increases with
increasing age away from the ridge.