Pillows EarthCache
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At this location, you can observe what is known as a "pillow lava" formation. When lava comes from the earth's crust under water, the difference between the temperature of the lava and the cooler temperature of the water causes the outer layers of the lava to harden, forming a skin. When the pressure of the magma becomes sufficient to rupture the skin, a new pillow forms near the vent. This process produces a series of interconnecting lobes that are pillow-like.
When a pillow is formed, the exterior of the pillow cools faster than the inside of the pillow, so the exterior is very fine grained, with a glassy texture. The magma inside the pillow cools more slowly, so the grain would be coarser.
An indication that this formation might be part of the Canadian Shield is if the rock appears to be green, part of the "Abitibi Greenstone Belt" found in Northern Ontario. Geologists would then date the Canadian Shield rock as being very ancient, 2.5 to 2.7 billion years, a time scientists say that there was very little free oxygen.
Lava pillows have also been created when there is a volcanic eruption beneath a glacier. In those cases, the lava melts the glacier above, which would again cause the molten lava to be under water and form in pillows even more quickly because of the cool temperatures.
When you closely inspect the pillows at this site, you will observe that on the rock surfaces, there are many tiny, rounded holes. The holes were likely caused by gasses escaping from the vent from which the lava flowed. The gasses then rose through the lava and hardened in the rock face.
To log this earthcache, please e-mail the cache owner with your answers to the following questions:
1. Do you believe this is an ancient formation, part of the Canadian shield, or a relatively recent formation? Explain why you came to that conclusion.
2. Based on the presence of the small holes, what does that indicate about the conditions in which the pillows formed?
3. From the presence of holes on the exterior of the pillows, what does that say about the speed at which the pillows formed?
4. Where in Canada might you expect to find pillow lava that formed because of volcanic activity beneath glaciers?
Though not required to log this cache, you are encouraged to take a picture of yourself or your GPSr at this location.
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