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The 15 volcanoes that comprise the eight principal
Hawaiian Islands are the youngest in a linear chain of about
125 volcanoes that stretches for about 3,600 miles across
the north Pacific. The chain is remarkable for its length
and the number of volcanoes that comprise it, the bend that
separates the older Emperor Seamount Chain from the younger
Hawaiian Ridge, and the age of the volcanoes, which is
systematically younger towards the southeast. The age of the
oldest dated volcano, located near the northern end of the
Emperor Seamount Chain, is 81 million years, the bend
between the two chains, 43 million years, and the oldest of
the principal islands, Kaua'i, a little more than 5 million
years.
The assembly line that forms the volcanoes is driven by a
"hot spot," or plume of hot material, deep within the Earth
that partially melts to produce magma as it rises beneath
the Pacific Plate. As the plate moves west-northwest, each
volcano moves with it from its place of origin above the hot
spot. The age and orientation of the volcanic chain records
the direction and rate of movement of the Pacific Plate. The
pronounced 43-million-year-old bend between the Hawaiian
Ridge and the Emperor Seamount Chain marks a dramatic change
in plate direction.
Each of the volcanoes in the chain evolved through a
sequence of life stages. When a new volcano forms, eruption
rates gradually increase over a period of several hundred
thousand years, attain their peak for perhaps 500,000 years,
then decline rapidly. It may be several million years before
the final activity at any volcano is over. As the eruption
rates change, so does the composition of the lavas. The
lavas erupted during each stage are chemically distinct,
thereby allowing identification of the different stages. The
relationship of the top of the volcano to sea level
determines the style of eruptive activity, e.g., submarine
eruptions form pillow lava; explosive eruptions occur in
shallow water, may continue after the volcano has grown
above sea level, and form ash deposits; and subaerial (above
sea level) eruptions form ropey pahoehoe and blocky 'a'a
flows.
- The initial stage of volcano growth is the submarine
preshield stage, characterized by infrequent,
small-volume eruptions. Pillow lava constructs a
steep-sided volcanic edifice with a shallow summit
caldera and two or three rift, or fissure, zones
radiating from the summit. Rift zones are a prominent
feature of Hawaiian volcanoes during all but the final
eruptive stage. This stage lasts for perhaps 200,000
years, yet produces only a small portion of the final
volume of the volcano. As the volcano grows, the
composition of the lava changes, and eruptions become
more frequent and voluminous.
- When the transition is complete, the volcano enters
the shield stage. The shield stage is subdivided into
three substages.
- The submarine substage is characterized by the
continuing eruption of pillow basalt.
- The explosive substage begins when the summit of
the volcano reaches shallow water and magma and sea
water mix to produce explosive eruptions. These
ash-generating eruptions occur intermittently for a
period of several hundred thousand years. During this
and the subsequent subaerial substage, summit calderas
repeatedly collapse, are filled with thick, ponded
flows, only to collapse again. When the volcano has
risen enough above sea level, explosive eruptions
cease.
- The subaerial substage begins. The low-profile
"shield" shape of Hawaiian volcanoes is formed (named
after the shape of a warrior's shield). Eruption rates
and frequencies peak during the shield stage, and
about 95% of the volcano's volume forms during a
period of roughly 500,000 years.
- The postshield stage caps the shield with a carapace
of lavas that have low silica and high alkali contents.
Not all Hawaiian volcanoes go through this stage (e.g.,
Ko'olau and Lana'i), whereas other volcanoes have erupted
thick lava sequences (e.g., Wai'anae and Mauna Kea)
during this stage. The high fountains of these eruptions
produce large amounts of cinder and short, thick, pasty
'a'a flows that steepen the slopes of the volcano as they
pile up near the vents around the summit region and along
the previously active rift zones. These lavas commonly
fill and overflow from the final caldera. During this
stage, the eruption rate gradually decreases to zero over
a period of 250,000 years or less.
