Volcanoes in
Hawaii
Stacey Eng
The Volcanic
Formation of the Hawaiian Islands
The Hawaiian Islands, as we know them today, are, collectively, a
paradisiacal model of perfection. It’s
hard to imagine that, in the recent past, most of the sixteen hundred-mile
length of the islands first emerged from the Pacific as massive, fiery, volcanic
cones. With the aid of
paleontology, geologists have been able to hypothesize exactly how long ago the
Hawaiian Islands came into existence. The
most probable hypotheses say that the rift, or fault, in the Pacific floor
through which volcanoes emerged opened about twenty-five million years ago.
However, the islands we see today are much more recent than that.
The oldest of the islands, the Leeward Islands, probably formed between
five and ten million years ago, which places the formation of these islands in
the Pliocene period.
As mentioned above, the Hawaiian Islands are
mostly of volcanic origin. Island
building begins with submarine eruptions, which reach the ocean surface.
From these primary explosive eruptions flows
undersea lava which consists of “pillow lavas” (large rounded chunks),
pumice (foam-like glassy lava rich in gas bubbles), and ash. In the second stage of island formation, an above water
crater spouts lava from rifts on the side of the cone. This lava is mostly composed of basalt, which is the most
common type of lava in the Hawaiian Islands.
The third stage of the development features the
collapse of the summit, or highest point on the volcanic cone, to form a caldera.
Calderas, depressions which are much
wider than they are deep, occur when the summit falters and sinks as liquid lava
(magma) beneath the summit melts the core of the volcano.
Volcanoes in the caldera stage continue to emit lava flows, both from the
summit and from lateral rifts. Mauna
Loa, and Kilauea (both located on the island of Hawaii), two of the most famous
volcanoes in the Hawaiian Islands, are good examples of volcanoes at this stage.
The great activity of these two volcanoes is evident from the fact that
Mauna Loa itself has produced four billion cubic yards of lava between 1850 and
1950.
During the fourth stage, lava fills and overflows the caldera, forming a
rounded summit. At this point, the
activity of the volcano slows down and lava flows tend to be thick and composed
of chunky fragments. Nonetheless,
thousands of feet can be added to the mountain during this stage. Lava fountains
may burst out of rifts on the main dome, and form cinder cones, which dot the
surface of the volcano. These
cinder cones are considered to be the “last gasps” in a mountain’s
eruptions since the majority of building activity is over by this stage.
The most famous volcanoes at this stage are the Kohala Mountains (north
end of the island of Hawaii), Mauna Kea, and Hualalai.
Another example is Haleakala although it did not achieve a dome-like
shape but retains a gigantic crater.
In the fifth stage of island building, the volcanic cone is attacked by erosion.
A sea bluff is formed as the surf eats away at the base of the mountain.
Rainfall is heavier on the highest parts of the forming island, where
streams of water cut deep valleys and leave sharp ridges.
These characteristics are strikingly evident on the islands of Kauai and
Oahu. Shorter volcanic cones,
usually not exceeding four thousand feet, erode into broader valleys with wider,
rounded ridges, such as on the island of Kahoolawe. Portions of all the major Hawaiian Islands could be said to
represent the fifth stage of island formation.
However, some islands have reached the sixth stage, which is said to
occur when the mountains are worn down even further, becoming minor rocks barely
breaking the ocean’s surface. Major
shifts in the earth’s crust could even cause these islands to sink.
If this stage proceeds relatively slowly, it is possible for a coral reef
to build up on the fringes of the island. Most
of the time, the formation of these reefs can keep pace with the sinking of an
island, so that barrier reefs thousands of feet in thickness can accumulate. An example of an island in which a remnant lava pinnacle is
combined with a broad fringing reef is French Frigate Shoal, in the Hawaiian
Leeward chain.
The seventh stage of island formation involves a minor renewal of
volcanism. On portions of an island above the sea, a few small cones or
lava flows may be formed. These
lavas can be identified by their mineral content.
If an eruption occurs underwater, ash is produced. The lateral cones of this stage are small in size.
Portions of West Maui, for instance, are currently in the seventh stage
of island formation. The eighth and final stage of island building occurs when
above-water lava remnants have been completely eroded away or submerged, and
only the coral reef remains. Parts
of Hawaii in this final stage are the oldest islands in the Hawaiian Chain.
The Leeward Islands represent the degradation of a volcanic island with
its atolls.
We are several million years too late to witness the eruption of any
Hawaiian island from beneath the ocean surface, but this phenomena still occurs
in many other parts of the South Pacific. Examples
of recent submarine eruptions in the Pacific are Tonga in 1967 and the Bonin
Islands in 1946. When lavas from a
submarine eruption contact the ocean, they form round balls called pillow lavas,
which are one to five feet in diameter. These pillow lavas result from the rapid cooling of lava in
water. A visitor to the Hawaiian
Islands could view pillow lavas at the base of Wailua Falls, where lava once
spread over the Wailua River.
