Volcanos and lava flows are part of the West Maui geologic story, but not today.
The pictures and videos of the recent 2018 Kilauea eruptions on the Big Island of Hawaii are mesmerizing, striking and even scary. Seeing Kilauea erupt can help us imagine what West Maui was like two million years ago. The West Maui geologic story starts there.
All Hawaiian Islands have the same origin but were staggered in time, with the oldest to the northwest and the youngest to the southeast. There is a “hot spot” or mantle plume deep in the earth that is stationary and as the shallower East Pacific tectonic plate moves northwesterly across this spot, molten rock is formed and rises upwards until it erupts through the surface. This phenomenon creates a linear island chain, like luggage on a long conveyor belt. Geologists don’t understand exactly why there is a stationary “hot spot” but we do know that new islands form, and older islands rapidly erode and subside until they are below sea level. We know that this part of the story has been going on for at least 80 million years as we can trace the Hawaiian chain of islands, atolls and then seamounts under the Pacific Ocean all the way to a subduction zone near Russia, with a bend in the line when there were changes in the plate boundaries 40 million years ago.
The Pacific Ocean showing the entire the Hawaiian Island chain. The bend in the line is thought to be when there were changes in the plate boundaries about 40 million years ago. Illustration from Science News March 2015.
For the Hawaiian Islands, northwest is always older than southeast, and Maui is therefore older than the Big Island of Hawaii. They are similar in origin, but with separate formational histories. They are different pieces of luggage on the conveyor belt. And West Maui is also older than its’ gigantic East Maui cousin Haleakala, although they likely had related activity when lavas from both volcanos merged together in their connecting isthmus. The last volcanic event on West Maui is thought to be about 320,000 years ago and for Haleakala/East Maui the estimated date is 1790 based on human observation.
West Maui is an ancient shield volcano. Although a separate feature, it is part of a mega complex or massif that includes East Maui, Moloka’i, Lana’i, Kaho’olawe and the connecting areas around them. It is thought that at one time all of this area was above sea level and bigger than the Big Island of Hawaii. Today, the mostly submerged massif structure creates large areas of relatively shallow waters around these islands. Shallow water means warmer water, making it perfect for whales during the winter season. So now you know why whales love West Maui!
A whale playing with the West Maui shield volcano slopes in the background. West Maui is part of a larger volcanic complex which creates shallower and warmer waters, perfect for whales!
The West Maui volcano had three distinct island building volcanic events. The first and main event oozed out lava from a central caldera from 2.00+ to 1.25 million years ago. The rock from this event is called the Wailuku Basalt and is the dominant surface rock over West Maui. Molten lava spread out radially and created the oval island shape with smooth slopes we see today. There were likely thousands of eruptions. Then around 1.25 million years ago, the lava changed chemistry, perhaps due to cooling as it moved away from the heat source. It became more alkaline and viscous, flowing slower and in thicker layers. The resulting deposits are called the Honolua Volcanic series and are found today mostly on the north side of west Maui, but also some to the south. These rocks form the Kapalua highlands and jagged northern coastline as well as the ridge that supports the wind turbines near Ma'alaea Harbor. 600,000 years of erosion and calm followed, until the Lahaina volcanic series erupted in a few places starting about 400,000 years ago. This was West Maui’s last gasp. Since then there has been no additional activity; only more erosional carving to create the beautiful landscape we see today. The West Maui Volcano is categorized as extinct.
Around the Honua Kai resort and Kaanapali beach, many familiar features relate directly to the geologic story. Looking back at the West Maui mountain, you have a spectacular view of the now incised shield volcano. The rocks you see are from the main shield building event, the Wailuku Basalt, and you can imagine how the mountain formed by piling up layers and layers of slowly oozing lava flows, just like what we see erupting today on the Big Island of Hawaii. Black Rock (Keka’a Point) at the Sheraton Hotel is part of the Lahaina volcanic series and was one of the last eruption events in West Maui. The town of Lahaina also sits on a larger deposit of these recent cinder cone and lava rocks, which are visible looking downwards from the by-pass road.