- The end of the postshield stage is followed by a
period of erosion and subsidence, during which deep
canyons may form along the flanks of the volcano. As the
islands subside, fringing coral reefs grow. Following
this erosional period, the volcanoes may go through one
last eruptive phase, the rejuvenated stage. During this
stage, lavas with very low silica and high alkali
contents may erupt, commonly through the reefs formed
during the erosional period; the flows are funneled down
stream valleys, such as Manoa Valley on Oahu. Lavas
erupted during the rejuvenated stage are abundant on
Kaua'i and Ni'ihau, common on Ko'olau Volcano, Oahu, and
occur on East Moloka'i, West Maui, and Haleakala, East
Maui. Rejuvenated-stage activity has very low eruption
rates and may occur over several million years.
- Following the rejuvenated stage, the islands undergo
a long period of erosion and subsidence that eventually
reduces them to sea level. With continued subsidence, the
islands become coral atolls, such as those that lie west
of La Perouse Pinnacles, the westernmost and oldest
subaerial remnant of a former Hawaiian island. These
coral reefs may die if conditions are not favorable.
After the reef dies, the volcano continues to subside.
Once below sea level, these flat-topped, coral-capped
volcanoes are called "guyots." Although a few volcanoes
apparently never grew above sea level, nearly all the
volcanoes in the Hawaiian-Emperor volcanic chain older
than 30 million years are guyots. Most of these volcanoes
stood thousands of feet above sea level in their youth,
as the volcanoes that make up the present-day islands do
today.
The abundance of giant landslides around the principal
Hawaiian Islands indicates that such landslides are an
integral part of their life history. Fortunately, such
catastrophic events occur infrequently - - only once every
several hundred thousand years. Aspects of the geology of
individual volcanoes, such as the apparent absence of summit
calderas, are readily understood within the context of such
giant landslides. New undersea data collected within the
last decade, combined with previous observations and mapping
on the islands, have led to a revolution in our
understanding of the geologic processes that built and then
modified the Hawaiian Islands.
Island of Hawai'i
The Island of Hawai'i consists of five coalesced
volcanoes, a submarine volcano that has already subsided
below sea level, and another, Loihi Seamount, that has yet
to grow to sea level. The following is a brief history of
each of these volcanoes, starting with the youngest and
progressing to the oldest.
Link
to U.S.G.S. map of recent earthquakes on island of
Hawaii.
Loihi,
which is in transition between preshield and shield stages,
has infrequent small eruptions, and has had small earthquake
swarms nearly every year since 1980. The flat-topped summit,
about 3,180 feet below sea level, is connected to a
well-developed south rift zone and a less pronounced north
rift zone. There are two deep collapse pits, similar in size
to Halema'uma'u at Kilauea's summit, located in the southern
part of the summit region. Hot water escapes from vents near
its summit and along its upper south rift zone. These
observations, and the recovery of fresh, glassy lava
fragments, indicate that Loihi is an active volcano destined
to emerge as an island within the next 200,000 years, grow,
and coalesce with Hawaii.
Kilauea
Volcano, in the explosive substage of the shield
stage, and the most active volcano on Earth, has erupted 60
times since 1840. Its lava flows are interbedded with ash
deposits produced by infrequent explosive eruptions, most
recently in 1790. Eruptions can occur anywhere at the summit
or along the east or southwest rift zones. The south flank
of the volcano, bounded by the two rift zones, slips towards
the ocean at rates of a few inches per year on a flat-lying
fault about 6 miles deep. Large earthquakes, such as those
that occurred in 1975 and 1989, are associated with
large-scale movement along this flat-lying fault.
Mauna
Loa Volcano, nearing the end of the shield stage, is
declining in its eruption rate. Only three of its 36
eruptions since 1843 have occurred since 1950. In addition
to the two prominent rift zones, repeated fissure eruptions
have occurred randomly on the northern and northwestward
flank of the volcano. In order to grow to its present
estimated volume of roughly 10,000 cubic miles, Mauna Loa
must have erupted, for much of its lifespan, at four or five
times its historic rates. Like Kilauea, the southeastern
flank of Mauna Loa slips slowly towards the ocean on a
flat-lying fault that generates large earthquakes. The west
flank also slips during large earthquakes. The flanks of
Mauna Loa have spawned at least six catastrophic landslides
that can be recognized as blocky debris on the sea floor
adjacent to the island. The Alika slide off the west flank
of Mauna Loa Volcano is inferred to have formed about 105 ka
(thousand years before present); it appears to be the
youngest of the large landslides around the Hawaiian
Islands.
