Hawai'i - A Chain of Shield Volcanoes
A lava pond formed over the new vent of Pu'u O'o, and
its frequent overflows built a broad, low shield that
reached its maximum height of 55 m in less than a year.
From HVO photo
gallery. |
The Islands of Hawa'ii are a series of shield volcanoes that
have built up from the ocean floor. Some of the islands are
composites of extinct volcanoes, while Maui's Haleakala and
four of the big island of Hawaii's volcanoes - Mauna Loa,
Loihi, Hualalai and Kilauea - have erupted in the past 2,000
years.
List of National Parks exhibiting Hawaiian Geology
Shield volcanoes are the result of multiple eruptions, most
of which have produced fluid basaltic lavas, which flow and
harden. Due to the low viscosity of these lavas, the flows
are relatively thin and travel far, building up slowly to
produce a volcanic profile like that of a shield lying on
the ground.
Most of the volcanoes' lavas flows from a central vent,
or group of vents, although side vents are common. Basaltic
lava flows also tend to form lava tubes, which allow the flow
of lava to extend much further than would otherwise be possible.
The tube protects the lava from cool air temperatures. Heat
loss is the enemy of flowing lava; it causes crystals to form
in the melt, inhibiting the movement of the lava, which becomes
thicker and slower until it stops and totally crystallizes.
With its sides and top insulated, lava flowing through tubes
can remain melted for longer and travel much further. On Hawai'i
some lava tubes feed lava directly into the sea. Click here
to learn more about the Kilauea lava tubes at the HVO web
site.
Scientist takes advantage of a skylight to measure height
of lava flowing in a lava tube, Kilauea Volcano, Hawai`i.
From HVO photo
glossary. |
Underwater basaltic lava flows form things called pillow
lavas. These are forms that have been quenched quickly, leaving
them in globular, tooth-paste-like forms. Close to the sea
level, lava that comes into contact with water shatters, forming
piles of glassy shards. As these build up lava flows and hardens
on top of these shards, kept out of the water by the thickness
of the deposits from previous eruptions. However the layers
of glassy shards are not stable, and can create hazards after
many eruptions. The ground which is formed above the shards
appears stable; smooth thick cliffs of unbroken lava benches
look over the sea all around the coast of Hawai'i. People
don't realize that the ground is prone to landsliding. Bench
collapse, or the sliding of an entire section of the coast
into the ocean, is a major hazard at Hawai'i Volcanoes National
Park. Also hazardous are the plumes of volcanic gasses. Volcanic
fume consists primarily of water vapor, carbon dioxide, and
sulfur dioxide. If you visit, don't walk through the plumes,
which can be poisonous!
To
learn more about the hazards of Hawaii's volcanoes from the
HVO website, click here.
Some of the largest volcanoes in the
world are shield volcanoes. In northern California and Oregon,
many shield volcanoes have diameters of 3 or 4 miles and heights
of 1,500 to 2,000 feet. The floor of the ocean is more than 15,000
feet deep at the bases of the Hawaiian Islands. As Mauna Loa, the largest
of the shield volcanoes (and also the world's largest active
volcano), projects 13,677 feet above sea level, its top is
over 28,000 feet above the deep ocean floor. |
