The four islands between Oahu and the Big Island of Hawaii are the remnants of a large volcanic edifice, referred to as Maui Nui, literally Big Maui. At its height some 1.2 MY ago, it was about 50% larger above sea level than the current Big Island of Hawaii. Following that peak the normal cycle of landslide, erosion and subsidence divided Maui Nui into its current four separate islands of Molokai, Lanai, Maui and Kahoolawe. This piece will give a short geologic history of those islands.
Volcanic activity on all the Hawaiian Islands has four stages of activity classified like that on Kauai. These include preshield (sub aerial) which is primarily alkali lavas; shield (aerial); postshield (alkali); and rejuvenated (alkali). For all these islands, the preshield and shield building lavas comprise the vast majority of erupted materials. Preshield builds the island from the ocean floor. Shield building builds the island above the ocean surface. Postshield takes place at the end of the shield building stage and normally fills the caldera. Rejuvenated takes place some hundred(s) of thousands of years after the main eruption stops and is characterized by cinder cones, phreatomagmatic and similar eruptions. Rejuvenated eruptions emit by far the smallest amount of lavas by volume in construction of a Hawaiian Island.
It is also important to note that all the Hawaiian Island sit in a pretty strong band of trade winds blowing generally east to west. This means that the upwind side of the islands is very wet, generally covered with tropical rain forests. Rainfall increases as the water-laden air mass is forced higher up against the volcano. On the lee side, the air mass is drier, and drops successively less rain as you get farther away from the central volcano. On most islands, the windward side has sufficient rainfall to support tropical rain forests, while on the lee side, it supports vegetation more appropriate to the deserts of West Texas. Cactus is not uncommon on some of the islands.
This distribution of rainfall is important in the evolution of life on Maui Nui and explains a lot about what is seen on the islands today.
Initial growth of the island appears to be a single shield volcano underlying Penguin Bank, which sits between the island of Molokai and Oahu. Molokai sits some 42 km east of Oahu. The shield volcano underlying the Bank started building some 2.2 MY ago and was briefly connected to Oahu as the next expression of the hot spot that created the string of Hawaiian Islands. Today, the top of the volcano sits some 60 m below the surface of the Pacific and is topped with coral.
The next island to start forming was Molokai, which is the northern most of the remaining group. The western most shield volcano started forming West Molokai some 1.89 MY ago. The western volcano started some 1.52 MY ago. Normal shield growth only takes some 200,000 years for each volcano given estimates of island growth for Kauai and Hawaii.
Yet another set of vents erupting out of three rift zones formed the island of Lanai south of Molokai with major activity between 1.46 – 1.2 MY ago. Lanai was connected to Molokai by the shared volcanic edifices. Lanai is a single shield volcano.
Simultaneous with eruptions forming east Molokai and Lanai, the volcano forming West Maui was active some 1.6 MY ago. It is now considered extinct. The East Maui volcano – Haleakala – became active some 800,000 years ago and had its last eruption of rejuvenated magma some 200 – 500 years ago.
The final active volcanic center, and smallest island is Kahoolawe which is a single shield volcano active some 1.03 MY ago. It now sits some 10 km south of Maui.
The Pacific Plate continued to move and with it, the activity continued to move southeast relative the existing islands. The oldest volcano making up the current island of Hawaii is Kohala, which was active some 1.00 MY ago, breaching the surface some 500,000 years ago and erupting for the typical 200,000 years. It sits some 50 km southeast of Maui.
At its largest some 1.20 MY ago, Maui Nui was some 14,600 km2 above water.
Now that the growth of the larger island of Maui Nui is moderately nailed down in terms of shield building, next comes its destruction via a series of landslides, subsidence and erosion.