Hualalai
Volcano, an active volcano in the postshield stage,
last erupted in A.D. 1800-1801, once about 700 years ago,
and three times between 900 and 1,200 years ago. It erupts
every few hundred years, on average; the flows of 'a'a and
pahoehoe advance quickly due to its steep slopes. The final
summit caldera is buried, and spatter and cinder cones align
along well-developed northwest and south rift zones. At
about 105 ka, the postshield-stage pumice cone Pu'u
Wa'awa'a, and its associated 900-foot-thick flow at Pu'u
Anahulu are the oldest exposed lavas on Hualalai. The
youngest shield lavas are about 128 ka. Hualalai is a
potentially dangerous volcano that is likely to erupt in the
next 100 years.
Mauna
Kea, a dormant volcano in its postshield stage, last
erupted about 4,500 years ago. Lava flows and cinder cones
have buried the final summit caldera. Although a few flows
have funneled down stream beds and reached the coast, its
youngest lavas are thick and pasty and formed large cinder
cones and short flows. Its oldest exposed lavas are about
250,000 years old. Many Mauna Kea eruptions were explosive
and produced widespread ash deposits. These ash-producing
eruptions may have been triggered by water/rock interaction
as lava encountered glaciers at the summit of Mauna Kea.
Three periods when mountain glaciers covered the summit
region have been identified. The thick ash cover on its
flanks is derived mostly from Mauna Kea eruptions, but
perhaps three feet of it may be from explosive eruptions on
Kilauea about 39,000 years ago. Mauna Kea could erupt again,
although it is unlikely, because postshield-stage eruptions
become less and less frequent before they cease
altogether.
Kohala Volcano is extinct; it consists of shield
lavas, including the oldest lavas from the island of Hawaii,
dated at about 460,000 years old, and postshield-stage lavas
as young as 60,000 years old. Its northwest rift zone
extends through an elongate summit depression, apparently an
extensional basin at the headwall of the Pololu landslide
that moved towards the northeast. Waipi'o and Pololu valleys
have formed along faults that bounded this landslide, which
occurred before eruption of postshield-stage lavas began
about 260,000 years ago. The thick ash cover on Kohala
Volcano is derived mostly from Mauna Kea, although some is
probably of local origin.
Mahukona, the first volcano to form part of the
island of Hawaii, is located off the northwest shore of the
island; it is now completely submerged. Mahukona has a
prominent west rift zone, but the location of its summit and
any other rift zones are now buried beneath flows from the
adjacent Kohala and Hualalai Volcanoes. It apparently became
extinct at the transition between the shield and postshield
stages about 450,000 years ago. A stairstep series of
drowned coral reefs occur on the northwest- and
southwest-facing slopes of the volcano. These reefs mark
former shorelines and record the subsidence of the island
from about 465,000 years ago until the present. At its
zenith, Mahukona probably rose no more than 820 feet above
sea level; it was completely submerged shortly after 435,000
years ago.
The Island of Hawaii will change shape in the future as
Kilauea extends its southern and eastern shorelines and as
Loihi grows to sea level and eventually coalesces with the
island. At the same time, however, continued subsidence will
reduce the size of Kohala, Mauna Kea, Hualalai, and Mauna
Loa Volcanoes. At the present rate of subsidence of about
1/8 inch per year, Kohala will become a separate island
about 350,000 years from now. Between 600,000 and 850,000
years from now, Hualalai and Mauna Kea will become separate
islands; exactly when that time will come cannot be
estimated without knowing when Mauna Loa will stop
erupting.
Island of Maui Nui
Before the island of Hawaii formed, Maui Nui was an even
larger island. It consisted of six or seven coalesced
volcanoes, including Haleakala, West Maui, Kaho'olawe,
Lanai, East Moloka'i, West Moloka'i, and Penguin Bank, which
was apparently a volcano separate from West Moloka'i.
The youngest of the volcanoes that made up Maui Nui is
Haleakala.
