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OPAE-ULA HYPOGEAL HABITATS
Volcanic islands are made up of successive porous lava flows. Faster moving pahoehoe-type lava flows resemble superheated tar and under a hardened insulating surface, fluid lava races down the slopes. As the flow ends, the tube drains out and small and large caverns may form. There are extensive expansion cracks and air spaces separating each flow. In A-A lava flows, slower moving semi-hardened rocks lumber along like the treads of a bulldozer with the top of the flow falling forward to be covered by the volume following. There are a lot of air spaces between the jagged chunks of sharp lava.
Successive volcanic eruptions formed the Hawaiian Islands and measured from the ocean floor, our volcanoes are the largest mountains in the world. Occasionally lava flows cooled slowly and allowed all air to escape to form dense rock layers that captured pockets of water that were later tapped as shallow cap rock wells. Large and small air spaces in the lava become future pathways for the rain that falls on the mountains to flow in underground rivers to the sea. Near the ocean, the fresh water from the mountains and seawater mix to form a brackish water table. Salinity is continually changing and water gets saltier with depth and less salty farther from the ocean.
Opae-ula can survive in ocean, brackish, or fresh water! They are amazing creatures that evolved in tropical Hawaii in a very porous underground watertable. "Hypogeal" describes this vast labyrinth of underground connections and interstices that constitute their subterranean habitat!
Most of us go through life assuming that the ground below us is solid, but it's not! In Hawaii it varies depending upon the specific lava flows, and while there are thick layers of very dense lava, Hawaiian engineers will tell you that in some places, 5% to 25% of the mass may be air spaces. When below sea level, these air spaces fill with water and it becomes a "subway" system. Opae-ula have been discovered thousands of feet from the ocean!
HYPOGEAL THEORIES & CLUES
Openings in the lava allow us to view the water table and opae-ula can be found in anchialine pools, collapsed lava tubes and in sinkholds in uplifted fossil reefs. This is the "epogeal" portion that are windows and entrances to the opae-ula world. Because we cannot observe what is happening, in the hypogeal habitat, there are large gaps in our understandings. Although the body of knowledge is rapidly expanding, most of the knowledge is due to observations of the epogeal habitat or research tanks.
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