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Pillow Lavas I Have Known
Maine's bedrock geology abounds with variety, ranging from sedimentary rocks to high-grade metamorphic rocks, and intrusive igneous rocks of all types. Some of the more fascinating rocks are the volcanic rocks. While Maine currently has no active volcanic zones or even old volcanic peaks, it nonetheless has many areas that are underlain with volcanic rocks (Figure 1).
This month, we will look at a peculiar type of volcanic rock called a pillow lava. When molten lava erupts underwater, it commonly forms mounds of elongate lava "pillows" by repeated oozing and cooling of the hot lava. As the lava oozes, it forms a flexible crust in a blobby pillow shape. More lava expands this flexible crust until it breaks, lava oozes out, and another pillow forms. As new pillows pile onto old, the bottoms of the new, warm pillows mold their shapes to the tops of the older cooled and hardened ones, forming characteristically convex tops and concave bottoms. While the rock type is most often basalt (a dark volcanic rock rich in iron and magnesium), other types of lavas can form pillows as well. To the geologist, pillow lavas are clear evidence that the lava erupted under the sea (Click to see a video clip of modern pillows forming). For geologists in Maine, pillow lavas show us where ocean basins existed hundreds of millions of years ago.
Examples of Maine pillow lavas
Unless otherwise noted, most of these sites are difficult to get to and on private property.
1) These photos are examples of pillow lavas in the Canada Falls Member of the Frontenac Formation near Canada Falls and Seboomook Lakes in northwestern Maine. They are slightly more than 400 million years old. In the cross-sectional view on the left, the group of pillows have been tipped on edge so that the rounded tops are on the right side and the concave bottoms on the left. The photo in the center shows some elongated pillows that have been flattened. Still, it is possible to see that the rounded tops are on the right side and the concave bottoms on the left. The photo on the right shows the internal structure common in some pillows. The outer crust is generally fine-grained and evenly textured. The center here shows former gas bubbles that are now filled with calcite. Geologists call open gas bubbles that are preserved in the rock vesicles. When they are filled with a mineral they are called amygdules. (Photos: R. Marvinney)
7) Pillow lavas of the Quoddy formation as mapped by Olcott Gates at Eastern Head in Trescott. Pillows are shown in cross-section on the photo on the left of a sea-worn outcrop. Each pillow shows a quickly cooled thin dark crust. The interiors of some pillows have vesicles. Pillows stand out in relief in the right photo. The broken one in the center shows the typical radial fracture pattern formed as the pillow cools, and vesicles. (Photos: D. Allen)
Gates, O., 2001, Bedrock geology of North Haven and Vinalhaven Islands, Maine: Maine Geological Survey, Open-File Map 01-352.
Hussey, A.M. II, and Marvinney, R.G., 2002, Bedrock geology of the Bath 1:100,00-scale quadrangle, Maine: Maine Geological Survey, Geologic Map 02-152.
Additional web sites:
Text by R. Marvinney
Originally published on the web as the January 2003 Site of the Month.
Last updated on April 19, 2012
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