I first became aware of Easter Island (Rapa Nui, Isla de Pascua) around 60 years ago via the Thor Heyerdahl Aku Aku: The Secret of Easter Island. This was a fascinating description of a Norwegian archeological expedition to Easter Island in 1955 – 1956 to investigate the island, its massive statues, and the rise and fall of civilization on the island.
As there have been multiple expeditions and books about the expeditions written over the last 70 years, I will only point the reader to those resources rather than go into them in detail. My purpose here is to describe Easter Island as fundamentally a volcanic island, far, far from both South America and Micronesia. The inhabitants originally came from Micronesia.
The island is one of most remote inhabited islands, lying over 3,500 km west of Chile in South America. Pitcarin Island is the closest inhabited island to the west at 2,075 km. The island itself is constructed from three shield volcanoes erupting magma from the Easter hotspot. This hotspot is erupting through the Nazca Plate, creating the Sala y Gomez Ridge, a line of seamounts stretching 2,700 km east to South America.
The island is generally triangular measuring 25 x 12 km with an area of 163 km2. Its population in 2017 was 7,750, half of whom claim to be related to the original Rapa Nui inhabitants.
Climate is classed as a tropical to subtropical rainforest. Temperatures range from 15 – 28 C. Winds in its location tend to keep temperatures relatively cool. Precipitation averages 112 cm/yr with occasional heavy rainfall and rainstorms. The island is not on the track of cyclones.
Original inhabitants arrived from the west somewhere between 800 – 1200 AD, perhaps as early as 350 AD. More recent estimates have generally moved the arrival date closer to 1200 AD. The people themselves are related to those of eastern Polynesia. Over time, the population grew, topping out somewhere around 15,000 a couple hundred years before Europeans discovered the island. They built the giant statues during times of large populations.
Inhabitants likely arrived via canoe from the Gambier and Marquesas Islands, some 2,600 and 3,200 km west. A modern-day voyage with reconstructed canoes in 1999 took 19 days.
Unfortunately, growing a large population stressed the ability of the island to support it, leading to the extinction of the subtropical forests that originally covered the island, many species of plants and indigenous birds. Rats that arrived with the colonists also contributed to this. The colonists also brought their religion, tribes and warfare, creating a toxic stew of too many humans crammed together on an island that was too small to support the numbers, doing to humans what they have always done to one another.
Loss of the trees and multiple other species meant that the inhabitants were no longer able to go fishing via boat. Bird populations also crashed. Over time, the human population crashed from its high of around 15,000 to 2,000 – 3,000 by the time of European discovery in 1722. Most of the population was killed or removed by 1860. Peruvian slave raiders took 1,500 in 1862. When the abducted were repatriated a few years later, they brought smallpox with them. Whalers introduced tuberculosis around 1867. The island was annexed by Chile in 1888. In the 21st Century, inhabitants have become embroiled in fishing disputes mainly with Chinese who are targeting tuna populations used by the residents for food.
Multiple books have been written about the rise and fall of civilization on Easter Island. All are strongly recommended. I have already linked Heyerdahl’s Aku Aku. He also wrote Easter Island: Mystery Solved in 1999. His first extensive book Kon Tiki: Across the Pacific in a Raft in 1950 detailing an expedition that demonstrated how Polynesians could cross large distances in rafts built of locally available materials.
Easter Island is the emergent portion of the NE trending submarine Rano Kau Ridge rising 2,800 m from the sea floor. The ridge is around 50 km long, mostly SW of the island. It is built on three volcanoes.
The largest, Terevaka edifice is central to the island with the satellite Poike and Rano Kau vents on its E and SW slopes, giving the island a triangular shape. The seaward portions of Poike and Rano Kau have been extensively eroded by the sea, creating cliffs. Inland erosion is minimal and volcanic structures are well preserved. There are multiple smaller volcanic features and cinder cones, including multiple lava tubes and caves.
All three volcanoes have similar ages, roughly from 780 – 410 ka for main shield building. Rejuvenated volcanic activity took place 360 – 110 ka. Volcanic activity on the island has largely ceased inactive since the end of rejuvenated volcanism. There was report of steam from the Rano Kau crater wall photographed in the first half of the 20th Century. There are lava flows on the southern flank of Terevaka thought to be as young as 2,000 years old.
Rano Kau has a clear summit caldera, over 100 monogenetic cinder and spatter cones, and a pair of tuff cones, Rano Raraku and Maunga Toa Toa. Most of these radiate from the Terevaka summit aligned along three rifts. The island has an area of 166 km3, with an estimated subaerial volume of 22 km3. Total volume of the entire complex from the seafloor is estimated at 2,200 km3, with Rano Kau at 180 km3, Terevaka at 340 km3, and Poike at 290 km2. This is comparable with other volcanic islands with less than 10% of the total volume above sea level.
Rano Kau tops out 300 m above sea level. The shield is a pile of thin basalt lava flows (one to several tens of meters thick). It is topped with a well-developed circular caldera 1.6 km in diameter, 200 m deep. There was a single lava flow erupted around the same time as the caldera collapse. The collapse and outflow lavas are dated 350 – 340 ka.
The final phase of Rano Kau activity took place 240 – 110 ka. It created several monogenetic vents and rhyolitic intrusions aligned alng a 6.5 km long NE-SW trending fissure bisecting the volcano. Offshore to the SW are multiple cryptodome islets Motu Nui, Motu Iti, Motu Kao Kao. The sheet intrusion of Te Kari Kari is at the base of the SW marine cliff. There is an explosive crater on the northern caldera rim associated with an obsidian rich breccia. To the NE is the 500 x 350 m Te Manavai phreatomagmatic explosive crater, ringed by perlitic obsidian-rich breccia and the Maunga Orito dome.