The connection between Molokai and Penguin Bank was broken some 700,000 years ago. The connection between Lanai and Molokai opened up some 600,000 years ago. The connection between Kahoolawe and Maui opened up some 150 – 200,000 years ago. Maui, Lanai and Molokai have been intermittently connected since then, mostly due to variable sea levels due to glacial episodes during ice ages. One article estimates the islands were connected as recently as 18,000 years ago due to the lower sea levels during the last great ice age.
Note that the reason the islands are no longer connected is because of the subsidence of the entire structure at the rate greater than 3 mm / year. There is a drowned reef line around Molokai some 400 m below the surface that rises around 30 m from that depth.
There are three main landslides / slumps associated with the greater structure. The difference between a landslide and a slump is velocity. Landslides move very quickly, often creating tsunamis, typically spreading debris tens to hundreds of kilometers on the ocean floor. Slumps are much slower, do not necessarily create tsunamis, and do not necessarily deposit material on the ocean floor or nearly as far along the floor should the debris reach that far.
The first and largest slide was the Wailua Slide, which appears to have taken the northern half of the East Molokai shield volcano down to the bottom of the Pacific with it. It covers some 13,000 km2, is just under 195 km long and 40 km wide. The failure took place when the volcano stood some 1,300 m higher than today.
Around 2,500 km3 of Molokai fell off toward the north. Its source is a 40 km wide amphitheater that took most of the northern half of the East Molokai volcano. The landslide was relatively high speed as debris climbed out of the Hawaii Deep, the depression the mass of the Hawaiian Islands created in the Pacific Plate as magma is transferred from the mantle to the surface. During its runout, the debris flow climbed some 300 m uphill before it stopped.
The landslide appears to have taken place around 1.4 MY ago and generated a tsunami between 600 – 700 m locally on Molokai and Lanai. This height is verified by the presence of corals high in the mountains of the island.
Note also that due to subsidence, the scarp face the landslide released from is now well below the surface of the Pacific. http://www2.hawaii.edu/~nasir/083Molokai.ppt
A second slide is the Hana Slump, which appears to take place from an active underwater rift zone called the Hana Ridge. This ridge reaches east from Maui and crosses the northern end of the Big Island. At its end, there was a debris slump covering some 4,900 km2, measuring some 85 km long by 110 km wide. Given the proximity of multiple debris slumps north from the Big Island and the Kohala Volcano, the area is fairly complex. Age of this slump should be on the order of 500,000 years. I have not found any associated massive tsunamis with this feature. http://www.agu.org/books/gm/v128/GM128p0011/GM128p0011.pdf
The final massive landslide if the Clark Slide out of Southwest Lanai. This feature is bound mostly by the Penguin Bank to the north and proceeds mainly south and west in two lobes. It is tentatively set at around 650,000 years old based on a successive series of coral reefs around Lanai, the lowest some 1,000 m below the surface of the Pacific. The head of the avalanche has the typical amphitheater shaped source where part of the island calved off downslope to a bottom some 3.5 km below the waves. Original dating of tsunami debris attributed to the slide had to be reevaluated as the debris was dated at some 105,000 years ago which has now been tied to the Alika debris avalanche off the west side of the Big Island. http://walrus.wr.usgs.gov/reports/reprints/Moore_JGR94.pdf
Molokai is roughly rectangular in shape, with the two volcanoes on the ends with a saddle in the middle. It is oriented east – west. The island is 670 km2 in area, hosts a population of some 7,300 and tops out at the eastern volcano at some 1,500 m.
Both volcanoes were built by layered basaltic lava flows once the edifice broke the surface of the Pacific. The layers measure from less than a meter to some 25 m thick. They are separated by soils and ash some centimeters to one third a meter thick. The Eastern Molokai Volcano is topped with a caldera that used to measure some 7 km in diameter. There has been massive erosion on the flanks before and after the flank collapse that removed the north half of the eastern volcano exposing numerous dikes on both volcanoes.
There are cinder cones and spatter cones on the western slope of East Molokai from the final stage of volcanism. There are the remains of at least one cone created by a phreatomagmatic eruption at Mokuhooniki a couple kilometers off the eastern end of the island.
