Everything is in Motion – An Island in the Middle of the Ocean

by Martin Thiel, Bernhard Steinberger & Tim Kiessling

Did you know that the geological history of Rapa Nui depends on two types of volcanism? There is on the one hand the Mid-Ocean Ridge, which crosses the South Pacific in more or less North-South direction. The Mid-Ocean Ridge volcanism continuously forms the oceanic plates, which in the South Pacific are growing on average by 10 centimeters per year to both sides of the ridge (Figure 1). There are two plates formed, the Pacific Plate to the west, and the Nazca Plate to the East. As the plates grow, they slowly move away from the Mid-Ocean Ridge; the Nazca Plate moves at 5-10 centimeters per year towards the east. At the current distances it takes the newly formed parts of the Nazca Plate about 50 Million until they reach the South American continent, where the plate slowly slides underneath the South American continental plate, producing the intense earth quakes observed along the Pacific coast of the continent.

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The other type of volcanism that was fundamental for the creation of Rapa Nui is the hotspot volcanism, which is quite different from the shallow Mid-Ocean Ridge volcanism. At the hotspot, there is a huge magma cell deep down in the mantle of the earth, which rarely changes its position. When enough magma has accumulated at the hotspot, this melted rock soup (magma) breaks through the plate and creates a lava mountain on top of the plate. When sufficient lava has piled up, this mountain even rises above the sea surface. This is the case with Rapa Nui and Motu Motiro Hiva (Figure 2). The hotspot that has created these islands is called the “Easter Hotspot”.

While it was previously thought that Rapa Nui was several million years old, a recent study by Vezzolli & Acocella (2009) showed that extensive volcanism at the Easter Hotspot started some 800,000 years ago, giving birth to the three volcanoes that formed Rapa Nui (Poike, Rano Kau, and Terevaka). Thus, Rapa Nui is geologically very young, and it is assumed that Motu Motiro Hiva is a little bit older than Rapa Nui. To the east of Motu Motiro Hiva are many seamounts that were deposited by the Easter Hotspot on top of the Nazca Plate (Figure 2). At the moment of their formation, some of these seamounts where islands that reached above the sea surface, but as the Nazca Plate moved further eastwards, away from the Easter Hotspot, it cooled and the mountains were slowly sinking down. When they sank below the sea surface, these mountains became “seamounts”, forming the Easter Seamount Chain, which stretches from Rapa Nui almost 2000 km to the east. The oldest seamounts are the ones farthest to the east, and the youngest ones are the ones right above the Easter Hotspot, namely Rapa Nui and Motu Motiro Hiva (Figure 3). As the oceanic plate was formed before the islands and seamounts, these are typically younger than the surrounding plate (compare the age data in Figure 3).

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Currently the Easter Hotspot is located about 500 kilometers east of the Mid Ocean Ridge. As volcanic activity only occurs when sufficient magma has accumulated below the oceanic crust, the hotspot only erupts every million years or so to create seamounts or islands. This has not always been like this. Between ~50 and 30 Ma, the hotspot was located so close to the Mid-Ocean Ridge, such that all or most of its related volcanism occurred directly and continuously at that ridge. The combined formation of new plate together with hotspot volcanism created the Nazca ridge, which is a continuous ridge instead of individual seamounts.

However, both the Pacific and Nazca plate also had a northward component of motion, and because of its geometry (sketched in Figure 4) the Mid-Ocean Ridge moved away from the hotspot, and subsequently its distance to the hotspot increased. Now volcanism occurred, on one hand, directly at the hotspot, and hot material flowed towards the Mid-Ocean Ridge causing volcanism between hotspot and ridge, and increased volcanism at the ridge. At the time when this happened, between 30 and 20 Million years ago, this led to rather widespread volcanism. One can recognize several parallel chains of seamounts, and lines of volcanic features at an oblique angle to the Easter Seamount Chain (Figure 3) that have probably formed above flow channels from the hotspot to the Mid-Ocean Ridge (yellow line in Figure 4).

Once the hotspot and the Mid-Ocean Ridge had moved sufficiently apart, the continuous growth of the Nazca Plate at the Mid-Ocean Ridge and the hotspot volcanism were mostly separated. Now, hotspot volcanism became episodic and only when sufficient magma had accumulated did the hotspot erupt and break through the plate. Over the past 15 Million years, this process has led to the formation of the Easter Seamount Chain, of which Rapa Nui and Motu Motiru Hiva are the youngest parts. Thus, while the geologic history of Rapa Nui itself is comparatively young (with an age of 0.8 Million years it can be considered a geological baby), that of the surrounding areas is old and has many interesting tales to tell.
Vezzoli, L., & Acocella, V. (2009). Easter Island, SE Pacific: An end-member type of hotspot volcanism. Geological Society of America Bulletin 121: 869-886.

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