Above-water volcanic eruptions are a spectacular sight.
On Mauna Loa and Kilauea, events continue with such great frequency that
a major, dramatic display is offered about every five years, and minor eruptions
can be seen almost every year. The
ground is rocked by powerful earthquakes. In
an eruption of Kilauea, in December of 1959, more than twenty-two hundred
earthquakes were recorded. Another
natural wonder which occurs during the eruption of a terrestrial volcano is the
fire fountain, which is a stream of hot lava forced out of a narrow crevice.
A fire fountain, while spectacular, produces only relatively small
volumes of lava because the aperture through which it emerges is small.
During a volcanic eruption, lava will emerge from the mountain in either
a solid (chunky) state or a liquid state. The
Hawaiian word for chunky lava is aa, or a-a, and is the word used universally by
geologists everywhere to describe this type of lava.
Aa consists of basalt, and forms sharp angular blocks and rough
fragments, which contain trapped bubbles of gas.
When active, an aa flow is relatively cool but an aa flow glows a bright
orange and can move along like a river turning to ice.
The remnants of aa flows can be viewed all over the Hawaiian Islands in
places such as the South Kona District on the west coast of the island of
Hawaii. In dry areas, an aa flow
can remain bare for decades, even hundreds of years, although many are readily
covered with vegetation. If an aa flow penetrates into a forested area, the vegetation
will not be set on fire. The lava
will leave several isolated pockets of vegetation known as kipukas.
Numerous kipukas may be seen on the Saddle Road, between Hilo and
Kamuela. If an aa flow reaches the
ocean, it may add onto the coastline. Or,
lava blocks can form natural quays and bays, which ancient Hawaiians used to
form fishponds. Fishponds made of aa blocks can be seen along the south shore
of Molokai, in the north Kona district of Hawaii.
Aa flows are not exclusively different from liquid flows.
Some lava flows can even be composed of both types of lava.
Pahoehoe is the Hawaiian word used to describe lava emitted in the liquid
state, this is also the universal term for this type of lava.
Pahoehoe flow are much fewer and far between than aa flows.
Because of the liquid state of pahoehoe, many curious formations can
result from its flow. On the island of Hawaii, many of the areas penetrated by
pahoehoe are now covered by forests and other vegetation. A peculiar phenomenon to witness when visiting Hawaii is the
“lava tree.” This results when
pahoehoe lava invades a forested area, encasing many trees, and then flows away.
The lava encasement hardens around the tree as it cools and the tree
either rots away or is burned. There
is a state park dedicated to these “lava trees” called the Lava Trees State
Park in the Puna District. Another
interesting result from pahoehoe flows are a natural lava bridge, which results
when the pahoehoe covers soft deposits such as ash beds or volcanic debris
called tuff. The tuff is eroded
away leaving the top layer of the pahoehoe forming a natural bridge. Pahoehoe flows can also form lava tubes.
A pahoehoe flow cools more rapidly at its surface than in its center, so
if pressure builds within the flow, the center may be released leaving the
outside as a hollow cylinder. These
tubes are sometimes so large that they were often used as refuge caverns by the
Hawaiians during wars and battles. The
Thurston Lava Tube in Hawaii Volcanoes National Park in Kilauea is the largest
in the Hawaiian Islands.
Description of Volcanic Flows
from Garrett Hongo's Volcano pages 119-20
Once, out with
scientists from the Hawaiian Volcano Observatory, I stood on the roof of a tube
while lava still flowed through it. I
was beguiled by skylights, holes where the roof had luffed off, fragments that
had torn themselves away from the surface so there were viewports into the tube.
We’d found two-one on a clinkery surface and the other on smooth, ropy pahoehoe.
I studied one, drawing myself closer, feeling the heat wafting up past my
face, my arms, toasting the cotton fibers of my Levi’s.
I stood too close, enraptured, and one of the scientists pulled me back. I was trying to gaze in, looking down on the little red river
I could see sliding by through the portal of the skylight.
‘Don’t stand over it,’ he cautioned.
I moved back, seeing from his immobile face that he wasn’t kidding.
‘It’s about two thousand degrees Fahrenheit,’ he said.
‘It would sear your eyeballs.” He
nodded and I gulped.
I saw a little band of
orange run quickly--the way seawater surges through a narrow, pipelike channel
cut through tidal rocks. The
skylight could have been a tiny blowhole spouting fiery spume, but the viscid
matter instead ran steadily in its channel, sending heat shimmers up from the
gray folds of hardened rock. It was
an endless sauna out there--but its currents encircled me, putting me into a
little cell of convection. So long
as I stood near the beautiful thing, I’d have to steel myself against a mild
agony.
