Researcher uncovers surprising cause of the demise of Easter Island indigenous population

Easter Island Credit: Yale

A new paper by a team of researchers including a Virginia Commonwealth University anthropology professor sheds new light on what led to the downfall of the indigenous population of Easter Island — also known as Rapa Nui — prior to European contact in 1722.

The study, "Variation in Rapa Nui (Easter Island) Land Use Indicates Production and Population Peaks Prior to European Contact," published in the Proceedings of the National Academy of Sciences, found that the population's demise likely had more to do with pre-existing environmental conditions than with environmental degradation by the indigenous people.

Christopher Stevenson, Ph.D., an assistant professor of anthropology in the School of World Studies in the College of Humanities and Sciences, recently discussed his team's findings with VCU News.

Your paper explores what led to the collapse of the prehistoric population of Rapa Nui. What did you conclude?

Our paper evaluates a longstanding debate and examines whether the prehistoric population of Rapa Nui experienced a significant islandwide demographic collapse prior to European contact in A.D. 1722. We have used dates from hydrated obsidian artifacts recovered from habitation sites as a proxy for land use over time. The analysis suggests region-specific dynamics that include abandonment of western coast and interior locations. These temporal land-use patterns correlate with rainfall variation and soil quality. The environmental stresses are regionally specific rather than islandwide. Significant erosion around the entire island generated by deforestation has been hypothesized by other researchers as a cause for economic failure. This does not hold up to scrutiny.

How would you characterize the significance of your findings?

This analysis demonstrates that the concept of a sudden "collapse" is a misleading characterization of prehistoric human population dynamics. As a result, we see our approach as useful in the study of other prehistoric societies for which a very rapid demographic collapse has been proposed in prehistory.

Although small, islands can be complex ecosystems that are structured by climatic variation and underlying geology. Human activities are constrained by these parameters and can even cause the ecosystem conditions to change appreciably through their manipulation of the landscape.

Were you surprised by what you found? In what ways?

The results of our research were really quite surprising to our team. Indeed, in the past we’ve published articles about how there was little evidence for pre-European contact societal collapse. When we proposed the [National Science Foundation-funded] project we were expecting that we would gather data supporting the notion that change did not occur until after European contact. In contrast our research suggests that significant changes in land use occurred prior to European contact. The connection between those changes in land use and notions of environmental degradation and subsequent societal collapse are limited.

There is no evidence that massive physical erosion took place on Rapa Nui before European contact and it is unlikely that physical erosion caused productivity decline and societal collapse. While we do not have direct population data, it is clear that people were reacting to regional environmental variation on the island before they were devastated by the introduction of European diseases and other historic processes. In short, our research does not support the suggestion that societal collapse occurred prior to European contact due to physical erosion and productivity decline, but it does indicate that use of less optimal environmental regions changed prior to European contact.

What did your research methodology entail?

We used a method called obsidian hydration dating to date the human creation of volcanic glass tools found at habitation sites. These dates were a proxy for the relative number of people on the island throughout prehistory. We also used an analysis of soil nutrients to look at the fertility of various landforms (e.g., coastal vs. upland) at different elevations up to 500 meters above sea level.

You've written several books on Rapa Nui. What led to your interest in this topic?

This is a continuation of my interest in looking at the development of agricultural systems in small island societies and how that impacts social and economic decisions made in prehistory.

What will you be working on next?

We will be applying some of the same concepts to the Canary Island agricultural systems which have a much higher diversity of geological ages and climatic conditions.

Source: Vcu

Magma pancakes beneath Indonesia's Lake Toba: Subsurface sources of mega-eruptions

Lake Toba, Indonesia

The tremendous amounts of lava that are emitted during super-eruptions accumulate over millions of years prior to the event in the Earth's crust. These reservoirs consist of magma that intrudes into the crust in the form of numerous horizontally oriented sheets resting on top of each other like a pile of pancakes.

A team of geoscientists from Novosibirsk, Paris and Potsdam presents these results in the current issue of Science. The scientists investigate the question on where the tremendous amounts of material that are ejected to from huge calderas during super-eruptions actually originate. Here we are not dealing with large volcanic eruptions of the size of Pinatubo of Mount St. Helens, here we are talking about extreme events: The Toba caldera in the Sumatra subduction zone in Indonesia originated from one of the largest volcanic eruption in recent Earth history, about 74,000 years ago. It emitted the enormous amount of 2,800 cubic kilometers of volcanic material with a dramatic global impact on climate and environment. Hereby, the 80 km long Lake Toba was formed.

