Dispatches from Sea >>
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| March 2007 |  Laura Preston, educator, UNH/Salem High School, Salem, NH. |
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| April 2007 | |||||||
April 16, 2007
Today’s feature: Subsurface Geology
So, yesterday we reviewed volcanics. Did you do your homework and look at the related links?!
Today, I want to discuss the physical location of the magma (molten rock at more than 1200˚C) beneath us, the movement of magma through the subsurface, and why it looks the way it does when it comes to the seafloor surface. We’ll also take a peek at what the scientists on this cruise have found so far.
Scientists studying this area previously found that there are magma chambers in the crust about a mile beneath the surface of the ridge axis. How does that magma get to the surface? It’s because of seafloor spreading. When the two plates spread apart, cracks are created in the crust (kind of like pulling on each end of a chocolate-covered candy bar, when the chocolate on the surface cracks apart revealing the carmel underneath). On the ocean floor, these cracks are called fissures.
 A typical seafloor fissure. |
Sometimes these fissures are not very deep, and you can see to the bottom of them. But sometimes, if the fissures are deep enough, and the pressure of magma in the magma chamber is great enough, the magma escapes from the magma chamber, rises up and fills the fissures, even forcing new cracks to open as it rises. Sometimes this magma rises all the way to the surface to erupt on the seafloor as lava that forms a new eruption. Magma that rises from the chamber, but does not make it to the surface solidifies within the crack; this is called a dike. Dikes and lava flows make up the upper 500-5000 m of ocean crust.
So, the geologists look at the lavas on the seafloor to decide where and when dikes rose to the surface. “Fresh” lava tells them a conduit exists nearby and that it was active in the recent past (0 – 5ooo years). “Older” lavas mean that the area is no longer being fed by dikes, but was fed in the more distant past. When they look at the surface features of the seafloor, they can understand what is going on beneath the seafloor. You could say they are like detectives, putting all the pieces together to solve the mystery. Pretty cool stuff!
 Fresh lava “buds” on the seafloor. |
 Older, sediment covered lava. |
By the way; did you know that only 3% of the seafloor is currently mapped?
In the next few days we will explore ground truthing.
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