Hawai'i
Ring of Fire
Hot Spots
Decompression Melting
The Hawai'i Hot Spot
The Hawaiian Archipelago is in the middle of the Pacific.
The eight major islands, in order:
The eight major islands are constructed of 17 large (shield) volcanoes:
Hawai'i
Mau'i
Kaho'olawe
Lana'i
Moloka'i
O'ahu
Kaua'i
Ni'ihau
All of the large islands are volcanic in origin.
Currently, five volcanoes
are active:
See the Active Hawaiian Volcanoes Page at SOEST, the School of Ocean and Earth Science and Technology at UH-Manoa.
Loih'i is 4000 m tall, its summit is 950 m below sea level.
See the Lo'ihi Page at SOEST.
The Big Island of Hawaii is the largest island, with about 60% of the land
mass of the archipelago.
The island chain extends to the northwest from the Big Island.
The northwestern Hawaiian Islands mostly are atolls, e.g. Kure and Midway.
All of the Hawaiian Islands are volcanic.
There are over 100 volcanoes, most of which are submerged (seamounts).
See the Formation
of the Hawaiian Islands Page at SOEST.
Magma - molten rock.
Lava - magma erupted onto the surface.
All lava is magma, but most magma is not lava.
90% of magma never reaches Earth's surface and solidifies inside Earth.
There are three primary types of volcanoes in the Pacific region:
Mid-ocean ridge volcanoes for where the sea floor spreads at the East Pacific Rise.
Thousands of volcanoes in the Pacific region form near the continental margins.
Either as continental volcanoes or volcanic island arcs.
Pacific rim is called the Ring
of Fire, because of the abundance of volcanism.
Examples of Island Arc Volcanoes:
Examples of Continental Volcanoes:
Most of these volcanoes are associated with subduction.
Subduction is the process where oceanic lithosphere subducts under continental
or oceanic lithosphere.
Subduction occurs at convergent plate boundaries.
The exact method of magma generation during subducting is uncertain.
Possible mechanisms of magma generation at lithospheric plate
boundaries include:
Most volcanism occurs along plate boundaries, but volcanism in the interior
of lithospheric plates occurs commonly.
These intra-plate spots of volcanism occurs at over areas of Earth's interior
that are hotter than normal.
These regions are called hot
spots.
Hot spots are regions of Earth's mantle that are hotter than the surrounding rock.
Approximately 100 hot spots are identified on Earth.
Hot spots can occur on the sea floor or on the continents.
Hot spots are characterized by a voluminous outpouring of lava.(e.g. Deccan
Traps, Columbia River Basalts,
or Yellowstone)
The voluminous amounts of lava are associated with the very high temperatures
of hot spots - 1200 - 1400oC.
The chemical composition of the magma associated with hot spots tends to be
slightly different than volcanoes at plate boundaries.
Along with the high temperature, this suggests that hot spots likely originate
deep in the Earth.
The structure of Earth can be viewed in two ways:
Density Structure of Earth
Structure based on density and composition:
The crust is on the outside and overlies the mantle.
The mantle volumetrically is the largest part of Earth.
The core is at the center of Earth.
The crust is the least dense.
The mantle rock is more dense than the crust.
The core is the most dense.
The crust/mantle boundary, called the Moho,
varies from 5-70 km deep.
The core/mantle boundary is approximately 2900 km deep.
The radius of Earth is approximately 6400 km.
Oceanic crust is relatively thin, and continental crust is relatively thick.
The average composition of each layer is distinct:
Mechanical Structure of Earth
When discussion the outside of Earth moving, geologists use the mechanical structure:
This structure is based on strength and viscosity.
Viscosity is a measure of the resistance to flow.
Highly viscous material resists flowing, whereas lower viscosity material flow
easily.
The lithosphere includes all crust and upper mantle to an average depth of
80 km.
The lithosphere is thinnest (a few kilometers) at the MOR, where it is created.
The lithosphere becomes thicker with increasing distance from the MOR.
Lithosphere has a relatively high viscosity.
Lithosphere is relatively strong, i.e. it tends to resist stress to the point
of fracture.
The asthenosphere extends from approximately 80 km - 300 km deep.
The asthenosphere has a relatively low viscosity.
The asthenosphere is relatively weak, i.e. it tends to flow with stressed.
The lower mantle, the mesosphere, also tends to flow when stressed, but likely has a lower viscosity.
Melting occurs at the top of the asthenosphere.
Primary source of Earth's interior heat is radioactive decay.
Geothermal gradient - the increase in temperature with increasing depth.
Geothermal
gradient averages about 30oC/km in the upper crust.
Temperature in the core is over 5000oC.
The geothermal gradient decreases in the mantle to about 0.1oC/km.
Most rocks melt at temperature of 1000oC or greater.
The temperatures for most of Earth's interior or greater than 1000oC;
however, most of Earth is solid rock.
The pressure of the overlying rock prevents most rocks from melting.
Only the outer core is mostly liquid, the rest of Earth is solid rock.
In the outer core, the temperature is high enough and the pressure low enough
to result in melting.
Why does some melting occur in the upper asthenosphere?
The process is called decompression melting.
Rocks are poor conductors of heat.
As hot, solid, less dense rocks ascend, the rocks reach areas of lower pressure
where melting occurs.
Once the hot, solid rocks rise to the asthenosphere, near the base of the lithosphere,
the temperature remains high enough and the pressure now is low enough that
a small percentage
of rocks melt.
Commonly only 1-4% of the rock melts.
The magma that is generated is less dense than the surrounding rock, so it rises to the lithosphere.
The Hawaiian hot spot forms a mantle
plume.
The hot, less dense rocks rise to about 60 km deep, where a small percentage
melts.
The magma continues to rise, because it is less dense than solid rock, and can
erupt to form a volcano.
Currently the Hawaiian hot spot underlies the Big Island of Hawaii.
Some geologists suggest that magma that formed the Hawaiian Islands comes
from rocks that originate as deep as the core/mantle
boundary.
Where as most of the magma generated near plate boundaries comes from shallower
in the mantle.
Hot spot is estimated to be 80-100 km wide as it rises.
The head of the hot spot can mushroom to a width of 100-200 km wide.
There is
only one Hawai'i Hot Spot, under the Big Island of Hawai'i.
If there is only one hot spot in Hawai'i,
why does a chain of islands form?
The Hawaiian hot spot is relatively
stationary.
However, the Pacific lithospheric plate drifts to the NW.
The section of the East
Pacific Rise (MOR), where is sea floor in the Hawaii region was formed,
is off the west coast of the Americas.
A volcano forms over the hot spot, then drifts away.
Subsequently, a new volcano forms over the hot spot.
The process creates a chain of islands.
Why discrete islands form is not known, but several mechanisms are possible:
Evidence does show that the plumbing systems bend as the Pacific Lithosphere Plate drifts.
The changing characteristics of the Hawaiian Archipelago support the hot spot
theory of formation.
With increasing distance from the hot spot:
Islands are progressively older to the NW, active volcanoes under Big Island.
Island are progressively more eroded to the NW and, past Kauai, mostly are
atolls.
Beyond Kure, the Emperor Seamount chain extends to the Aleutian trench off
Asia.
Entire chain is 6000 km long, the oldest volcanoes are 80 million years old.
The older seamounts likely were subducted or aggregated into Asia.
The kink in chain of volcanoes is about 43 million years old.
The reason for the kink is unknown.
The two most likely explanations are
End of the notes for the first quiz
