GEOLOGY
The Hawaiian Islands belong to a linear chain of volcanoes that stretches for approximately 3,800 miles
(6,100 km) from the central to the northern Pacific, almost to the Aleutian Islands. About halfway up its
length, the chain bends, dividing the older Emperor Seamounts (guyots)—now sunken to great depths—
from the younger Hawaiian Ridge. From northwest to southeast, each volcano is progressively younger.
The oldest dated seamount, near the northern end of the chain, was formed about 80 million years ago,
while dinosaurs still roamed Earth.
What forces formed these islands, the farthest archipelago from any continent, over such a long span of
time? Why are they lined up across the Pacific? Earth scientists believe that the Hawaiian Islands owe their
existence to two geological phenomena: a "hotspot" and plate tectonics. Beneath the active volcanoes of
Hawai'i Island, a plume of hot rock anchored hundreds of miles deep in Earth upwells to form a hotspot
beneath the Pacific Plate, one of several dozen tectonic plates that comprise the planet's crust. Most of
the world's active volcanoes are located at the edges of these tectonic plates, as exemplified by the
Pacific "ring of fire." But the Hawaiian hotspot, like many of the other hundred or so hotspots around the
globe, is positioned within the plate, where it has apparently remained fixed over the past 40 million
years. As the hotspot has continuously fed magma through the crust to fuel countless volcanic eruptions,
the Pacific Plate has drifted west-northwestward about 3.5 inches (9 cm) a year. Like a conveyor belt, the
plate has rafted each new volcano (about 129 in all, including the 15 volcanoes making up the major
islands of Hawai'i) away from its place of origin, across the Pacific Ocean. Thus the age and orientation of
the volcanic chain records the direction and rate of the plate's movement. The pronounced bend between
the Hawaiian Ridge and the Emperor Seamounts marks a dramatic change in the plate's direction,
probably caused by the collision between the Eurasian and Australian-Indian Plates that ultimately
created the Himalaya Mountains.
The Evolution of a Hawaiian Volcano
Each volcano in the chain has evolved, or is evolving, through essentially the same sequence of life
stages. When a new volcano forms, the rates of lava eruption gradually increase over several hundred
thousand years, attain their peak for perhaps 500,000 years, and then decline rapidly. As eruption rates
change, so does the composition of the lavas. Also, the position of the volcano's summit relative to sea
level determines the style of eruptive activity. Several million years may pass before activity at a volcano
ends and the volcano becomes extinct.
A volcano's life starts deep underwater, with occasional eruptions of relatively small volumes of alkalic
basalt lavas, rich in sodium and potassium, beginning at the seafloor. What is known about
this submarine preshield stage comes from recent research on Lō'ihi Seamount, the only volcano whose
preshield lavas have not been buried beneath later lavas. The Hawaiian chain's youngest volcano, Lō'ihi
lies about 3,200 feet (975 m) below sea level off the southeast coast of Hawai'i Island. Samples of lava
from the young volcano are the "pillow" lavas typical of deep underwater eruptions. Submarine eruptions
are building a steep-sided volcanic edifice with a shallow caldera, or basin-shaped depression, at the
summit. Two rift zones radiate from Lō'ihi's summit and will remain prominent features of the volcano
during all but the final eruptive stage. As a young volcano grows, small landslides cut into the steep
slopes, scarring its flanks. The submarine preshield stage lasts for perhaps 200,000 years yet produces
only a fraction of the volcano's final mass.
An increase in the frequency and volume of eruptions and a change in lava composition to tholeiitic
basalt with higher concentrations of silica and lower concentrations of sodium and potassium signal the
transition to the next stage of formation. During this transition, which Lō'ihi seems to be currently
under-going, tholeiitic and alkalic lavas erupt alternately over long periods, forming interbedded layers.
At the end of the transition, the volcano enters the submarine phase of the shield stage. During this
phase continued eruptions of pillow basalt build up the volcanic edifice almost to sea level. When the
volcano's summit has grown to the point that it is only shallowly submerged, magma and seawater mix
to produce explosive eruptions, and the explosive phase of the shield stage begins. These ashgenerating eruptions persist until the volcano has grown to perhaps 4,000 feet (1,200 m) above sea level,
occurring intermittently for up to several hundred thousand years.
When the volcano attains a sufficient height above sea level, explosive eruptions cease, signaling the
onset of the subaerial phase of the shield stage. Subaerial (above sea level) eruptions form flows of either
ropy pāhoehoe or blocky 'a'ā. Pāhoehoe lavas are generally produced by low-volume, sustained
eruptions, while 'a'ā flows are produced by high-volume eruptions that are usually limited in duration.
