Transcript Pacific Ocean - University of Hawaii

Subduction crack on the
Pacific Ocean crust
Magma is created at the subduction wedge
Volcanoes are thought to form in three settings: where tectonic
plates are diverging (for instance at mid-ocean ridges); where
tectonic plates are converging (in island arcs, for example);
and in "hotpots" (a generic term for volcanic activity that
cannot be attributed to plate tectonic movements. Hotspots are
generally thought to be formed by hot, buoyant plumes rising
rapidly from the boundary between Earth's core and the
mantle.
The "new" volcanoes, which are actually between one and
eight million years old, are not at plate boundaries. But neither
were they formed by deep plumes. The team thinks the minivolcanoes were created when cracks formed in the Earth's
crust during the elastic bending of the northwestern Pacific
plate, which is diving under the Kuril and Japan trenches.
They think partially melted material from the upper mantle
squeezed out of the cracks, to form the volcanoes.
Origin of
Intra-Plate
Volcanism
Hawaiian
Islands on the
Pacific
Oceanic Plate Hot Spot Model
Fixed Plume
coming from
Core-Mantle
Boundary, ~
3000 km (1800
miles) deep down
under in the
interior of Earth
There is lack of
strong geothermal,
geophysical, and
gravity, geomagnetic
radioactivity
evidences that
suggest that the
magma is ascending
from the Core-mantle
boundary.
Composition of
Hawaiian volcanic
basaltic rocks is very
similar to that of the
sea floor oceanic
basalts. There is a
lack of super heavy
elements, radioactive
minerals, and even
absence of Potassium
‘S’ Seismic waves pass minerals which are
normally under the
suggestive of deep
Islands. ‘S’ wave do not rooted magma.
travel through molten
material
A new type of volcano has been discovered in the western Pacific Ocean. The findings may
reduce the strength of a popular theory of “hotspot” volcanism, researchers say.
http://volcano.und.nodak.edu/dn9629.html
Naoto Hirano at the Tokyo Institute of Technology, Japan, and colleagues have discovered
miniature volcanoes – between 0.005 cubic kilometres and 1 km3 in size – near the underwater
Japan Trench. These volcanoes, dubbed “petit spot” because of their size, cannot be accounted for
by any of the conventional theories of volcanism.
The team thinks the mini-volcanoes were created when cracks formed in the Earth's crust during
the elastic bending of the northwestern Pacific plate, which is diving under the Kuril and Japan
trenches. They think partially melted material from the upper mantle squeezed out of the cracks,
to form the volcanoes.
“I was unbelievably excited to discover this volcanism,” Hirano says. “The possibility had been
proposed in the past, but had never been adequately documented.”
Buoyant plumes
Volcanoes are thought to form in three settings: where tectonic plates are diverging (for instance
at mid-ocean ridges); where tectonic plates are converging (in island arcs, for example); and in
“hotpots” (a generic term for volcanic activity that cannot be attributed to plate tectonic
movements. Hotspots are generally thought to be formed by hot, buoyant plumes rising rapidly
from the boundary between Earth’s core and the mantle.
The "new" volcanoes, which are actually between one and eight million years old, are not at plate
boundaries. But neither were they formed by deep plumes.
When Hirano’s team analysed the trace element geochemistry and compositions of noble gas
isotopes in their volcanic flows, they concluded that the material originated in the asthenosphere
– a layer of the mantle directly under Earth’s crust. Samples of the flows were collected by
Universal explanations
“These findings are probably the best evidence to date that not all chains of mid-plate volcanoes
are formed by plumes,” says Marcia McNutt of the Monterey Bay Aquarium Research Institute in
Moss Landing, California, US, who wrote a commentary on the paper published in Science.
“These findings do not mean that plumes don’t exist at all – there may be some volcanic chains
that are caused by plumes, but they are no longer a universal explanation.”
The question remains whether this new type of volcanism could explain other volcanic hotspots.
There are lava fields near Samoa and in Hawaii for which a flexing of the tectonic plate had been
suggested as a cause. But these suggestions had not been documented well, and the studies were
largely discounted by other researchers, according to Hirano.
Now, in the light of the new research, scientists will go back to observations for other volcano
chains with a more critical eye, McNutt believes. “They’ll look to see what observations were
ignored or swept under the rug because they couldn’t explain the finding in the context of plume
theory.”
Journal reference: Science (DOI: 10.1126/science.1128235 and 10.1126/science.1131298)
There are geochemical differences
between shield volcanoes and lavas
from the two parallel trends, Kea
and Loa, defined by young
Hawaiian volcanoes.
Lanai
There is lack of geothermal, geophysical,
and geomagnetic evidences that suggest that
the magma is ascending from the Coremantle boundary. Composition of Hawaiian
volcanic basaltic rocks is very similar to that
of the oceanic basalts. There is a lack of
super heavy elements, radioactivity, and
even absence of Potassium minerals which
are suggestive of deep rooted magma.
Kaholawai
Hawaiian Islands
Volcanic Islands
and Volcanic Fields
Pacific Ocean
Map shows the North and South Arch lava fields in red, in relation to the
Hawaiian Isles. The axes of the trough and arch are encircling the isles.
Older volcanoes and eruptions are mixed in with younger ones in no
particular order on the ocean floor and on the islands. This seems to
contradict the idea that a hotspot created this chain of islands.