A few feet away was
another skylight, larger and more like a hatch had been cut through the roof of
the living tube. A thick orange
light radiated from it, making visible the outlines of stalactites that hung
down into the moving liquid. I
watched for a while, trying hard to see the visible pulse of the land.
A column of light spewed from the hole.
Downslope from where I stood, ropes of bluish gray lava piled up like
short pliant chains of pearls hiding the sags of flesh on an aging neck.
The scientists shooed
me along, and I turned away, an upraised hand shielding my eyes against the
heat. I realized then I was
standing on the roof of a tube, earth shoving its way beneath me like a baby
sliding along its mortal red canal.
from
Garrett Hongo's Volcano: A Memoir of Hawaii (New York: Vintage, 1995), pages 153-54
It was nearly sunset, and the regular rains of the afternoon had let up,
though there were still a low-lying fog and patches of cloud cover that the sun
did not quite manage to break through. A
slanting light came straight at me from behind the vast field of lava trees.
They looked then like black mesas densely packed and backlit by the
setting sun. There profiles made me think of Monument Valley and the
Painted Desert, except that the scale was much smaller, the space more intimate.
It was such a congenial panorama—a stark profile repeated again and
again all across the lava field still glistening from the last shower of rain in
the late sunlight. When I walked a little farther ahead, the angle of light
changed, or the intensity of light itself shifted.
I was looking at a few trees standing over swirled mounds of lava made
hazy in the misty light. I strolled
and strolled, empty of breath, gaping as I passed under the torn arms of the
first tree I could touch.
I stepped up to it, finding definition in what appeared from far away as an almost
smooth, featureless surface. The
formation was a small tower, a kind of chimney built up around what had once
been an ‘ohi’a tree maybe fifty
years old. Lava had clothed the
living tree in a winding sheet that burned it until it smoked at one end and
spat a sizzle of juices out of each of its limbs and branches. The whole tree might have erupted in flame at one point,
incinerated almost completely in the grip of the hardening stone.
What was left, I could see, was a hollow where the trunk had been, a
column partially filled with a cutaway of what had once been its cross section,
a radiance of blackened phloem like a charcoal anemone inside the small cave of
the absent trunk. I stepped back to measure it, sizing it against my own body,
finding it half again as much as I was, a monster except for its gentleness as
infant rock.
It was a dark, black thing, a torso lifting itself out of mire.
There were niches in it, burbles in the rock, fungal growths pocked up
its trunk. It looked at me and
winked from under its charcoal skin, and the mineral world seemed made into
flesh that day, rock into a softer presence alive with comedic insinuation, like
the gigantic caterpillar in Wonderland. And
yet it was the shadow of all matter—lavas wound in a coil that preserved a
distorted semblance of a tree’s shape, like the dark cast a high sun makes of
the body when one stands between it and the surface of the earth.
A white limb hung from out of the tree’s
hollowed center. It was blobbed
with lava at one end, burned like used kindling at the other, a bone that had
snapped off
and then exploded into burning when the mold was being formed.
I picked it up and swung with it in one hand, turning my wrist like it
was a miter, feeling its weight, a heavy counterbalance at the end of the light
stick. There was a fine ash from
the old burning, a silver slip of color shining within my palm.
A chorus of voices came from down in the vale of
shadows, then a jet plane’s echo sounded from where the black flamingoes had
been at rest. I tested the snapped limb against owl-shaped hornitos, swung it
against rounded driblet spires rising like black mushrooms at my feet and around
the dark lava tree.
What was memory and what sense furled together in
spirals. Before me was a plain
stripped of its comfort of forest. Kilauea,
an innocent shadowland, tilted itself gently skyward into fists of wind, crimson
dreadlocks of nimbus clouds ganjaed over the sponsoring grove of relict trees.
from Garrett Hongo's Volcano: A Memoir of Hawaii (New York: Vintage, 1995), pages 129-30:
At one point, we had to
jump off the live flow, dipping down to a level of older, lichened over ‘a’a
that was friable and broke easily like gypsum as we walked a few steps over it.
We were walking alongside a large ‘ohi’a
that had been downed, burned a bit by the flow, which must have stripped it of
leaves and bark first, then ploughed on through, bringing it down so the log
fell on its side next to the flow. I
looked back, up toward Kjargaard, who was trailing us.
He stood among smoking tree branches, little gnarls of white wood shaped
like nerve bundles riding on top of the flow.
Below him, where the moving rock had ground over the downed log, there
was a red, vagina-shaped hole glowing with the fires of escaping gases from the
spot where the butt end of the tree trunk had burned away and left the mark of
its absence. If the flow had
stopped right then, it might have become a tree mold lying horizontally to the
ground. As it was, the’a’a
was going to grind on, consuming all with its slow collisions of making and
unmaking.
Hawaiian Volcanoes on the Web
http://www.soest.hawaii.edu/GG/HCV/haw_formation.html