On the other end of the island, the Poike volcano is 6 km wide. It covers 15 km2 of the eastern end of the island. It was active 780 – 410 ka. The lava flow sequence is exposed by 100 m high sea cliffs. The 367 Puakatiki summit is topped with a 175 m wide circular crater a few meters deep. The shape suggests that the Poike shield also collapsed, creating a summit crater which was subsequently filled and buried by the more recent Puakatiki lava cone active some 360 ka. It was originally a separate island that merged with the main island as lavas from both shields overlapped.
The youngest volcanic activity at Poike created three small trachytic lava domes, Maunga Vai a Heva, Maunga Tea Tea, and Maunga Parehe. These are also aligned along a NE-SW trending fissure. The feeder dike of this fissure is exposed in the northern sea cliff at Hanga Tavaka.
Terevaka is the largest of the three central shields. It is 14 km in diameter, covering some 70 km2, with a volume of 28 km3. It was active 770 – 360 ka. Initial construction of the main shield stopped with collapse of the summit caldera, some 4 x 3 km in diameter. The scarp has inflowing younger lavas and at least one smallish inward-facing avalanche scar. The caldera is so completely filled with newer lavas that it is difficult to determine its original volume.
Renewed intracaldera activity built aligned cinder cones, phreatomagmatic tuffs and breccias, and extrusion of lavas. Lavas from these eruptions filled the caldera. Pahoehoe lavas flowed to the N coast creating lava tubes. The cinder cones formed on the western caldera rim from NNE-SSW fissures. These date 300 – 190 ka. The S and SE flanks of the shield are covered by extensive younger lava flows, cinder and spatter cones. These are often related to and fed by fissures. The products of this activity cover most of the area between the three shields. The younger eruptive fissures date 240 – 110 ka.
The Rano Raraku and Manuga Toa Toa tuff cones date around 210 ka and are somewhat older than the oldest lava flow surrounding the two cones. The crater of Rano Raraku contained a freshwater lake. The most recent lava flows at Hiva Hiva near the west central coast are thought to be less than 2,000 years old.
All the large stone Moai figures are carved from the light and porous tuff of Rano Raraku. The carving was abandoned when more dense fragments were found. These were turned into stone hammers and chisels. The Puna Pau crater contains a very porous pumice used to create the Pukao hats on the figures. The Maunga Orito obsidian was used to make mataa spearheads.
Caldera formation took place at all three shields at the end of the growth phase. The only remaining well exposed caldera is the summit of Rano Kau. Unlike the Hawaiian Islands, massive flank collapse landslides to not appear to have taken place in the past from the flanks of the shields. There are at least a couple smaller landslide scarps pointing in toward the caldera on two of the shields.
Magma supply was prolific during the shield growth stage and did not allow for any evolution of magma. Once the supply stopped, central calderas collapsed, and subsequent magmas erupted during the rejuvenated stage varied in amounts of time-related evolution.
Easter Island is part of the Sala y Gomez Ridge, a 2,700 km ridge punching though the Nazca Plate. It is located 350 km E of the East Pacific Rise, a spreading center. Movement of the Nazca Plate is around 15 cm/yr, with younger seamounts to the west. The current hotspot location is estimated to be W of Easter Island, fueling the construction of the Ahu, Umu, Tupa submarine fields, and the Pukao and Moai seamounts. The underlying plate is only 4 – 2 Ma. This makes the Easter hotspot an example of a near-ridge hotspot.
There is an ongoing argument about the source of the Easter – Salas y Gomez magmas. Most of the research tends to support the notion of an underlying mantle plume / hotspot, in this case located rather close to the East Pacific Rise. The competing theory is a tear in the Nazca Plate is it is formed at the East Pacific Rise moving to the east. There are over 500 seamounts associated with this system. It is prolific and very young.
The main features of Easter Island are similar to those of other intraplate hotspot volcanic systems such as Hawaii, Reunion, Galapagos, Tahiti, Canarias, madeira, Azores and Ascension. The geodynamic similarities are the age of the underlying oceanic crust and plate velocity. Local volcano-tectonic features include mean eruptive rate, over all composition, maturity of rift zones, summit calderas, and subaerial lateral flank collapses.
An important parameter in all of these is the overall eruptive rate. This is where the Easter hotspot is different, with what the authors believe is the lowest rate of any hotspot volcano, a third to seventh of that of Hawaii, which represents the opposite end of the spectrum. The other oddity is that the Easter plume is better defined than the Hawaiian one. This may be related to the proximity of the fast-spreading East Pacific Rise.
The low eruptive rate of Easter Island may be related to the poor development of local rift zones, described as poorly defined or immature. These were large enough to allow magmas fueling rejuvenated volcanic activity post-shield to reach the surface, but not sufficiently well developed to carve large chunks of the island off its flanks in debris avalanches. In contrast, Hawaii has the highest eruptive rate, well developed and mature rift zones, and significant lateral collapses from its islands. Interestingly, time for construction of the main shields is similar for both Easter and the Hawaiian Islands at 400 – 200 ka. The final difference is that the high eruptive rate of Hawaii only produced tholeiitic lavas during all phases of its activity while Easter Island was more sequential, allowing some evolution of magma.
Easter Island is a great example of intraplate hot spot volcanism. While magma supply built Easter Island shields at about the same rate as Hawaiian shields, the magma supply was limited, and the shields did not get nearly as large. The limited magma supply also shows up with failed rift volcanism during the rejuvenated volcanism stage. There was simply not enough total magma volume to create massive rifting structures with multiple flank collapses like we see on the Hawaiian Islands. Volcanic activity on Easter Island appears to have mostly stopped over the time of human habitation.