Field Trip: Hawai'i
J-Term, January 2006
In 2006, the Geology department, seeking sun and clear blue water, left behind ice-locked Wisconsin to explore the sunshine geology of Hawai'i. Hawai'ian geology is fascinating and has been extensively studied - Hawai'i is referenced in any intro geology textbook. The Hawai'ian islands are considered the prime example of shield volcanoes, a common form of volcano. So the opportunity to see in person what is, literally, textbook geology, was an opportunity hard to pass up.
View along the Na Pali coastline, on the island of Kauai. Tall cliffs exist here due to a major fault collapse on a portion of the island. Part of the island literally slid into the ocean. Imagine the tsunami that could create! Shield volcanoes, like those of Hawai'i, form as layer upon layer of lava extrude onto the sea floor. This means that there are zones of weakness between each layer of basalt (the rock type which makes up Hawai'i). The boundary between layers can be especially weak if weathering allowed a layer of sediment to build up on top of the last basalt flow. If the stresses get to be too great, sheets of the island can break off along those boundaries.
A hike through Kiluaea crater on the big island of Hawai'i. Craters like these often form on the summits of volcanoes. As previously mentioned, shield volcanoes such as Hawai'i form through volcanism, slowly building up from the sea floor. The Earth's crust is made of giant plates which move relative to one another. Most volcanoes occur where these plates meet, where they collide and where they pull apart. Hawai'i is hundreds of miles from any plate boundary, however. Why is there a volcano?
Hawai'i formed over a hotspot, an unusual point under the crust where conditions are so hot that magma is produced without dividing or colliding plates. The magma here is "primitive." It is thin and runny, and produces a rock known as Basalt. As mentioned, the hotspot occurs under the plate. The plate moves very slowly over the hotspot. Just as candle flame will leave a burnt streak on a piece of paper waved slowly over it, one volcano forms after another after another, forming a chain of islands. This is how the Hawai'ian islands formed. The Big Island of Hawai'i, then, is the island currently over the hotspot. It is the youngest of the Hawai'ian islands, and the islands get progressively older as one moves to the northwest.
Active outgassing of volcanic vents in Kiluaea crater is visible.
Haleakala crater, on Maui, is one of the largest craters in the world. It was formed, in a large part, by erosion rather than volcanism. In the periods between volcanic events, erosion is the dominant geologic force, after all.Small cindercone volcanoes can be seen rising from the floor of the crater. These "parasitic" volcanoes form mounds made out of chunks of a porous rock called scoria. Scoria is similar in composition to Basalt.
The Waimea canyon on Kauai is part of the "Grand Canyon of Hawai'i." The relief of this island creates some very strange microclimates: Although the browns in this valley make it look somewhat dry, just a few miles distant is the wetest place on Earth.
Lecturing outside. Bright. Sunny. T-shirts. In March. Can't do that in Green Bay!
Papakolea beach on Hawai'i's Big Island is one of Hawai'i's famous greensand beaches. As the cindercone in the background is eroded, green crystals of a mineral called 'olivine' are exposed. Olivine is found in rocks formed from volcanoes like Hawai'i, but not found in volcanoes such as Mt. St. Helens. More differences between these volcanoes are discussed below. This crystals quickly oxidize to look dark and dull, but many green crystals can be found on the beach.
The Hawai'ian volcanoes are considered relatively benign because they produce lava which is thin and runny, unlike the thick lava responsible for the explosive eruptions of volcanoes such as Mt. St. Helens and Mt. Vesuvius. These differences are related to the methods by which each volcano formed. The thin lava which flows over the Hawai'ian landscape produces two variants of basaltic rock when it hardens. At the end of a lava flow, when the lava is moving slowly, the rock that is cooling crinkles and crumples, and spikey variant forms. It is aptly named 'Aa' ("ah-ah"). The second variant, termed 'Pahoehoe' ("Pa-hoy-hoy") refers to lava which cooled higher in a flow, where the lava was still runny. This produces a smooth, ropey texture. Flow bands of Pahoehoe can be seen in this photo. As they curve around, they look somewhat like a turtle shell. Hawai'i produces rocks which look like turtles. . .
...And turtles which look like rocks!
We hope to see you on our next field trip.