Hawaii is a Paradise for Scientists As Well
A "Hot Spot" in more ways than one
As winter drags along, images of tropical islands might come to mind unbidden. A snowbird's dreams of the sun, sand and warm tropical breezes of Hawaii can make these islands a favored escape, real or imagined. Scientists have been drawn here for many years as well, not for the beaches, but because of the islands' fiery volcanic beginnings, extreme topography, extreme isolation, and long and readable history of geologic, ecologic, and human change.
And it all stems from a relatively rare phenomenon - a volcanic "hotspot."
That hot spot was in the headlines recently when the same process that provided the ideal location for monitoring a crucial indicator of global change also threatened the continuity of those measurements.
In the 1950s, Charles Keeling was searching for a location to monitor the concentration of carbon dioxide in the atmosphere. Among other sites, he focused on the top of Mauna Loa on the big island of Hawaii, the most massive shield volcano in the world.
He recognized that locating a laboratory on the summit of this isolated mountain, more than 13,600 feet above the sea and thousands of miles from any continent, would allow sampling the well-mixed upper atmosphere of the Northern Hemisphere unaffected by local sources. That choice, decades of rigorous sampling, and the determined efforts of colleagues and family members to maintain his legacy, have led to the compilation of the "Keeling Curve" - the environmental icon of our time - a single set of continuous measurements summarizing the human-induced changes in the chemistry of the atmosphere that are driving climate change.
This volcanic mountain and the measurements made there were in the news because Mauna Loa, which had not erupted in 40 years, was acting up. Fresh lava flows had cut off the power supply and shut down the lab.
Acting quickly, the staff from the Mauna Loa site switched collections to a University of Hawaii observatory location on the adjacent volcano of Mauna Kea, so the continuity of this crucial data set could be maintained. Even if a few missed measurements occurred, the record itself is so consistent year to year and across the seasons that the significance of the data would not be compromised. Consistency in the pattern of change in carbon dioxide concentration is one of the most powerful messages from these measurements.
While the Keeling Curve is answering a fundamental global change question, the Hawaiian Islands also provide an invaluable setting for many other fields of study. Let's start with a basic question that frames the rest: what is the world's largest shield volcano doing in the middle of the Pacific Ocean? And if volcanoes are building the big island, how did the other, smaller islands get there?
The majority of active volcanoes in the world occur along boundaries between major tectonic plates. This is famously captured in the "Ring of Fire" where the Pacific Ocean and Plate are being "squeezed" by the conflicting movements of the North American, Eurasian and Indo-Australian plates.
But the Hawaiian volcanoes are in the middle of the Pacific Plate - far from any tectonic boundary. How does that happen?
At a few places in the world, molten lava, effectively melted rocks from the Earth's crust or mantle, erupts directly to the surface. How, why, and where these occur seems to be a bit of a mystery. The consensus is that there are 40 to 50 hot spots scattered around the world, Hawaii being one of the most famous. A characteristic of oceanic hot spot volcanoes is that they are generally not explosive, as plate-margin volcanoes can be, but are formed by consecutive flows of lava that build on a growing base, yielding a gently sloping or "shield" volcano.
Not all hot spot volcanoes reach the surface, and those that do will tend to sink (or subside) because of their massive weight and will eventually erode back to sea level. Charles Darwin may have been the first to recognize that some oceanic atolls, lagoons surrounded by coral reefs, may result from the erosion of volcanoes back to sea level. Those that never build high enough to reach the surface may remain as "seamounts," essentially underwater mountains. The highest mountain on Earth is actually a hot spot seamount that more than broke the surface — Hawaii’s Mauna Kea, measured at more than 30,000 feet tall from the seafloor 18,000 feet beneath the surface. Estimates of the number of seamounts range anywhere from 14,500 to over 100,000.
I have always wondered about the number and wide distribution of small islands in the Pacific Ocean. Apparently, most are the result of oceanic hot spots.
That leads to some other questions. Will Mauna Loa erupt and grow forever? Why or why not? And are the other islands near the big island of Hawaii part of this story as well?
There is much more to the full Hawaiian archipelago than we can see above sea level. The visible islands occupy less than one quarter of a long chain of islands, atolls, and seamounts that appear to be marching off to the northwest, directly toward the Aleutian Trench, an active subduction zone where the Pacific Plate is sliding under the North American Plate near the boundary with the Eurasian Plate.
Putting these facts together, the full story involves the formation of new, large, volcanic islands over a stable hot spot, with the newly formed islands being carried to the northwest by the migrating Pacific Plate. The islands subside and erode as they age, becoming atolls and then eventually seamounts as they approach the Aleutian Trench. There, fittingly completing the cycle, they are buried and reduced again to molten lava. The entire passage from hot spot to trench takes perhaps 60 to 80 million years.
