Portlock Point - Oahu, HI
Posted by: 
Raven
N 21° 15.571 W 157° 42.484
4Q E 634044 N 2351417
Just 10 minutes from downtown Honolulu, this dramatic shoreline bluff was created from layer upon layer of consolidated volcanic ash which was cemented together over time to form basaltic tuff, then eventually weathered by the ocean surf.
Waymark Code: WMVQGE
Location: Hawaii, United States
Date Posted: 05/18/2017
Views: 2
Hidden away in a small posh neighborhood in Hawaii Kai, just 10 minutes from downtown Honolulu is a magnificent Oahu coastline which can't be spotted from the road, known only by a few people for its naturally manicured sea cliffs and ocean spewing cave. To get to this waymark, you will have to follow Portlock Rd. from Kalaniana’ole Hwy (Hwy 72) and eventually make it to Lumahai Rd. Park near N 21 15.614 W 157 42.447, then take a short but steep trail down to a sloping 60 foot high bluff.
At the posted waymark coordinates on the bluff itself, you will notice several layered rock formations. Underneath you, as you stand on those rocks, the ocean is pounding the entire coastline around you (including a specific spot where the waves created an undercut cave which repels the water back, causing enormous explosions of white water and mist -- hence another the name of this site: "Spitting Cave of Portlock").
The geology of the rocks all around you are best described by an
Earthcache posted at this specific site by "sillygirl & jrr", a local geocaching couple:
"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. [...]
This [particular area] 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. [...]
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."