View from the Honua Kai Resort looking east towards the now ancient West Maui Shield Volcano. The incised valleys are caused by post volcanic erosion. The clouds are just clouds and not volcanic activity, although one could imagine!
The old West Maui airport was located along North Kaanapali beach because it was a large flat area. You can read more on the Airport Beach informational sign at the Kahekili beach park north of Black Rock. Honua Kai resort was built at the northern edge of the former airport land. The ground surface here is alluvium and dunes, but underneath is the gently sloping Wailuku basalt lava flows. You can see them in outcrop by looking up slope at the Coffee Plantation Farms development where iron oxidation has caused the red coloration.
Just north of the Honua Kai resort there is Honokowai Stream, a distinct drainage line that traces all the way up to the main mountain core. An erosional feature now, but possibly coincident with an ancient lava flow that defined its’ shape. Google earth images show that Honokowai point, at the northern end of the Mahana resort and just south of the stream today, is a broad arc shaped extension to the shoreline, shaped like a river delta. From this feature northward there are distinct differences in the beaches and the extending ocean floor. Take a walk from Honua Kai north on the Lower Honoapi’ilani road and you will see and feel it. The beaches here are smaller and nestled in coves.
Aerial photo of May 2018 Kilauea Volcanic eruption on the Big Island of Hawaii, illustrating what West Maui looked like about 2.0 million years ago - minus the houses! USGS Photo.
North of Kahana, the topography changes even more and this is where the post shield building Honolua volcanic series is preserved. The rocks there are lighter colored and show banding or flow planes. These lavas flowed from smaller cones or domes radially scattered downslope from the original caldera. The rugged coast line and steep cliffs of the Honulua volcanics are in stark contrast to the gentle slopes and large beaches to the south where the Wailuku Basalts are found.
Photo of West Maui shoreline at Oneloa Beach showing the late stage Honolua Volcanics. Note flow banding in the gray rocks at the left.
So, does the recent volcanic activity on the Big Island connect under the earth to Maui? Will the west Maui area erupt too someday? The answers are no and no. The 2018 Kilauea eruption, although spectacular, is affecting a small part of the Big Island on the east/southeast side, 130 miles from West Maui. And to complete the story, there is a new island already being formed under the sea southeast of Kilauea called the Lo’ihi seamount. The Hawaiian Island geologic story has been happening for a long time and it isn’t changing.
References and for more information:
Park, Michael F. and H. Wang, 2005, Bathymetric Atlas of the Main Hawaiian Islands, PIFSC Data Report R-05-002. https://www.pifsc.noaa.gov/cred/himap/shoals.php
Sherrod, D.R., J.M. Sinton, S.E. Watkins, and K.M. Brunt,Geologic Map of the State of Hawaii, Sheet 7, Island of Maui, US Geological Survey. https://pubs.usgs.gov/of/2007/1089/Maui_2007.pdf
Siegel, Ethan, May 2018, What's Happening At Kīlauea In Hawaii? 16 Questions With A Front-Line USGS Scientist. https://www.forbes.com/sites/startswithabang/2018/05/29/whats-happening-at-kilauea-in-hawaii-16-questions-with-a-front-line-usgs-scientist/#79030694badf
Sinton, J.M., 2006, Maui Field Trip, Department of Geology and Geophysics, University of Hawaii, 37pp. https://www.soest.hawaii.edu/GG/resources/docs/Maui_2006.pdf
Stearns, H. T., and G.A. Macdonald, 1942, Geology and Groundwater Resources of the Island of Maui, Hawaii. Bull. Hawaii Div. Hydrog. 7, 344 pp.
Sumner, Thomas, March 2015, Plate loss gave chain of Pacific Islands and seamounts a bend – Shift in mantle flow repositioned magma plume responsible for Hawaii. Science News Magazine.https://www.sciencenews.org/article/plate-loss-gave-chain-pacific-islands-and-seamounts-bend
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