|Why does the seafloor not disappear since part of
it continues to subduct along the ring of fire?|
|Question Date: 2018-02-07|
You are correct that the seafloor is destroyed at
subduction zones, but it is simultaneously
being created at mid-ocean ridges.
A mid-ocean ridge is where new crust, or
seafloor, is being made by rising magma within the
Earth’s interior. These mid-ocean ridges are
located along the seafloor and the rising magma
erupts along seams on the ocean floor.
These regions are thus also sites of seafloor
spreading ; this is where the crust being
created at the ridge is spreading apart because as
new magma rises and continually erupts, it pushes
older oceanic crust away from the ridge. This
older oceanic crust may get pushed towards a
subduction zone where it will eventually be
consumed and recycled back into Earth’s interior
see figure 1 .
Figure 1: Seafloor spreading at a mid-ocean
ridge(where new crust is being created) and it’s
destruction at a subduction zone.
As an example, the Mid-Atlantic ridge is the most
well-known and longest ridge in the world
extending for about 10,000 miles from the Arctic
Ocean to the southern tip of Africa
see figure 2 .
Figure 2: Linear extent of the Mid-Atlantic Ridge
Although most mid-ocean ridges lay along the
seafloor, a small portion of the Mid-Atlantic
ridge actually resides above sea level. This
portion of the ridge cuts through the country of
see figure 3
Figure 3: The portion of the Mid-Atlantic Ridge
that cuts through Iceland.
see figure 4
Figure 4: The land trace of the Mid-Atlantic
ridge in Iceland in which the
people linking hands span between the two edges of
see figure 5
Figure 5: A diver swimming between the a
portion of the ridge submerged in
Great question! You're correct that at subduction
zones like the Ring of Fire, the oceanic crust
gets subducted underneath of continents, or
sometimes even other pieces of oceanic crust. But
the other half of the equation is that oceanic
crust is also continuously remade along Mid-Ocean
Ridges, which are underwater volcanoes. These are
the longest chains of volcanoes on Earth that
continuously make new seafloor. This process of
new oceanic crust creation and movement away from
the Mid-Ocean Ridge is called "seafloor
If you go to Google Earth, you can see
these ridges really clearly in the middle of the
different ocean basins. One of the most famous
ridges is called the Mid-Atlantic Ridge and it
runs north to south along the middle of the
Atlantic Ocean. So new oceanic crust is made in
the "middle" of oceans along Mid Ocean Ridges, and
it's destroyed where oceanic crust meets another
tectonic boundary and subducts.
That is great that you are wondering about the big
picture! Some plate boundaries, like at subduction
zones, are absorbing sea floors, but at other
plate boundaries sea floor is being created . The
biggest example where new sea floor is being
created is right in the middle of the Atlantic
Ocean. This is called the Mid-Atlantic rift. Two
plates are moving apart (diverging), and new magma
is coming up to fill the newly created space. This
creates new seafloor.
This big system where the Atlantic is creating
more crust while the pacific is absorbing crustis
often compared to a big conveyor belt.
New crust is constantly being made at the East
Pacific Rise, which is a rift off the coasts of
Central and South America. There is another rift
that creates new crust that circles Antarctica.
The reason why the seafloor doesn't disappear is
because new seafloor is continually being made at
these rifts to replace the old seafloor that gets
That said, the Pacific Ocean is becoming smaller
as the rate at which seafloor is being subducted
along the Pacific Ring of Fire is faster than the
rate at which these rifts are producing it. At the
same time, the Atlantic Ocean is getting larger
because the Mid-Atlantic Ridge, which is yet
another rift, is making crust but there is no
Atlantic counterpart to the Pacific Ring of Fire
to subduct it.
The continental plates covering the earth bump
against each other. One plate gets pushed up and
makes mountains. The plate next to it gets pushed
under - it subducts. Volcanos in the ring of fire
are pushed up parts of plates.
Great question! The seafloor certainly does
disappear at subduction zones along the Ring of
Fire. Here, the seafloor (and all the oceanic
crust beneath it) plunges down under the
continents into Earth’s mantle, to be recycled.
Here is the thing: for all the seafloor and
oceanic crust that disappears during subduction
into the mantle, an equal amount of new seafloor
and oceanic crust is made somewhere else to
replace it. The new seafloor is made far away
from the Ring of Fire, in the middle of the
Pacific Ocean. This is because the sinking of the
seafloor into the mantle that happens at the edges
of the Pacific Ocean essentially pulls the
seafloor apart in the middle of the ocean.
Where the seafloor is pulled apart, a gap or
rift is created, and this rift is immediately
filled in by melted mantle which wells up below
the rift. The melted mantle, or lava,
erupts along the rift and freezes solid when it
comes into contact with seawater. This frozen
lava becomes the new seafloor. Since new
seafloor stays hot for a long time, it is more
buoyant (floats higher) than the older seafloor,
forming a high ridge that looks like an underwater
mountain chain. We call this ridge where new
seafloor is created a ‘mid-ocean ridge’.
The same process happens in the middle of the
Atlantic ocean, where the ridge is called the
So you can think of the seafloor as a giant
escalator that is moving downstairs. The mid-ocean
ridge where new seafloor is created is the top of
the escalator, and the subduction zones where old
seafloor disappears is the bottom of the
escalator. Even though it is always moving, the
length of the escalator stays the same. Just like
the size of the Pacific Ocean can stay the same,
even though some seafloor is disappearing, and new
seafloor is being created.
picture of process
Here is a link to a video that illustrates this
When the seafloor subducts it does disappear
from the surface. It moves deeper into the Earth,
into the mantle. But in the Pacific ocean there
are also ridges, called spreading centers,
where new seafloor is created. In the southern
Pacific ocean in the Ring of Fire there are some
spreading ridges. So even though old seafloor
is subducted there is always some new seafloor to
take its place!
It is difficult to subduct a ridge though. There
used to be another plate with old seafloor which
subducted beneath North America until its ridge
met the North American plate. Instead of
subducting, the plate on the other side started to
move north in approximately the same direction as
the coast. And that sideways movement is
actually what helped form the San Andreas Fault.
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