Postshield lavas are rare on the older, western volcano. However they make up a substantial volcanic unit on the eastern volcano. The majority of the final volcanic episodes built cones and flows on the north shore of Molokai, one of the episodes forming a peninsula hosting a former leper colony. There was also at least one lava tube associated with this structure. http://pubs.usgs.gov/misc/stearns/Molokai.pdf
Early settlers mismanaged livestock on the island to the point where the dry western end of the island was mostly stripped of vegetation. Since then, has been poorly regrown and suffers substantial erosion.
Maui is the largest remaining island of this grouping. It is just under 1,900 km2. The eastern volcano, Haleakala tops out at over 3,000 m. Maui hosts a population of over 140,000 and sits some 50 km west of the Big Island of Hawaii.
As with Molokai, the western volcano is the oldest while Haleakala is the youngest vent. The isthmus connecting the two volcanoes was built chiefly by lavas out of Haleakala. Both mountains are constructed by multiple layered lava flows during the shield building stage, interspersed with thin layers of soils and the occasional ash deposit. http://pubs.usgs.gov/misc/stearns/Maui.pdf
The Hana Slump is the closest known landslide associated with Maui, though the velocity of the collapse does not appear to be significant. Note that Maui is a lot like the original shape of Molokai before the eastern volcano calved and slid north.
As with all Hawaiian Islands on the windward side, Maui generates a significant amount of rain as it wrings water out of the trade winds forcing them up the slopes of the volcanoes. On the windward side of Haleakala, over 750 cm of rainfall yearly is not uncommon. On the lee side, there are places that average 40 – 50 cm per year or less. Droughts are not uncommon with significantly less rainfall than that.
West Maui Volcano is the oldest, having gone through all three aerial stages of Hawaiian volcanism. Its highest point is some 1,700 m above the shoreline. It is constructed primarily of layered basalt flows which are heavily weathered and present spectacular cliffs to the ocean. Following the main shield building phase, it went through a series of postshield eruptions which filled the 5 km diameter main caldera mostly with lava. The final phase was the rejuvenated eruptions which ended some 500,000 years ago.
The East Maui Volcano, Haleakala is the most recently active Hawaiian volcano not associated with the Big Island. It has at least ten eruptions over the last thousand years, and numerous over the last 10,000 years. Magma being erupted is of the rejuvenated variety. The most recent eruption is thought to be somewhere between 250 – 500 years ago. The volcano is thought to have been originally about a kilometer taller than it currently is. Most of that loss of height is thought to be due to erosion.
The volcano itself is once again a layered basalt mountain, though this one is claimed by some as the largest single volcano in the chain in terms of sheer volume. It is topped with a crater that measures some 3.5 by 12 km. Interestingly enough, geologists do not write of this as a caldera, as it is breached on either end by valleys carved by erosion. Recent activity in the crater has created an array of spatter cones, cinder cones, basalt flows in the crater and flows reaching downhill to the north and south of it to the sea. http://hvo.wr.usgs.gov/volcanoes/haleakala/
Other rejuvenated volcanism within the last 1,500 years has taken place at the western edge of one of the rifts that make up Haleakala, with flows southwest to the sea. There is another region at the far northeast corner of the island associated with the east rift zone has many small cinder cones that have covered an area of the island with new lava. The east rift zone slopes down into the Pacific and becomes the Hana Ridge mentioned earlier as a source of a slump along its northern slopes. It is named after Hana, which is on the eastern tip of Maui. Haleakala is considered an active volcano for planning purposes. http://hvo.wr.usgs.gov/volcanoes/haleakala/newmapping.html
Lanai is the remains of a single shield volcano which erupted from three rifts arrayed around the central vent / caldera. It is roughly egg-shaped and sits some 15 km south of Molokai and 14 km southwest from Maui. As such, it is well shielded and protected from the trade winds and the moisture they carry. The island is some 364 km2 in area and supports a population of just over 3,100 per the 2010 census.