Other factors that partly determine the type of flow are the slope of the ground and the physical
properties of the erupted lava, such as its temperature, composition, amount of suspended crystals, and
quantity of bubbles. Lava that flows into the ocean shatters into sand- and gravel-sized fragments that
blanket the submarine slopes.
During all three phases of the shield stage the summit caldera repeatedly collapses, fills up with
thick, ponded flows, and then collapses again. The frequency and rates of eruption peak during the
shield stage. Over a period of roughly 500,000 years, the volcano accumulates about 95 percent of
its mass, taking on the "shield" shape (named for its resemblance to a warrior's shield) characteristic
of Hawaiian volcanoes. But the volcano also loses mass as weathering and erosion relentlessly wear it
down. Its seaward, unsupported flanks are unstable and slip toward the ocean, creating large faults
and causing major earthquakes and, infrequently, catastrophic landslides.
Such landslides can be enormous, as evidenced by the vast fields of blocky debris recently discovered
and mapped on the seafloor around the major Hawaiian islands. These submarine deposits indicate
the past occurrences of at least 17 major landslides and suggest that such events happen roughly
every 100,000–200,000 years. In some cases, "slumps"—slow-moving
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Geology
landslides—can be distinguished from "debris avalanches," sudden, catastrophic landslides.
Not all Hawaiian volcanoes undergo the post-shield stage, during which a cap of alkalic basalt
overlays the shield. High fountains spew out cinders, and the volcano's slopes become steepened as
short, pasty 'a'ā flows pile up near vents around the summit and along the previously active rift
zones. Spatter cones, cinder cones and, sometimes, pumice cones are typical vents. Over perhaps
250,000 years the eruptions taper off and then cease. The final caldera of a volcano that undergoes
the postshield stage is inundated by alkalic lavas and eventually buried beneath the cap.
Following the postshield stage the forces of weathering and erosion gain the upper hand on the
volcano. During this erosional stage deep canyons incise the flanks, often along faults previously
created by landslides. Pressed down by the volcano's enormous weight, the underlying lithosphere
(Earth's outer layer) flexes downward, and the volcano and the island to which it belongs subside or
sink. As this occurs, fringing coral reefs grow at the shoreline, and sediments from the reef
accumulate in lagoons. In some-places around Hawai'i, ancient reefs are preserved offshore, in series
like stairsteps. Each reef originally grew at an old shoreline during an era of global climate cooling,
when the polar ice caps enlarged, pulling sea level down. The dual process of island subsidence and
falling sea level kept the shoreline relatively stable, allowing the reef to grow. When global
temperatures increased, the polar ice caps shrank and sea level rose again, drowning the reef, which
could not grow fast enough to keep pace with the rising sea and the still-sinking island. Thus, on the
slopes of rapidly subsiding young volcanoes, the deepest reef is the oldest, and each shallower reef is
progressively younger.
After this period of weathering and erosion, a volcano may undergo the rejuvenated stage, a final
eruptive stage during which strongly alkalic lavas are discharged—often through previously formed
reefs—creating ash cones or rings. Lava flows are funneled down stream valleys carved out during
the previous erosional stage. Lavas characteristic of this stage exist not only on several of the major
Hawaiian islands, but also on their underwater flanks and the surrounding seafloor. Eruptions during
this stage are infrequent and volcanic output minimal, but activity may occur over several million
years.
The long duration of the rejuvenated stage on Kaua'i and Ni'ihau suggests that lavas of this stage
could erupt again on West Maui, East Moloka'i, and the Ko'olau Range on O'ahu, or could begin to
erupt on West Moloka'i, Lāna'i, Kaho'olawe, or even Kohala on Hawai'i Island.
As the rejuvenated stage wanes, erosion and subsidence predominate once again, eventually
reducing an extinct volcano, or volcanic island, to sea level. With continued subsidence, a volcanic
island is transformed to an atoll: a ring of coral surrounding a lagoon, such as those that lie west of
La Pérouse Pinnacle, the westernmost and oldest subaerial volcanic remnant of a former Hawaiian
island. If the water becomes too cold for active growth because of northward drift of an extinct
volcano, or if sea level fluctuates, as discussed above, the reefs that make up the atoll die. A volcano
continues to subside after its reef dies. Once below sea level, a flat-topped, coral-capped volcano is
called a guyot. 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.