Located a few hundred yards off the east coast of Ohau at Lāʻie Point is an islet called Kukuiho‘Olua, one of five islet’s along this coast, measuring two acres in size and reaching a height of 20 ft.
In addition to Kukuiho‘Olua, there are four other small islands in the vicinity of the point. To the north, the northernmost island is Kihewamoku, with Mokuaia Island (locally known as “Goat Island”) and Pulemoku Island lying between Kihewamoku and Kukuiho’olua. To the southeast of Kukuiho’olua and directly east of Laniloa Point is another islet named Mokualai Island.
These islets are a lone refuge for many native seabirds, plants, and insects. Because of their ecological importance, the islets are part of the Hawaii State Seabird Sanctuary system. These islets are not open to the public.
Logging Requirements
1. Please send your answers before logging this earthcache
2. If you are in a group, only one member needs to submit the answers however each person will need to post a photo showing they were on site. A group shot is acceptable.
3.Logs without sending answers will be deleted
Questions:
- From the information on the cache page, what type of rock formed Kukuiho‘Olua Islet?
- From the information on the cache page, what is another name for a sea cave?
- From the information on the cache page and your observations at the posted coordinates, describe the erosional forces that are impacting Kukuiho‘Olua Islet? What factors cause significant erosion?
- According to the Youtube video link on the cache page in the Sea Arch section, which part of the arch is the most unstable part?
- The height of Kukuiho'Olua Islet is reported to be 20 ft tall. Based on your observations at the posted coordinates, how thick do you believe the top of the sea arch roof is?
- MANDATORY: Photos showing you or a personal item, with Kukuiho'Olua Islet and the arch are required to log this earthcache.
Video Geologic History of Hawai'i
The above link is a good resource to view for a better picture of the geology of Hawaii. For the purpose of this earthcache, I will try and condense the information to the relevant geology of La’ie Point and Kukuiho‘Olua Islet.
When people think about Hawaii, they think of volcano’s. As with all of the Hawaiian islands, the Island of Oahu started out as two, sub-marine volcano’s, Waianae volcano and Koolau volcano. Gradually repeated eruptions built two volcanic islands into one large island. Both volcano’s were built over three sets of fissures that intersected at a summit crater. The highly fluid basaltic flows built up shield shaped cones.
In terms of the geologic history of the planet, the Hawaiian Islands are pretty young. While the oceanic crust they reside on are almost 90 million years old, the oldest of the Hawaiian Islands, Kauai, is a mere 5 million years old.
Reef-building and sedimentary deposits
La’ie Point and the Islets off the point including Kukuiho’olua Island developed during the Pleistocene and Holocene periods and were formed from fossil reefs and carbonate sediments, primarily lithified beach dunes, thought to be less than 150 ka ( 150 thousand years ago). The dunes are thinly bedded and resulted when sand was blown and concentrated on shore during a glacial low stand in sea level making these islets and La'ie point sedimentary rock.
Sedimentary rock
- Sedimentary rocks are formed on or near the earth's surface, in contrast to metamorphic and igneous rocks, which are formed deep within the Earth. The most important geological processes that lead to the creation of sedimentary rocks are erosion, weathering, dissolution, precipitation, and lithification
- Calcareous rocks are a sedimentary carbonate rock usually limestone and dolostone. This rock forms in a stable continental shelf environment. This type of rock my be pure carbonate or may include chert, hematite or other detrital material including sand and clay.
Sea Caves
Sea caves, also called littoral caves are a type of cave formed by wave action of the sea. The driving force in cave development is wave action and occur both above and below the waterline. While waves batter coastlines all over the world, for a sea cave or littoral cave to form, there needs to be a weakness in the rock. This weakness can result from a fault, fracture, or in bedding planes where the bedding separates or parts, due to a difference in hardness between the layers. This is primarily the case with sedimentary rocks, however sea caves can form in any type of rock including metamorphic and igneous rock.
Sea Arch
When a cave forms in a headland with exposure to water on both sides, erosion can expand the cave until it punches through the headland so you are left with a sea arch. Initially this hole can be small however with continued water forces eroding the headland taking advantage of the weakness in the rock, this arch can grow larger.
https://www.youtube.com/watch?v=00Khn0RSpYA
Some historic and geologic background about Kukuiho‘Olua
As far back as 1919, erosion was visible on Kukuiho‘Olua in the form of two small caves on either side of the islet, in particular on the windward facing side of the islet. Over time the two caves continued to erode until the erosion punched through both caves creating an opening over 70 years ago. During February 24-26, 2016 storm, large waves resulting from the unique El Niño conditions washed out the large boulder that had lain within the arch since its initial formation, significantly increasing the open area beneath the arch. As a result, a distinctive sea arch is visible from Lāʻie Point.
References:
https://imagesofoldhawaii.com/kukuihoolua/
https://www.mdpi.com/2076-3263/11/3/147
Stratigraphy and Whole-Rock Amino Acid Geochronology of Key Holocene and Last Interglacial Carbonate Deposits in the Hawaiian Islands1