Geoscientists were interested in finding out: How can the gigantic amounts of eruptible material required to form such a super volcano accumulate in the Earth's crust. Was this a singular event thousands of years ago or can it happen again?

Researchers from the GFZ German Research Centre for Geosciences successfully installed a seismometer network in the Toba area to investigate these questions and provided the data to all participating scientists via the GEOFON data archive. GFZ scientist, Christoph Sens-Schönfelder, a co-author of the study explains: "With a new seismological method we were able to investigate the internal structure of the magma reservoir beneath the Toba-caldera. We found that the middle crust below the Toba supervolcano is horizontally layered." The answer thus lies in the structure of the magma reservoir. Here, below 7 kilometers the crust consists of many, mostly horizontal, magmatic intrusions still containing molten material.

New seismological technique

It was already suspected that the large volume of magma ejected during the supervolcanic eruption had slowly accumulated over the last few millions of years in the form of consequently emplaced intrusions. This could now be confirmed with the results of field measurements. The GFZ scientists used a novel seismological method for this purpose. Over a six-month period they recorded the ambient seismic noise, the natural vibrations which usually are regarded as disturbing signals. With a statistical approach they analyzed the data and discovered that the velocity of seismic waves beneath Toba depends on the direction in which the waves shear the Earth's crust. Above 7 kilometers depth the deposits of the last eruption formed a zone of low velocities. Below this depth the seismic anisotropy is caused by horizontally layered intrusions that structure the reservoir like a pile of pancakes. This is reflected in the seismic data.

Supervolcanoes

Not only in Indonesia, but also in other parts of the world there are such supervoclcanoes, which erupt only every couple of hundred thousand years but then in gigantic eruptions. Because of their size those volcanoes do not build up mountains but manifest themselves with their huge carter formed during the eruption -- the caldera. Other known supervolcanoes include the area of the Yellow-Stone-Park, volcanoes in the Andes, and the caldera of Lake-Taupo in New Zealand. The present study helps to better understand the processes that lead to such super-eruptions.

Source: Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences

Kilauea, 1790 and today

The Island of Hawai'i, USA.

Scores of people were killed by an explosive eruption of Kīlauea Volcano, Hawai'i, in 1790. Research presented in GSA Bulletin by D.A. Swanson of the Hawaiian Volcano Observatory and colleagues suggests that most of the fatalities were caused by hot, rapidly moving surges of volcanic debris and steam that engulfed the victims. Deposits of such surges occur on the surface on the west summit area and cover an ash bed indented with human footprints.

The footprints, made by warriors and their families, appear along a major trail in use at the time. Today, the area is one of the most visited parts of Hawai'i Volcanoes National Park.
The explosive eruption resulted from the violent interaction of groundwater with hot rocks. Such explosive eruptions have happened frequently in Kīlauea's past and will probably occur in the future when the caldera collapses down to the water table, some 600 m (2000 ft) below the summit of the volcano.

The 1790 eruption of Kīlauea was explosive, and its major impacts were in the summit area of the volcano. The eruption taking place now at Kīlauea is effusive, says Swanson, producing a flow of lava that erupts without explosion. This flow is erupting from a site named Pu'u 'Ō'ō on the east rift zone, far from the summit area, and lava has to flow many kilometers (several miles) before reaching inhabited areas.

Explosive eruptions are very hazardous; the 1790 fatalities bear witness to this fact. Lava flows are not very hazardous to life but can be exceedingly destructive to property. Explosive eruptions are brief but terrifying. Lava flows often last for months or more and are captivating to the viewer. Kīlauea has both types of eruptions, but not at the same time.

Violent explosive eruptions from the summit of Kīlauea are geologically common. They are generally clustered into periods lasting a few centuries. It has been about 200 years since the most recent major explosion, which culminated about 300 years of frequent explosive eruptions. In the past 200 years, Kīlauea has produced many lava flows similar to the present one; small explosions took place in 1924 and, on an even smaller scale, during the past 6 years.

The general public is unaware of Kīlauea's explosive nature, because the volcano has erupted mainly lava flows in recent times. Kīlauea will almost certainly become explosive at some future time, producing conditions similar to those of 1790. However, according to Swanson, there is no reason to think that a period of violent eruptions will resume any time soon. The public can probably expect more lava flows in the near future, such as those of the past three decades from Pu'u 'Ō'ō.

Source: Geological Society of America