Hawaii Geology and Geography
Lava TypesAs you drive around the island you can't help but notice the various lava formations and lava fields that crisscross the island. If you pay especially close attention you're likely to observe there are two distinctly different types of lava flows. Around the Kona area you'll quickly notice how clunky and jagged the lava is. In most of Hawaii Volcanoes National Park, especially at the end of Chain of Craters Road, you'll notice the lava is very smooth, pillowy, and even ropy. These two unique flow types owe their origins to two distinctly different types of lava flows, Pahoehoe and A'a.
The word Pahoehoe (pronounced Pa-hoi-hoi) rolls off the tongue in the same fashion that it flows out of the volcano. It is smooth, and as the surface cools it forms ropy swirls and smooth hills. Pahoehoe flows slowly and steadily forward using bulbous toes to lead the way. As the toe (lobe) cools, it creates a thin layer. The hot lava continues to advance by breaking through the layer. The unique cracking sound of the cooled crust popping off often bewilders first time visitors. The surface texture of pahoehoe flows can vary greatly. The viscous nature of pahoehoe makes it the perfect medium to create a variety of designs in the cooled lava.
The other type of lava is called 'A'a. 'A'a (pronounced "ah-ah") is the rough and tumble sister of pahoehoe. It rolls forth in clumps of broken lava called clinkers. The clinkers continue to pile up until the lava has cooled leaving a sharp, brittle pile of lava rock. It is difficult to find sure footing on the rubbly surface. The term 'a'a is actually said to have come from the ancient Hawaiians who would exclaim 'ah ah' as they walked over it with their bare feet. While the top of the flow may just look like a pile of rocks, underneath is a dense lava core which drives it onward. The crumbly surface clinkers are able to hitch a ride on top of the dense lava for awhile, but eventually they topple off the end and are rolled over. This process creates layers of fragments on the top and bottom of the advancing flow. It's almost like a glacier of fire.Most of the lava that erupts in the islands begins its life as pahoehoe. Along its journey a variety of factors can make it change into 'a'a. The thickness of the lava and the resistance of the path it takes can make this change occur. The thicker the pahoehoe flow, the less resistance is required to turn it into 'a'a because it is moving so slowly. Conversely, the thinner and more free flowing the pahoehoe flow, the harder it is to encounter the amount of resistance required to turn it into 'a'a. Once a pahoehoe flow has transitioned to 'a'a, there is no going back. Pahoehoe can turn into 'a'a, but 'a'a can never turn into pahoehoe.
Two other famous volcanic by-product are Pele's hair and tears. Both formed in fountaining eruptions, Pele's hair are thin strands of volcanic glass caught in the air during a eruption. Pele's tears are solid tear-shaped glass particles formed in the same way.
Subsidence & Erosion of the Hawaiian Islands
Though the Big Island of Hawai'i may seem incredibly large compared to it's predecessors in the chain, in all likelihood it is not that much larger at all (historically speaking). Just across the 'Alenuihaha Channel sits the island of Maui and it's greatest volcano, Haleakala. Geologists suspect that at one time Haleakala was not only joined to the West Maui Mountains, like today, but also was a single landmass combined with the islands of Lana'i, Moloka'i, and Kaho'olawe -- known as Maui Nui (literally, big Maui) (view map). The submergence of Maui Nui resulted as the volcanic body moved away from the Hawaiian hot spot. The lack of volcanic upbuilding combined with continued subsidence into the ocean floor eventually sank portions of the large island into the Pacific, providing us with the four separate islands we see today.
A similar fate awaits the Big Island in due time. As the hot spot "moves away" from the island (due to the Pacific Plate carrying the islands piggyback-style off to the north west) the Big Island too will fall victim to subsidence and erosion. Eventually the Big Island will likely find itself in a similar state to that of Maui Nui. It will become separate and smaller islands as the ocean encroaches on the flanks of each separate mountain. Such is the geologic circle of life beyond the Hawaiian hot spot.
For the next few thousand years however the Big Island will remain just that, big! It continues to represent an astounding 62% of the total land area of the Hawaiian Islands. And because Mauna Loa and Kilauea are currently still active and erupting volcanoes, the island of Hawai'i is still growing. The geologic future of the island is a work in progress.
A larger version of the rendered map above can be found here: Hawaiian Islands Map (Courtesy NGDC, GLCF, and DLR).
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Now all that said, the lava may occasionally thicken to a`a, blocking the vent. Or pyroclastic materials (cinders, pumice and the like) may block the vent. Either way, an explosion can (but rarely does) result. Kilauea has had two major explosions during recent human history. Native Hawaiians noted an explosion around 1790 which killed several soldiers of an army encamped near the volcano, while the other explosion occurred in 1924. The latter was studied extensively by volcanologists. The most recent explosion in March of 2008 is what creating the relatively small smoking vent inside Halema`uma`u Crater. It was the first explosion in Kilauea's main Halemaumau Crater since 1924, scattering debris over an area of about 75 acres of the park.