This line of volcanic remnants records the direction that the tectonic plate has been moving relative to the hot spot over geologic time. The bend in the Hawaiian Island/Emperor Seamount chain seen in the figure is thought to have occurred about 35 million years ago.
And your next question could be: If this is how it works, shouldn't there be a new island forming to the southeast of the big island? Yes, and there is. The recently renamed Kamaʻehuakanaloa (Hawaiian for "glowing child of Kanaloa," the god of the ocean) began forming around 400,000 years ago. It currently reaches 10,100 feet above the sea floor but still 3,100 feet below the surface. It is growing at about one foot every ten years.
In addition to volcanic geology, this same pattern of formation and movement has provided the setting for a unique study of the development of tropical soils and ecosystems over time.
My source for what follows is a wonderful book by Peter Vitousek, who has spent decades studying the integrated workings of vulcanology, plate tectonics, rock weathering and soil formation across the Islands, and their interaction with species diversity, plant growth and the cycling of nutrients required to sustain that growth.
The book is a wonderfully coherent and accessible telling of these interactions, but I can only focus on two parts of it here. The first is why the Islands are a near-perfect "model ecosystem" for studying the changes in ecosystem function over geologic time (up to 4 million years). The second is a summary of the story of those changes.
A perfect model ecosystem is a site or set of sites that are similar in all respects except one. While rarely found in nature, the Hawaiian Islands come very close. They were all formed in the same way from the same basic volcanic materials. They all experience the same climate. The one big difference among the islands is age. A gradient of sites used by Vitousek captures changes that happen across more than 4 million years.
But aren't tropical forests so incredibly diverse that the plants across these sites must be very different, reducing the value of these model systems? Not in this case. The Islands are thousands of miles from any large land mass. The chance for a new species to invade is very small. The first species that do arrive on isolated islands tend to evolve very rapidly to fill all available sites and conditions (niches). So the Hawaiian Islands were, before human arrival, populated mostly by species that evolved on site. Unlike the Amazon basin or other tropical forests, the number of species was relatively small, and almost all were "endemic" meaning they occur nowhere else.
With age since formation as the major variable across his study sites, Vitousek could test some classical hypotheses about how ecosystems function, especially in terms of the interactions between nitrogen and phosphorous (the first two elements in your NPK fertilizer) in limiting plant growth and ecosystem function.
Fresh lava is rich in nutrients, including phosphorous, but those nutrients are locked up in the new rocks, and are not available to plants. Even unpolluted rainfall is slightly acidic, enough to begin to "weather" those rocks, chemically dissolving them to release those embedded nutrients and make them available to plants. Once plants are established, they can accelerate the weathering process.
The one critical element missing in those rocks is nitrogen, essential for the construction of proteins and DNA. Nitrogen can be "fixed" from the atmosphere by certain microbe/plant combinations, but that is an energetically expensive and relatively slow process.
So the soils on the youngest site sampled by Vitousek, at 300 years since formation as lava, have not had sufficient time to accumulate much nitrogen or create organic matter, and so plant growth is severely limited.
In the middle of the age sequence, from 150,000 to 1.4 million years, biological activity has increased nitrogen availability and rocks are still young enough to release phosphorous by weathering, so that plant growth is highest. By 4.1 million years, few weatherable materials remain in the rocks, and the phosphorous in soils has been mostly transformed into an unavailable form. In these older soils, which are actually typical of most tropical forest ecosystems, low availability of phosphorous limits tree growth as well as the cycling of other elements.
The detailed results reported by Vitousek carefully track the change in limiting factors over time and the physiological responses by the dominant species in the face of those limitations. These are classic questions in ecosystems ecology that could be addressed across the unique time gradient in the Hawaiian study sites.
The Hawaiian Islands also tell a human story of human change with parallels to the environmental one.
When James Cook and the crew of HMS Endeavor "discovered" the Islands in 1778, they found a sophisticated Polynesian culture in place. The post-contact history of the Islands is a sad tale of both human suffering and species extinctions that is beyond the scope of this essay, but one question has long intrigued me: How did those fearless Polynesian explorers find islands as remote as Hawaii? It is a tiny speck in the middle of the vast Pacific, more than 2,300 miles from any previously settled islands.
As I was writing this essay I was fortunate enough to reconnect with Peter Vitousek. He and colleagues have just released a new book (citation below) that captures the incredible navigational abilities of Polynesians, the rapid spread of their culture across the vast Pacific, and how their adaptations to the variety of environments encountered might reflect on our own efforts at sustainability. I recommend it to you!
What follows here is again a too-simple summary of all this book contains.