An active volcano in its rejuvenated stage, it last erupted
about 1790 and has a poorly-established eruptive recurrence
interval of several hundred years. Three fissure, or rift,
zones extend to the northwest, east, and southwest. A large
summit depression, originally interpreted as a caldera and
later as an erosional feature, may have been formed from the
coalescence of the headwalls of two landslides to the north
and south of the summit. The volcano consists of
shield-stage lava (1.1 million to 900,000 years old),
postshield-stage lava (860,000 to 410,000 years old), and
rejuvenated stage lava (younger than 400,000 years old).
Haleakala is unique in that rejuvenated-stage vents are
aligned along the rift zones. Haleakala is a potentially
dangerous volcano that is likely to erupt again within the
next several hundred years.
Kaho'olawe, whose eastern half slid away during a
catastrophic landslide, is an extinct volcano that has
undergone the shield and postshield stages. Only a few vents
and flows represent the postshield stage, and a
west-southwest rift zone is identified by aligned vents.
West Maui Volcano is an extinct volcano whose
evolution includes shield (1.6 to 2.0 million years old),
postshield (1.5-1.2 million years old), and rejuvenated
stages. The rejuvenated stage is represented by only a
handful of vents and flows, located mainly near the town of
Lahaina. Numerous cones, domes, dikes, flows, and
pyroclastic deposits delineate the postshield stage. Deep
erosion has exposed nearly 4,900 vertical feet of the
volcanic stratigraphy on West Maui.
Lanai is an extinct volcano with only the shield
stage represented. Its lavas have been dated at 1.28 million
years - - too young, compared to the ages of nearby
volcanoes. A large landslide deposit on the sea floor
south-southwest of Lanai, named the Clark landslide, was
derived from Lanai or Penguin Bank.
East Moloka'i is an extinct volcano with lavas
erupted during the shield (1.5 million years old) postshield
(1.5-1.35 million years old), and rejuvenated stages
(570,000 to 350,000 years old). Its summit caldera was
bisected by an enormous landslide, named Wailau, that slid
to the north and deposited half-mile-sized blocks on the sea
floor as far as 100 miles north of the island. The landslide
apparently occurred during the late part of the shield
stage; the steep northern cliff marks its headwall. The sea
cliff on the north side of the volcano exposes about 4,000
feet of stratigraphic section, including lavas erupted
during the shield and postshield stages. The rejuvenated
stage is represented only by the lavas that formed the
Kalaupapa Peninsula.
West Moloka'i, an extinct volcano with lavas
erupted during the shield (1.9 million years old) and
postshield (1.8-1.75 million years old) stages, has no
exposed caldera complex. A series of normal faults that step
down to the east is located on the east flank of the
volcano. These faults probably mark the headwall of a
landslide that down-dropped the summit and eastern half of
the volcano towards the east. Such a landslide presumably
occurred before East Moloka'i Volcano had grown and
buttressed the eastern flank of West Moloka'i Volcano.
To the west of West Moloka'i Volcano, a broad shoal
called Penguin Bank appears to be a separate volcano
that has now subsided below sea level and is covered with a
coral deposit of unknown thickness. Lavas recovered from the
southern flank of Penguin Bank are all from the shield stage
and are distinct in composition from those of adjacent West
Moloka'i. Much of the original caldera complex may be strewn
across the sea floor as blocks in the Clark landslide.
About 300,000 to 400,000 years ago, Maui Nui, which grew
from west to east, subsided to form two islands, one
consisting of Penguin Bank, Moloka'i, and Lanai, and the
other consisting of Maui and Kaho'olawe. Kaho'olawe then
separated from Maui, and finally Lanai separated from
Moloka'i, both within the last 100,000 to 200,000 years.
Penguin Bank probably submerged within the last several
hundred thousand years. With continued subsidence at the
present-day rates, Haleakala and West Maui will become
separate islands in about 15,000 years. At its largest, Maui
Nui stretched from about 42 miles west-southwest of the
present west shoreline of Moloka'i to roughly 47 miles east
of the eastern tip of Maui. Although some of the oldest
western land may have subsided below sea level somewhat
before the easternmost part of the island formed, Maui Nui
probably had a maximum size of about 6,200 square miles,
some 2,150 square miles larger than present-day Hawaii.
