It is here that you will be treated to a magnificent site: a volcanic, layered shoreline, a reminder of Hawaii’s violent geological beginnings, along with spectacular blue water. The cave itself is caused by a point in the bluff where the ocean is pounding its way in, causing a small "dent" in the land. These undercut cliffs are what cause the "spitting." As waves pound the cave, they are repelled causing enormous explosions of white water and mist; a truly spectacular sight. Summer is a good time to visit this spot, as this is when the biggest waves come to the South shores of the island. However, even winter time waves can be impressive. From December to April, this spot makes an excellent place to look for whales, as you’re high up, with an expansive view of the ocean. At any time of year, you may be treated to sightings of Hawaiian monk seals, turtles, spinner dolphins and several species of birds.
Geologic Background
The island of Oahu was formed by two volcanoes, the Wai'anae Volcano in the west and the Ko'olau Volcano in the east. The Wai'anae Volcano formed the western third of Oahu and the Ko'olau Volcano formed the eastern two thirds of the island.
The Ko'olau Volcano began forming a few million years after the Wai'anae Volcano, forming a separate island a few miles to the east of Wai'anae. The two volcanoes continued erupting until they gradually filled the ocean between them with land and became one island. After the Wai'anae Volcano became dormant lava flows from the Ko'olau volcano formed the Schofield Plateau between them. Cinder cones, tuff rings and spatter cones from the Ko'olau Volcano formed some of Oahu's most well-known landmarks including Diamond Head Crater, Hanauma Bay, Koko Head, Koko Crater, Punchbowl Crater, and Mount Tantalus.
This spitting cave is located on Koko Head (Kuamookane), part of the Koko rift zone of Ko'olau Volcano. Koko Head is one of a chain of volcanic tuff cones along the Ka Iwi coast which were created when magma came into contact with sea water. The bluffs here were formed from layer upon layer of consolidated volcanic ash with different erosional properties. Koko Head defines the eastern side of Maunalua Bay. At 642 ft., Koko Head is somewhat dwarfed by its neighboring tuff cone, Koko Crater (Kohelepelepe), rising to 1208 ft. Koko Head has three significant depressions or old vents, the largest of which forms Hanauma Bay.
The cliffs at The Spitting Cave of Portlock were not caused by a typical flow of lava, but rather from layer upon layer of consolidated volcanic ash (bits of pulverized rock and glass). These layers, formed from multiple eruptions, became cemented together over time to form solid rock or tuff. There are several distinct types of volcanic tuffs, defined by mineral composition. Tuffs in the Hawaiian Islands are basaltic; when weathered they are filled with calcite, chlorite and serpentine.
More About Sea Caves
A sea cave, also known as a littoral cave, is formed primarily by erosion. Sea caves are found throughout the world, actively forming along present coastlines and as relict sea caves on former coastlines. The top of a sea cave will at times collapse and create a "blow hole" or "spouting horn."
Littoral caves may be found in a wide variety of host rocks, ranging from sedimentary to metamorphic to igneous, but caves in the latter tend to be larger due to the greater strength of the host rock. To form a sea cave, the host rock must first contain a weak zone. In metamorphic or igneous rock, this is typically either a fault or a dike. In sedimentary rocks, this may be a bedding-plane parting or a contact between layers of different hardness. The latter may also occur in igneous rocks, where waves have attacked the contact between the andesitic basalt and the agglomerate. Erosion is ongoing anywhere that waves batter rocky coasts, but where sea cliffs contain zones of weakness, rock is removed faster. As the sea reaches into the fissures thus formed, they begin to widen and deepen due to the tremendous force exerted within a confined space, not only by direct action of the surf and any rock particles that it bears, but also by compression of air within.
Blowholes (partially submerged caves that eject large sprays of sea water as waves retreat and allow rapid re-expansion of air compressed within) attest to this process. Adding to the hydraulic power of the waves is the abrasive force of suspended sand and rock. Most sea-cave walls are irregular and chunky, reflecting an erosional process where the rock is fractured piece by piece. However, some caves have portions where the walls are rounded and smoothed, typically floored with cobbles, and result from the swirling motion of these cobbles in the surf zone.
Logging Requirements
Email us the following information BEFORE you log a find for this cache:
1. How many distinct layers of ash can you count from sea level to the top of the cliff. (Answers may vary with tides.)
2. Which type of tuff is present in this area, and what are some of its secondary minerals?
3. Whether the cave is spitting during your visit. If it is, estimate the distance of the spray from the cave opening.
4. The time of your visit, stage of the tide, and if any weather advisories (e.g, high surf, wind, or small craft) were in effect. Click here for tide tables.
5. Optional: Post a photo of the cave in action.
Extra credit for those not afraid of the edge: Proceed to N 21 15.578 W 157 42.457 to feel the cave rumble when it spits.
Please include in your online log any wildlife sightings!
____________________
References
Anatomy of an Island: A Geological History of Oahu – by Gordon MacDonald, Kyselka MacDonald
Roadside Geology of Hawai'i – by Richard W. Hazlett, Donald W. Hyndman
Authigenic Zeolites in Zeolitic Palagonite Tuffs on Oahu, Hawaii – by Azuma Iijima, Kazuo Harada