The Hawaiian Islands were settled near the end of the "great age of discovery" between about 3,000 and 800 years ago during which Polynesians discovered and populated every major island in the Pacific from New Zealand to Easter Island to Hawaii. Hawaii, as the most northern island settled, was only discovered 800 to 1,000 years ago.
These essays often focus on technologies, and here are two that made this rapid expansion possible.
First, the Polynesians invented and refined the amazingly seaworthy double-hulled longboats capable of carrying people, plants, and animals in numbers sufficient to establish a new colony on any previously unoccupied island encountered - and all new islands would be unoccupied as the Polynesians were the first humans to reach these isolated outposts. The ability to feed and shelter the cultural microcosm represented by these ships for extended periods of time at sea demonstrates a high level of organization and planning.
Second, they developed incredible navigational skills that involved a deep understanding of winds, waves, and currents and a sophisticated and nearly lost sacred ability to navigate by the stars. Combining these two allowed wayfaring Polynesians to sail or row between islands and for some time allowed communication among them. Finding such small specks of land in the immense Pacific almost defies the imagination.
The parallel with the cycle of island formation and loss is with the evolution of Polynesian culture. The age of discovery ended when the full universe of Pacific Islands had been explored. Contact between Hawaii and others in Polynesian Pacific "ceased several centuries before European contact." Succession and change at a human scale as well as an environmental one, albeit with a much shorter timeline.
So these tropical Islands are much more than an appealing winter retreat. They have helped us understand vulcanism, plate tectonics, seamounts, atolls, soil development, plant physiology, ecology and the amazing seafaring abilities developed by the Polynesian culture. Dynamic change has been the norm for both the environmental setting here and the human story played out in that setting.
Acknowledgement: Thanks to Peter Vitousek for reviewing this essay and providing the connection to the new book on Islands and Cultures.
Sources
Background information on Mauna Loa can be found here:
https://www.usgs.gov/volcanoes/mauna-loa
https://en.wikipedia.org/wiki/Mauna_Loa
The map of the big Island is from:
https://www.usgs.gov/media/images/map-island-hawaii
Stories about the eruption of Mauna Loa can be found here:
https://www.nytimes.com/2023/01/06/us/kilauea-volcano-eruption-hawaii.html
https://www.washingtonpost.com/climate-environment/2022/11/29/mauna-loa-volcano-co2-keeling/
And about measurements being switched to Mauna Kea here:
https://keelingcurve.ucsd.edu/2023/01/10/update-on-co2-readings-commencing-at-maunakea/
The ring of fire map is from:
https://upload.wikimedia.org/wikipedia/commons/7/77/EQs_1900-2013_worldseis.png
Arrows and labels added by the author
Information on hot spots can be found here:
https://en.wikipedia.org/wiki/Hawaii_hotspot
https://education.nationalgeographic.org/resource/hot-spot-volcanism
The side view of the Hawaiian hot spot is from
https://en.wikipedia.org/wiki/Hotspot_(geology)
through the US Geological Survey
A good summary of the full cycle of formation and erosion of Hawaiian volcanoes is:
https://www.usgs.gov/observatories/hvo/evolution-hawaiian-volcanoes
A revisionist view of the formation of atolls is here:
Differing views on the number of seamounts include:
https://divediscover.whoi.edu/archives/hottopics/seamounts.html
100,000+ more than 1,000 feet hight
https://en.wikipedia.org/wiki/Seamount
14,500
The emperor seamount map and information on age of oldest seamounts is from:
http://www.geo.cornell.edu/hawaii/220/PRI/PRI_PT_hotspot.html
https://en.wikipedia.org/wiki/Hawaiian%E2%80%93Emperor_seamount_chain
Information on the new Hawaiian Island being formed can be found here:
https://geology.com/usgs/loihi-seamount/
https://en.wikipedia.org/wiki/Kama%CA%BBehuakanaloa_Seamount
The full reference for the first Vitousek book is:
Vitousek, P. 2004. Nutrient Cycling and Limitation: Hawai'i as a Model System. Princeton. University Press, Princeton, NJ. 223pp.
The new book on the exploration and settlement of Pacific Islands by Polynesians is:
Beamer, K., T.M. Tau, and P.M. Vitousek. 2022. Islands and Cultures: How Pacific Islands Provide Paths toward Sustainability. Yale University Press. New Haven, CT. 248pp.
Quotations are from this book.
Additional references on the navigational expertise of traditional Polynesians include:
https://www.pbs.org/wayfinders/polynesian2.html
http://www2.hawaii.edu/~dennisk/voyaging_chiefs/discovery.html
The image of the replica of a Polynesian longboat is from:
https://commons.wikimedia.org/wiki/File:Hokule%27aSailing2009.jpg