Island of O'ahu
O'ahu consists of two extinct volcanoes, Ko'olau to the
east and Wai'anae to the west. Ko'olau Volcano
consists of the eruptive products of the shield (2.5-1.7
million years old) and rejuvenated stages; no postshield
stage lavas are known. A caldera complex, filled with thick,
ponded lavas that have been altered by hot water, occurs in
the Kailua region on the northeast shore of the island. The
caldera was bisected by the catastrophic collapse of the
Nu'uanu landslide, which deposited numerous blocks on the
sea floor as far as 100 miles northeast of the island. The
largest of these blocks is about the same size as Manhattan
Island.
The rejuvenated stage lavas erupted mainly in the
Honolulu area, hence their name, the Honolulu Volcanics.
These vents and flows appear to be older than 100,000 years;
the best-dated vent, at Black Point, is 410,000 years old.
The flows and ashes of the Honolulu Volcanics have high
contents of sodium and potassium and low contents of silica.
Many of the vents erupted through a coral reef that
surrounded the island on the south side. These eruptions
tended to be explosive, and most vents along the coast are
ash, or tuff, cones, such as Diamond Head, Hanauma Bay, and
Salt Lake Crater. Flows erupted inland were funneled down
valleys, such as Manoa and Nu'uanu Valleys, thereby creating
flat valley floors.
Wai'anae Volcano consists of the eruptive products
of the shield (3.9-3.5 million years old) and postshield
(3.2-2.5 million years old) stages. The shield lavas are
overlain by a thick sequence of postshield stage lavas. A
post-erosional sequence of lava, once thought to be of the
rejuvenated stage, is 2.5 million years old and has been
reinterpreted as postshield. The erosional break that
separates these lavas from the earlier part of the
postshield stage has been attributed to a catastrophic
landslide to the southwest named the Wai'anae slump.
Island of Ni'ihau
Ni'ihau, an extinct volcano with shield- (4.9
million years old), postshield- (4.9 million years old), and
rejuvented- (2.5 million to 400,000 years old) stage lavas,
has no exposed summit caldera complex; it was apparently
removed during a large landslide towards the east. The
postshield stage consists of the erosional remnants of a
dike and of a single cone. Rejuvenated stage lava, with a
narrow range of compositions, covers about a third of the
island and forms a small ash, or tuff, cone, Lehua Island,
off the north shore. Numerous cones and flows of rejuvenated
stage lava also dot the deep sea floor west of Ni'ihau.
Island of Kaua'i
Kaua'i consists of at least one extinct volcano,
including shield- (5.6-5.0 million years old), rare
postshield- (4.94 million years old) and abundant
rejuvenated- (3.65 million to 500,000 years old) stage lava.
Kaua'i is unique among Hawaiian volcanoes with its enormous
caldera complex, no obvious rift zones, and a graben, or
downdropped block, on the south side of the caldera complex.
Rejuvenated-stage lava has covered much of the eastern half
of the island with a broad compositional range of basaltic
lavas. Some flows have filled canyons and diverted rivers,
only to be eroded through once again. Rejuvenated stage
lavas also erupted on the east and southeast submarine
flanks of Kaua'i.
Shield-stage lavas filled the caldera and the graben with
thick, ponded flows. The Makaweli graben probably formed in
response to a large landslide off the southern coast of the
island, that can be clearly identified by the levees it left
behind and the blocky debris on the sea floor. Other
landslides also affected the north and northeast flanks of
the island and, perhaps, the east flank, as well. The
caldera complex of a second smaller volcano may be
represented by thick, ponded lavas in the southeastern part
of the island. Deep erosion, weathering of the flows, and
voluminous subsequent rejuvenated-stage lava, make
unraveling Kaua'i's early history difficult.
Data Sources
Based on information from Volcano Watch by the U.S.
Geological Survey / Hawaiian Volcano Observatory ---
September 8, 1995 ; September 15, 1995; September 22, 1995;
and September 29, 1995.
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