Download mid-oceanic ridges

The student knows that plate tectonics is
the global mechanism for major geologic
processes and that heat transfer, governed
by the principles of thermodynamics, is the
driving force. The student is expected to:
• C) Explain how plate tectonics accounts for geologic processes
and features, including sea-floor spreading, ocean ridges and rift
valleys, subduction zones, earthquakes, volcanoes, mountain
ranges, hot spots, and hydrothermal vents;
• E) distinguish the location, type, and relative motion of convergent,
divergent, and transform plate boundaries using evidence from the
distribution of earthquakes and volcanoes
How Plate Tectonics Accounts For:
Sea-Floor Spreading is the
process in which the ocean floor is
extended when two plates move
apart. As the plates move apart,
the rocks break and form a crack
between the plates.
•Earthquakes occur along the
plate boundary.
•Magma rises through the
cracks and seeps out onto the
ocean floor like a long, thin,
undersea volcano.
How Plate Tectonics Accounts For:
As magma meets the water, it cools and
solidifies, adding to the edges of the plates.
•As magma piles up along the crack, a
long chain of mountains forms gradually
on the ocean floor. This chain is called
an oceanic ridge.
•When these boundaries form on the
continents, they are called Rift Valleys.
The boundaries where the plates move
apart are 'constructive' because new crust is
being formed and added to the ocean floor.
An example of an oceanic ridge is the Mid-Atlantic Ridge. It is
one part of a system of mid-oceanic ridges that stretches for
50,000 miles through the world's oceans.
How Plate Tectonics Accounts For:
As we’ve already learned, the
Earth's tectonic plates can move
apart, collide, or slide past each
other.
The Mid-Ocean Ridge system - the
Earth's underwater mountain range
- arises where the plates are
moving apart.
As the plates part, the seafloor
cracks. Cold seawater seeps down These hydrothermal vent
communities have evolved some
into these cracks, becomes super- of the most interesting
heated by magma, and then bursts communities of organisms on
Earth…perhaps showing us the
back out into the ocean, forming place where life on Earth
originated.
hydrothermal vents.
How Plate Tectonics Accounts For:
When two oceanic plates collide, the younger of the two plates,
because it is less dense, will ride over the edge of the older plate.
Oceanic plates grow more dense as they cool and move further
away from the Mid-Ocean Ridge
The older, heavier plate
The Marianas Trench, where the enormous
Pacific Plate is descending under the
leading edge of the Eurasian Plate, is the
deepest sea floor in the world.
The Challenger Deep at the trench’s
southern end measures nearly 7 miles
deep. If Mount Everest was set down in
the Pacific at this place, there would still be
well over a mile of water left above it.
plunges steeply through
the athenosphere, and
descends into the Earth,
where it forms a trench that
can be as much as 70
miles wide, more than a
thousand miles long, and
several miles deep.
Only three descents into the trench have ever been achieved. The first was the manned
descent by Trieste in 1960. This was followed by the unmanned ROVs Kaiko in 1996
and Nereus in 2009. Convergent boundaries are referred to as “destructive”.
How Plate Tectonics Accounts For:
Japan
20112008
China
Haiti 2010
One need only look at a map of
tectonic plates, and one of
earthquake activity, to see the
close association.
If we were to remove the clutter of the
minimal earthquakes, and only show
those equalling 7 or higher on the
Richter scale, you can see that all
major earthquakes have been
centered along tectonic boundaries.
An earthquake, also known as a quake, tremor or temblor, is the result of a sudden
release of energy in the Earth's crust that creates seismic waves. Earthquake intensity is
measured most commonly in terms of the “Richter Scale”, where each increase in whole
number, yields a10-fold increase in intensity. So, a magnitude 7 earthquake is 10 times
stronger than a 6, and 100 times stronger than a 5…etc.
How Plate Tectonics Accounts For:
Iceland Volcanoes
Volcanoes are generally found where
tectonic plates are diverging or
converging. A mid-oceanic ridge has
Three Sisters, East of Albuquerque NM, the newest
of volcanoes
caused
addition toexamples
the Rio Grande
Rift Volcanic
chain. by
divergent tectonic plates pulling apart;
the Pacific Ring of Fire has examples
of volcanoes caused by convergent
tectonic plates coming together.
By contrast, volcanoes are usually not created along transform boundaries, but they may
form where there is stretching and thinning of the Earth's crust in the interiors of plates,
such as in the East African Rift, and the Rio Grande Rift in North America.
This type of volcanism falls under the umbrella of "Plate hypothesis" volcanism.
Volcanism away from plate boundaries has also been explained as mantle plumes.
These so-called "hotspots", for example Hawaii, are hypothesized to arise from
upwelling of magma from the core-mantle boundary, 3,000 km deep in the Earth.
A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot
magma, volcanic ash and gases to escape from below the surface
How Plate Tectonics Accounts For:
Although Hawaii is perhaps the best known hotspot, others are thought to exist beneath
the oceans and continents. More than a hundred hotspots beneath the Earth's crust
have been active during the past 10 million years. Most of these are located under plate
interiors, but some occur near diverging plate boundaries.
Some are concentrated near the mid-oceanic ridge system, such as beneath Iceland,
the Azores, and the Galapagos Islands.
A few hotspots are thought to exist
below the North American Plate.
Perhaps the best known is the hotspot
presumed to exist under the continental
crust in the region of Yellowstone
National Park in northwestern Wyoming.
There are several calderas (large
craters formed by the ground collapse
accompanying explosive volcanism)
that were produced by three gigantic
eruptions during the past two million
years, the most recent of which was
600,000 years ago.
As you can see from prior
ash layers, the area affected
by a volcanic eruption would
be immense.
How Plate Tectonics Accounts For:
When two continents carried on
converging plates ram into each other,
they crumple and fold under the
enormous pressure, creating great
mountain ranges.
The highest mountain range in the world,
the snow-capped Himalayas, is an
example of a continent-to-continent
collision. This immense mountain range
began to form when two large
landmasses, India and Eurasia, driven
by tectonic plate movement, collided.
Because both landmasses have about
the same rock density, one plate could
not be subducted under the other.
The pressure of the colliding plates could
only be relieved by thrusting skyward.
The existence of linear mountain chains on
the Earth makes the Earth unique in the
solar system.
Although there is volcanism on Venus and
Mars and on some of the larger moons,
there is no evidence of linear mountain
chains.
Linear mountains suggest the movement of
a plate boundary and the existence of active
plate tectonics
We’ve already shown evidence of convergent and divergent
boundaries by looking at the earthquake/volcanic activity maps.
What about transform boundaries?
Since transform
boundaries are
“sliding” past one
another, they are
obviously a place of
great earth
movement, and
areas of high
Map of midoceanic ridges (red) and deep-sea
trenches (blue).
earthquake
activity.
Transform faults are shown in black, cutting the ridges.
Many earthquakes occur along transform plate boundaries.
Three types of transform faults:
• Ridge-ridge
are by far the most abundant. The active displacement on these
faults occurs only between the ridge segments, as shown
above. No movement occurs along the rest of the fracture zone.
• Ridge-trench
are much less common. They form an important connection
between diverging and converging plates. The longest
transform faults are all of this kind.
are also rare. These faults appear to connect trenches
together.
Transform faults can connect convergent and divergent plate boundaries in various
combinations. In all cases, the trend (or movement) of a transform fault is parallel to the
direction of relative motion between plates.
• Trench-trench