The central volcano tops out just above 1,000 m and supports a pair of calderas, one large and one small covering a combined 10 km2 in area.
Lanai sits on the lee side of Maui, so is drier than many of the other islands. The windward side of the island gets some 96 cm of rainfall yearly near the summit and 25 cm near the lee side coast. The primary crop is pineapple and sugarcane, both of which are being phased out in favor of tourism. Like Molokai, initial settlers allowed livestock to run wild for a time. This along with the drier climate made rebuilding of the plant coverage difficult.
Like Molokai, the shield building stage comprised of multiple layered basaltic lava flows interspersed with thin layers of soil and the occasional ash deposit. Because of the dryness of the island, spatter cones, pits and other normally observed rift zone and caldera lava sources are still easily found, with some 47 locations on the island. Think of Kilauea as the current example of this sort of volcanic activity.
There was no postshield magma emplacement found. And Lanai did not get any rejuvenated volcanic activity.
Early geologic surveys of Lanai found limestone, corals and other debris rather high in the valleys and the slopes of canyons carved by rainfall. These were misidentified as being due to massive changes in the height of the island above the ocean. As it turns out, the majority of these deposits were all brought ashore courtesy of tsunamis generated by the multiple massive landslides around this part of the Hawaiian chain. http://pubs.usgs.gov/misc/stearns/Lanai_and_Kahoolawe.pdf
Like Lanai, Kahoolawe is also the remains of a single shield volcano. It sits some 11 km south of Maui and 27 km east of Lanai. It is some 116 km2 in area and stands just over 450 m above the ocean. It has no permanent residents.
Like Lanai, Kahoolawe, was built by multiple layered basalt flows. Unlike Lanai, it is not high enough above the ocean to generate significant rainfall, averaging 25 – 60 cm per year, a semi-arid climate for this part of the world. It is also among the windiest of the Maui Nui islands and stands in the rain shadow of Maui. Wind is the primary erosive force on this island.
There is a central caldera on the eastern end of the island measuring around a kilometer in diameter. All three aerial types of volcanics are identified on this island, with the postshield mostly filling up the main caldera. Following an extended period of inactivity, rejuvenated magma eruptions did occur on several places on the island. There is no significant flank collapse associated with Kahoolawe, though tsunami debris is present on the entire island.
Goats were introduced onto Kahoolawe by Captain Cook in 1788, sheep in the mid-19th Century. These animals denuded the island of most of its native greenery, which is suspected to be mostly grasses, bushes and scattered trees. Once the vegetation was gone, the soil followed, being blown away by the constant winds. An estimated 1 – 3 m of soil was lost from the summit and left some 40 km2 of the island stripped all the way down to the weathering basalt lava flows. The goats and most sheep were killed off in 1918 and owners have started trying to reclaim the vegetation.
During WWII, Kahoolawe was turned into a gunnery range and impact area for Navy and Army Air Corps training. It was used for that purpose for over 30 years until Hawaiians decided it was sacred and the weapons training needed to stop. It was turned back over to the State of Hawaii in 1994 along with a military cleanup of exploded and unexploded ordinance. It has been turned into an Island Reserve for Hawaiian cultural purposes and agricultural remediation. This is probably the saddest single Hawaiian Island. http://pubs.usgs.gov/misc/stearns/Lanai_and_Kahoolawe.pdf
As we saw with the geologic history of Kauai, once the island is formed, it dissolves back into the ocean in a relatively short period of time. Total lifespan of these four islands has been around 2.2 MY from the first shield volcano built at Penguin Shoals west of Molokai. There were another five eruptive centers, making Maui Nui similar to the Big Island of Hawaii in construction. Want to see what the Big Island will look like in a few hundred thousands of years? Look no farther than 50 km west to the former Maui Nui.
Past VC Columns on Hawaii