Hi folks, Dr. Chapman here. Today we're
going to investigate Hawaii Volcanoes
National Park and hot spots. Hawaii
Volcanoes National Park is located on
the Big Island in the Hawaiian island chain.
All of the Hawaiian Islands are the tops
of giant shield volcanoes that rise from
the ocean floor. If you combine the
distance from the ocean floor with the
elevation of the volcanic peaks, these
volcanoes are taller than Mount Everest.
The hot basaltic magma that formed
shield volcanoes has a relatively low
viscosity meaning that they're more
runny and less sticky than other types
of magmas. Because of the
low viscosity lavas tend to flow outward
for long distances, which results in long
gentle slopes, which is where shield
volcanoes get their name. By contrast,
ocean island arc volcanoes like those
found on the Aleutian Islands in Alaska
and continental arc volcanoes like those
found at Cascades and the Pacific
Northwest are formed chiefly from
intermediate magma compositions like
andesite. More silica rich magmas are
also more viscous and these lavas don't
flow very far from the source after they
erupted. As a result, arc volcanoes have
very steep slopes. The high viscosity and
water content in arc volcanoes' magmas
also makes them much more explosive than
shield volcanoes. Giant shield volcanoes
like Mauna Loa and Mauna Kea were built
by many tens of thousands, perhaps a
hundred thousand individual lava flows,
each less than 10 meters thick. The
volcanoes were each built in about 1
million years ,which is not very long
geologically speaking. This
shows that while they're active the
Hawaiian volcanoes produce an enormous
volume of volcanic rock at very rapid
rates. Lavas can erupt at the summit of
volcanoes, like at the Kilauea crater, but
magma also spreads out beneath the
volcanoes through deep fracture networks
called rift zones. Volcanic rift zones
can sometimes be confused with rifts and
risk valleys like the Rio Grande rift.
But rifts and rift valleys formed by
extensional faulting related to tectonic
stresses like when two tectonic plates
are diverging.
Eruptions tend to concentrate along
rift zones and the intrusion of magma
itself helps to create and enhance
fractures as the rocks are tilted and
uplifted as the magma intrudes. So
there's a positive feedback loop between
eruptions and rift zone formation. The
elongate shape of Mauna Loa is related to
the growth of the volcano along two rift
zones that extend outward from the
central event. Magma traveling through
these rift zones or other underground
plumbing systems often
erupt on the flanks of the volcanoes. The
lava can get squeezed out of narrow
fissures and form spectacular lava
fountains. Or it can erupt from vents and
form lava pools or small cinder cones or
spatter cones. The Hawaiian Islands and
volcanoes formed above a region of hot
upwelling mantle called a mantle plume.
Mantle plumes usually cause the upper
mantle to melt and produce magmas and
volcanoes and so they're often called
hot spots. Although it's still debated,
hot spots like the Hawaiian hotspot are
thought to be more or less stationary.
The reason there's a chain of islands in
Hawaii is because the Pacific plate is
moving over the stationary hotspot. Kauai
is an extinct volcano that moved off of
the hot spot about five million years
ago. Whereas Kilauea on the Big Island is
an active volcano located directly above
the hotspot.
f we look at the sea floor bathymetry
we can see there actually many more
ancient volcanoes, which are located
beneath the ocean that extend past
Hawaii. These are called the
Hawaiian-Emperor seamount chain. These are old volcanoes that formed when the
Pacific plate was over the hotspot. The oldest volcano in the chain is the 82 million
year old Meiji seamount. It is
possible that there were older seamounts
or older volcanoes, but they were
subducted into the Kamchatka trench.
The youngest volcano associated with the
hotspot is not actually Kilauea, but an
underwater volcano called Loihi.
Geoscientists think that Loihi will be
above sea level and become its own
island and about 60,000 years.
The orientation of the Hawaiian Islands
and seamounts tells us which direction
the Pacific plate is moving (to the
northwest) and the age of the volcanoes
tells us how fast the plate is moving - 
about 10 centimeters per year. That's not
quite as fast as your hair grows. The
conspicuous bend in the Hawaiian-
Emperor seamount chain also suggests that
there was a change in the direction of
plate motion. Volcanic rocks dredged up
from the bend in the seamount chain are
about 47 million years old.
So geoscientists know when this plate
motion change occurred. Hot spots are one
of the main ways geoscientists can
determine absolute plate motion. Most of the
time we only consider the relative
motion between two tectonic plates. For
example, North America is moving westward
at about two centimeters per year
relative to Africa. Once the Hawaiian
volcanoes move away from the hotspot and
are no longer growing, erosion starts to
wear them down and the islands become
smaller and eventually disappear beneath
the ocean's surface. Wave action often
bevels the tops of the old volcanoes and
flattop seamounts are called guyots.
The old volcanoes are also lowered by
thermal subsidence. Subsidence is the
lowering of the land or sea floor
surface. Above the hotspot, the upwelling
mantle plume heats up the oceanic
lithosphere and it causes it to expand and
become less dense and this rises the
overall surface of the seafloor. After a
volcano moves off a hotspot, the crust
starts to cool down and it becomes dense
again causing the lithospheric plate to
sink or subside. Another theory is that
in addition to heating the lithosphere,
the upwelling mantle plume may actually
physically push up the lithosphere -
called dynamic uplift. At the same time
that the old volcanic islands are
sinking
coral reef communities are growing
around the islands and the upward growth
of coral reefs can keep pace with the
sinking of the islands. After the volcano
has been eroded or subsided beneath sea
level the coral reefs may remain, which
is called an atoll -  like the Midway
Islands are a part of the Midway Atoll
there was an active volcano over the
Hawaiian hotspot about 28 million years
ago. In as little as 10 million years
from now Hawaii Volcanoes National Park
and the Big Island may form an atoll
themselves or completely vanish beneath
the waves.
So visit while you can. Hey thanks for
watching. Check out some more videos and
share them with friends and family,
take care.