Oahu
Maui Nui
Hawaii
is not fixed
Magma is created at the Mantle wedge
Cracks on
the Pacific
Ocean crust
Oahu
Maui Nui
Hawaii
Cracks on
the Pacific
Ocean crust
is not fixed
Intra-Plate Volcanism on
the Pacific Oceanic Plate
Magma is created at the subduction zone
wedge
Origin of Intra-Plate Volcanism on the Pacific Oceanic Plate Hawaiian Islands – Plate
Tectonics Rifting Model, showing the flexing of the tectonic plate.
Two principal types of basalt in the ocean basins
Tholeiitic Basalt and Alkaline Basalt
Mauna Loa
Kilauea
Kilauea
Caldera
18481982
Halema’uma’u
Crater
Height of Eruption Column and Degree of Explosivity -Whereas the height of the eruption column can be measured
directly in observed eruptions, it can also be estimated in
ancient eruptions by measuring the geographic dispersal of
the airfall tephra.
The degree to which this tephra is fragmented provides a
means to measure the explosiveness of the eruption. The
diagram here demonstrates how these parameters will vary
with eruption type. For more detailed information on these
parameters, click on image. Note that hydrovolcanic
eruptions (above dashed line) are generally the most
explosive, but do not necessarily generate the highest
eruption columns.
Lava flows behave in many ways like flowing water - they seek the
lowest elevation, driven by the force of gravity. Here pahoehoe lava
which flowed down a crevice in an earlier flow.
Mahukona
There are geochemical differences between shield lavas from the two
parallel trends, Kea and Loa, defined by young Hawaiian volcanoes.
Kilauea Slump: On Nov. 8, 2000, the motion suddenly accelerated by factor of several hundred,
with some parts of the slope moving as much as half an inch over the course of one day. Their
computer models indicate that a 9-mile-by-3-mile slab located about 2 1/2 miles
underground slipped about 3 1/2 inches, pulling along the ground above it. The event was
equivalent to a magnitude 5.7 earthquake. a flank collapse would unleash with up to
1,200 cubic miles of rock sliding at speeds of up to 100 miles an hour for 20 to 40 miles
The Honolulu Series of Volcanic Eruptions is Recent, 100, 000 years
old with 15 Tuff Cones in northeast and southwest perpendicular to
3 million year old KoolausVolcanoc Series. If the Hotspot has been
active over 500,000 years under the Big Island, 300 km away from
Oahu. Other younger volcanoes are present on Molokai and Maui.
Did the Hotspot wander to Oahu and other Islands at the same time?
NO FIXED HOTSPOT, it is migrating along with the Pacific Plate.
Shallow oceanic crust is recycled by Plate Subduction to make
Younger volcanoes such as Diamond Head on the older Isles.
Any satisfactory theory for Hawaiian volcanism must explain
(or rationalize) the:
change in migration direction of the melting locus at the
bend,
association of the great bend with the Mendocino fracture
zone,
change in migration rate at the bend,
apparent commencement of the volcanic chain near a ridge,
absence of a “plume head”,
large variations in magmatic production, and a current
magmatic rate about 3 times greater than the next most
productive hotspots,
absence of a significant heat flow anomaly,
absence of lithospheric thinning,
absence of a strong high-temperature signal in the erupted
basalts,
production of very large volumes of magma even though the depth to
the top of the melting column is exceptionally large compared with
MORs,
spatial and temporal variation in the composition of erupted lavas on a
variety of scales,
remote location of Hawaii, near the center of a very large plate,
location of the oldest end of the chain with respect to the “Pacific
pocket”,
unique rift zones,
paired Loa and Kea trends,
seismic whole-mantle mantle structure that is apparently normal
compared with the Pacific ocean elsewhere, and
occurrence of a bathymetric swell (a moat and “arch”) along the eastern
two-thirds of the Hawaiian chain and wrapping around its southeastern
end, with alkalic basaltic volcanism occurring at some places along it.
In conclusion, Hawaii is not fully explained by any
current hypothesis. It is impressive that a region of the
Earth so extensively studied for so many years, by so
many Earth scientists with so many techniques could
remain so intransigent to full understanding.
Many of the numerous features that are not yet fully
understood, and the parameters of alternative
hypotheses, are not currently being studied, but they
offer exciting research opportunities.
Rising magma and
Magma rises into
High pressure causes
volcanic gases exert
reservoir beneath
rocks to break,
pressure
volcano
triggering earthquakes
Earthquake activity beneath a volcano almost always increases before an
eruption because magma and volcanic gas must first force their way up
through shallow underground fractures and passageways.
Pacific Ocean:
The Pacific Ocean is the largest of the world's five oceans
Location: the body of water between the Southern Ocean, Asia, Australia
and the western hemisphere
Area: 155.6 million square km, or about 15 times the size of the US. The
Pacific Ocean covers about 28 per cent of the global surface - larger than
the total land area of the world
Terrain: the ocean floor in the eastern Pacific is dominated by the East
Pacific Rise, while the western Pacific is dissected by deep trenches,
including the Mariana Trench, which is the world's deepest place
Deepest point: Challenger Deep in the Mariana Trench - 11,022m
The hybrid remotely operated vehicle, or HROV, will be
able to operate in two modes: as an autonomous, or
free-swimming, vehicle for wide area surveys, and as a
tethered, or cabled, vehicle for close-up sampling and
other tasks. (Image: ©Jack Cook, WHOI)
Ocean Intraplate Volcanism Ocean islands and
seamounts, Commonly associated with hot spots
Figure 14-1. After Crough (1983) Ann.
Rev. Earth Planet. Sci., 11, 165-193.