Hawai'i - Keynote pdf
Hawai'i Hot Spot - Keynote pdf
Lava - Keynote pdf
Shield Volcanoes - Keynote pdf
Rejuvenation Stage Eruptions
- Keynote pdf
The Hawaiian Archipelago is in the middle Pacific.
The most isolated islands on Earth.
The archipelago is 2400 km long.
The sea floor in the Hawai'i region is 5000 m deep.
Big Island of Hawaii is the largest island, 60% of the landmass of Hawai'i.
Archipelago extends to the northwest.
The Northwestern Hawaiian Islands are mostly atolls.
All of the large islands are volcanic.
Hawai'i consists of eight major islands:
The eight major islands are constructed from 17 large shield volcanoes:
Hawai'i
Maui
Kaho'olawe
Lana'i
Moloka'i
O'ahu
Kaua'i
Ni'ihau
Lo'ihi is the youngest of the large Hawaiian shield volcanoes.
Lo'ihi is 4000 m tall, but still 950 m below sea level.
It is growing 30 km to the southeast of Kilauea.
The Hawaiian Islands were formed by a hot spot.
Hot spot is region of Earth's mantle that is hotter than the surrounding rock.
Approximately 100 hot spots are identified on Earth.
Hot spots can form on the sea floor or on the continents.
Yellowstone is an example of a continental hot spot, which explains the heat
source for the geysers.
The trace of the Yellowstone Hot Spot shows that the North American Plate is
drifting over the relatively stationary hot spot.
Some geologists suggest that magma that formed the Hawaiian Islands comes from rocks that originate as deep as the core/mantle boundary.
The Hawaiian hot spot forms a mantle plume.
The hot, less dense rocks rise to about 60 km deep, where 1-4% of the rock melts.
The mantle is Fe, Mg silicate, as is the magma that is generated.
The magma continues to rise, because it is less dense than solid rock, and can
erupt to form a volcano.
Currently the Hawai'i Hot Spot underlies the Big Island of Hawai'i.
There is only one hot spot, so why does a chain of islands form?
The Hawaiian hot spot is relatively stationary.
However, the Pacific lithospheric plate drifts to the NW.
As the lithospheric plate drifts over the hotspot, a chain of islands is created.
The characteristics of the Hawaiian Archipelago support this formation theory:
Past Kaua'i, mostly are atolls.
Magma - molten rock.
Once magma is erupted onto the surface, it is called lava.
Magma can be
Tephra - magma or rock fragments ejected by gases.
Two primary types of lava in Hawai'i:
Pahoehoe can be smooth or ropey.
It commonly forms lava toes.
The ropey skin forms as the molten core flows and drags the solidifying skin.
'A'a has a molten core with clinker on the top and bottom.
'A'a tends to be more viscous than pahoehoe.
The two types of lava are the same chemically - Fe, Mg silicate.
The rock is called basalt.
The difference between 'a'a and pahoehoe might be related to flow characteristics
(i.e. how the lava flows).
Minerals in Basalt
Basalts from primarily from three minerals:
Olivine and pyroxene are Fe, Mg silicates.
Ca feldspar is a Ca-rich Al silicate.
The large, island-forming Hawai'i volcanoes are shield volcanoes.
They are called shield volcanoes because of their shape: wide compared to their heights.
Eruptions can occur anywhere on a sheild volcano: however, most eruptions occur in two regions:
Shield volcanoes tend to be elongate because the rift zone is linear.
The single location with the most numerous eruptions is the caldera region.
Therefore the caldera region forms the summit of a shield volcano.
The rift zones are areas of extensive fissure concentration.
Most shield volcanoes have 2 primary, offset rift zones.
Although the caldera is the single location with the most numerous eruptions,
more eruptions occur along the length of the rift zone.
Because of the eruptions along the rift zone, Hawaiian shield volcanoes commonly
are elongate parallel to the rift zone.
The rift zone forms a ridge, commonly extending in two directions from the
caldera.
The caldera of a shield volcano forms over the summit magma chamber.
Calderas are collapse features that form at the summit of a shield volcano.
The nearly vertical walls of a caldera are fault scarps.
A fault scarp is the surface exposed by faulting.
Calderas form when the magma chamber empties and the summit of a volcano collapses
into the summit magma chamber.
Calderas are formed by multiple collapse and refilling events.
Examples of calderas:
Pit craters are collapse features.
Form primarily in the caldera region and along the rift zone.
Pit crater walls are fault scarps.
Pit craters generally form from a single event.
Halema'uma'u Crater is a pit crater that formed in Kilauea Caldera.
Kilauea has two rift zones:
Pu'u 'O'o is the main vent in the current eruptive phase
After the formation stage of a Hawaiian shield volcano, it
generally enters a long period of extinction and erosion.
However, Hawaiian shield volcanoes experience renewed volcanism, after they have
drifted very far from the hot spot.
This return of volcanic activity is called the rejuvenation stage.
Rejuvenated volcanics have occurred on Maui, Moloka'i, Kaho'olawe, O'ahu, and
Kauai.
The source of the magma during the rejuvenated stage is unknown.
Declining stage eruptions ending on O'ahu approximately 1.8 million years ago.
The erosional period lasted about 1 million years, before volcanism returned
to O'ahu.
The first rejuvenated eruptions on O'ahu began about 800,000 years ago.
Most of the rejuvenated stage eruptions haved occurred in the Honolulu region
of O'ahu.
The volcanic deposited are called the Honolulu Series.
The eruptions tend to be much more explosive.
The eruptions occur along fracture systems that trend perpendicular to the rift
system of the Ko'olau Volcano.
The Honolulu Series is comprised of approximately 40 rejuvenated stage volcanoes:
Leahi (Diamond Head), Koko Head, Koko Crater, Manana (Rabbit) Island, Mokapu,
Salt Lake, Ka'imuki lava shield, Punch Bowl, etc.
The last eruptions of the Honolulu Series might be as young as a few thousand years old.
These explosive eruptions tend to form tuff cones.
Tuff cones were ash cones that solidified to tuff.
The explosive eruptions blasted the magma into ash when the molten rock encountered groundwater.
Tuff cones tend to be higher on the leeward side, because the Trade Wind blow most of the ash downwind.
Notes of the first exam are continued in Oceanography