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| Mauna Kea from the west, Nov 2015. |
From a geologic perspective, Hawaii is an ephemeral and dynamic anomaly.
The islands’ origins lie deep undersea where a large hotspot feeds a succession
of active volcanoes underneath the center of the Pacific plate. Measured
against geologic history, each island lives a very short life - they burst
forth from the sea floor and then quickly drown under the forces of erosion.
Hawaiian volcanoes are the largest on earth, by height (as
measured from the sea floor) and by total mass. In fact, their mass is so great
that they depress the Earth’s crust in their vicinity. Whereas the typical
depth of the seafloor might be 4.5 km, near Maui and the Big Island
The arc of the Hawaiian Islands
formed as a result of the northwest-ward movement of the Pacific plate,
currently estimated at a rate of 10 cm per year. New volcanoes form over the
hotspot with great rapidity, taking only about 0.3 million years between their
inception and the point at which they break the ocean surface. The Big Island Oahu formed about 3-4 mya, and Kaua’i
and Ni’ihau have the oldest rocks at about 6 million years old.
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| The Hawaiian archipelago. Map from Langenbeim and Clague (1987). |
To the northwest of the main Hawaiian Islands lies a series
of atolls (including Kure
and Midway), which are volcanic islands in their old age, hanging on to the sea
surface by their reef building corals. Farther into the north Pacific, the Hawaiian Island
chain bends northward and becomes the Emperor Seamounts which stretch towards Russia . The
bend in the path is dated to about 43 mya, and the change in course of the
Pacific plate is believed to be linked to the collision of the Indian
subcontinent with Asia at that time.
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| Topography of the Big Island, Hawaii, with elevations in feet. Mauna Kea (north) and Mauna Loa (south) both exceed 13,000 ft elevation. Map modified from USGS, "Ground Water Atlas of the United States. Source. |
Hawaiian volcanoes progress through 4 stages, starting with
the preshield phase when volcanic activity begins. Stage 2 is the shield stage
and is the most active period of volcanic activity when the vast majority of
the volcano’s volume is created. Magma rises from the hotspot located as deep
as 60-70 below the crust where it is stored in shallow reservoirs. During
volcano growth, erupting lava emerges both from the summit of the volcano and
from several lateral rift zones. The post-shield stage is next, and consists of
additional volcanic activity for about 0.1 to 0.3 million years after the
shield stage. Finally, after perhaps a quiescent period, there is the
rejuvenated stage, a period of variable length where some additional volcanic
activity may occur. The Big Island is the only Island
in the shield stage. Landslides and erosion gradually wear down the volcanoes.
There are five volcanoes comprising the Big Island :
Kohala, Mauna Kea, Hualalai, Mauna Loa, and Kilauea .
Only the latter two have erupted recently. Kilauea
is the most active volcano in the world. I hope to see lava flows when we visit
the National Park in a few days. A bit to the southeast of the Big Island Big Island
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| The five volcanoes of the Big Island. Map and key modified from Sherrod et al. (2007). |
References
Clague DA, Dalrymple GB. 1987. The Hawaiian-Emperor Volcanic
Chain. Part I.
Langenbeim VAM, Clague DA. 1987. The Hawaiian-Emperor
Volcanic Chain. Part II. Stratigraphic Framework of Volcanic Rocks of the Hawaiian Islands . USGS Professional Paper 1350.
Sherrod DR, Sinton JM, Watkins SE, Brunt KM. 2007. Geologic
Map of the State of Hawai’i .
USGS Open-File Report 2007-1089.
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