Transcript Geology of Hawaii

Volcanoes are also prodigious land builders as they have created the
Hawaiian Island chain. Kilauea and Mauna Loa, two of the world's most
active volcanoes, are still adding land to the island of Hawaii
Hawaii's Volcanoes Revealed
Subduction of oceanic
crust produces a Magma
Plume
Hawaiian volcanoes typically evolve in four stages as volcanism waxes
and wanes:
(1) early alkalic, when volcanism originates on the deep sea floor;
(2) shield, when roughly 95 percent of a volcano's volume is emplaced;
(3) post-shield alkalic, when small-volume eruptions build scattered
cones that thinly cap the shield-stage lavas; and
(4) rejuvenated, when lavas of distinct chemistry erupt following a
lengthy period of erosion and volcanic quiescence.
During the early alkalic and shield stages, two or more elongate rift
zones may develop as flanks of the volcano separate. Mantle-derived
magma rises through a vertical conduit and is temporarily stored in a
shallow summit reservoir from which magma may erupt within the
summit region or be injected laterally into the rift zones. The ongoing
activity at Kilauea's Pu‘u ‘O‘o cone that began in January 1983 is one
such rift-zone eruption. The rift zones commonly extend deep
underwater, producing submarine eruptions of bulbous pillow lava.
Origin of the Hawaiian Isles Volcanism
Hidden deep beneath the Earth's surface on the Pacific Ocean’s sea floor
lie one of the most constructive and yet least-understood natural
phenomena in the world – Volcanoes of the Hawaiian Island Chain. It
extends for 2,400 km (1,800 miles) from the extinct seamounts and atolls
in Northwestern Hawaiian Isles to active volcanoes in southeast in
Hawaii’s Big Island. The Pacific Oceanic plate is moving northwesterly.
Hawaiian volcanoes are formed by a column of magma - molten rock rising from deep within the Earth on the seafloor, erupting on the surface,
and hardening in layers down the sides.. This forms the familiar shield
shaped mountain we associate with volcanoes. The mechanism is termed
as the Hotspot Volcanism, due to convection in the Upper mantle. The
hotspot on the Pacific Oceanic Plate has been active for past 43 million
years. Hawaiian Volcanic Isles are also flanked by the South Arch
Volcanic field and North Arch Volcanic Field. The Hotspot is not fixed as
new data reveals and the formation of the islands is influenced by
subduction of shallow oceanic lithosphere. There are two geologic trends
of the Hawaiian volcanism, Kea-Kilauea Trend and Loa- Loihi Trend.
Life-cycle of Hawaiian hot spot volcanoes: The Hawaiian Islands
volcanoes are the product of a mantle hot spot in the middle of the
Pacific Plate. The hot spot's current activity is underneath the southern
end of the island of Hawaii and the next volcano in the chain, Lo'ihi
Seamount, is forming on the sea floor just to the south of Hawaii and
should emerge in another 200,000 years. The volcanoes undergo a
progression of eruption styles and chemistries as they age, from preshield stage (Lo'ihi), through the major shield-building stage (Kilauea),
to post-shield (Haleakala) and rejuvenated stages (such as Diamond
Head on Oahu when it erupted). As the enormous mountains build on top
of the ocean crust, the crust flexes downward and the islands slowly sink.
Erosion takes its toll on the islands: giant landslides have occurred off all
the islands, and some of the debris has traveled hundreds of kilometers
offshore. Most of the Hawaiian Islands have irregular shapes, not like the
round volcanoes. This is because their sides tend to collapse in gigantic
landslides, leaving chunks the size of cities scattered around the deep sea
floor near Hawaii. If such a landslide happened today it would be
devastating to the islands and due to tsunamis – Pacific Ring of Fire
Once a volcano has grown above sea level, subaerial eruptions produce
lava flows of jagged, clinkery ‘a‘a or smooth, ropy pahoehoe. If the
flows reach the ocean they are rapidly quenched by seawater and shatter,
producing a steep blanket of unstable volcanic sediment that mantles the
upper submarine slopes. Above sea level then, the volcanoes develop the
classic shield profile of gentle lava-flow slopes, whereas below sea level
slopes are substantially steeper. While the volcanoes grow rapidly during
the shield stage, they may also collapse catastrophically, generating giant
landslides and tsunami, or fail more gradually, forming slumps on
seafloor. Deformation and seismicity along Kilauea's south flank indicate
that slumping is occurring there today. Loading of the underlying Pacific
Plate by the growing volcanic edifices causes subsidence, forming deep
basins at the base of the volcanoes. Once volcanism wanes and lava
flows no longer reach the ocean, the volcano continues to submerge,
while erosion incises deep river valleys, such as those on the Island of
Kaua‘i. The edges of the submarine terraces that ring the islands, mark
paleocoastlines that are now as much as 2,000 m underwater, many of
which are capped by drowned coral reefs.
Direction Of Plate Movement
This diagram shows the relative topography along the Hawaiian chain. Note that with
The volcano grows by adding layer upon layer of new
pillow lava. Calderas are possible. Lava is alkalic in
composition, with highcontent of sodium and potassium
relative to the tholeiitic basalts of the shield-building
stage. It may or may not emerge above
Giant Lands
The large size of seafloor features, high activity of volcanism, and rapid rates of
subsidence make the Hawaiian islands an ideal place to study geologic processes.
The Hawaiian islands sink into the ocean at a measurable rate as the cessation of volcanism
that provides the magma to build the islands.
Once extinct, the Hawaiian Islands continue to erode until they slip
below sea level. The Emperor Seamount chain was once over the magma
plumeand probably looked much like the Hawaiian Islands, but the
volcanoes have since submerged. The Pacific Plate is carrying the entire
chain of islands and seamounts to the north-north-west as it drifts
slowly to the Aleutian Trench and its ultimate subduction.
The Hawaiian Isles volcanoes undergo a progression of eruption styles
and chemistries as they age, from pre-shield stage (e.g., Lo'ihi), through
the major shield-building stage (e.g., Kilauea), to post-shield (e.g.,
Haleakala) and rejuvenated stages (such as Diamond Head on Oahu
when it erupted). As the enormous mountains build on top of the ocean
crust, the crust flexes downward and the islands gradually sink,
sometimes rapidly (Mahukona).
Erosion takes its toll on the islands: giant landslides have occurred off
all the islands, and some of the debris has traveled hundreds of
kilometers offshore.
Once extinct, the islands continue to erode until they slip below sea level.
The Pacific Plate is carrying the entire chain of islands and seamounts to
the north-north-west as it drifts slowly to the Japan Trench and its
ultimate subduction.
Collapsed lava pillow on Puna Ridge, the
submarine extension of Kilauea's East Rift
Zone, April 2001, MBARI.
Equivalent lava flow on land, on Kilauea's
East Rift Zone, April 2001
Observations of submarine lava flows indicate volcanic processes that
contradict conventional wisdom. Discoveries of lava ponds and flood
basalts imply that lava may stay molten for a long time, despite the
almost infinite heat sink of the ocean. Fragmental rocks-ash, hyaloclastites, and volcaniclastic rocks, limu o Pele, and spatter imply that
explosive eruptions take place even in the deep sea, where hydrostatic
pressure is so great that steam bubbles should not be able to expand
This SPOT satellite image of the island of Oahu reveals the
extensive erosion that has removed much of the material on the
flanks of the Waianae and Koolau volcanoes. Note the stream
valleys that reach the crest of the mountains. The resistant rocks
mark the location of the rift zones of the volcanoes