Maine Perspective Front Page
Cutting Edge
University of Maine Research on the Frontiers of Science
Maine's Distant Geologic Past
A detailed study of a Maine granite is contributing new evidence to explain how dynamic magma chambers characterized Maine's distant geologic past. Daniel Lux, UMaine professor of geological sciences, is collaborating with David Gibson of the Department of Natural Sciences at the University of Maine at Farmington on an analysis of the gray building stone known as Mount Waldo granite.
Lux presented a paper on their research at the Northeast regional meeting of the Geological Society of America in March.
Mount Waldo granite is exposed at the surface over an area of about 150 square kilometers (58 square miles) just west of Bucksport, and extends into the earth to an estimated depth of about four miles. In the past, the stone was widely used for building material; abandoned quarries provide Lux and Gibson with a small window through which to understand the processes that created the granite more than 370 million years ago.
At that time, the locations of the tectonic plates that make up the Earth's crust were arranged very differently from today's pattern. The North American plate, on which Maine sits, was colliding with the smaller Avalon plate to the east. Remnants of the Avalon plate are found in coastal portions of modern-day Newfoundland, Nova Scotia and sections of the U.S. East Coast. The collision forced the leading edge of the North American plate beneath the Avalon plate, where elevated temperature and pressure caused melting. The resulting igneous activity was similar to what is happening today in the Pacific Northwest.
"We know that the granite exposed on Mount Waldo formed at a depth of about 10 kilometers (six miles) below the surface," says Lux. "Since then, overlying rock has eroded to expose the granite. My interest is what occurred within the magma chamber while the Mount Waldo granite was forming.
"The chamber once filled with liquid magma is now solid granite. Through our field and laboratory research, we are attempting to unravel the sequence of the growth and physical sorting of crystals within the magma chamber to understand what chemical exchanges might have occurred between crystals and liquid."
One group of geologists who study igneous processes have generally thought that magma, once concentrated in a chamber, simply cools and solidifies. However, recent evidence from other volcanically active areas, such as Yellowstone Park, suggests that some magma chambers are anything but stagnant. Lux and Gibson are adding to the latter view.
They report that crystals of the common rock-forming mineral, plagioclase feldspar, observed in the Mount Waldo granite, have distinctive growth histories. Initially some of the plagioclase crystals grew and then were partially dissolved, whereas others just grew. In the later stages of the crystallization process, the crystals were transported within the chamber and deposited together on the chamber floor.
In addition, Lux and Gibson have found distinct mineralogic layers, a feature not commonly observed in granites, which have significant differences in chemical composition. Taken together, these layers suggest that after the Mount Waldo magma was in place and cooling, new magmas with variable compositions were injected into the chamber.
"Mineral crystals within a magma behave in some sense like snow flakes in a storm cloud," says Lux. "Snow flakes are blown about as they grow. Each snowflake is different and their shape reflects growth in variable microenvironments before they accumulate on the ground. Similarly, crystals in granite start to form in a cooling magma chamber. New injections of hot magma may partially melt some of the growing crystals and transport them to different locations. The variable shapes and textures of the crystal reflect this dynamic environment."
The question is whether or not repeated injection of hot magmas is a fundamental process common to all magma chambers or restricted to Yellowstone magmas and other examples.
Lux and Gibson have worked together since the mid-'80s, when Gibson was a post-doctoral researcher at UMaine.
Seeking the Source of Sand Beach
A quiet backwater lagoon was once located where Sand Beach and the crash of ocean waves now attract tourists at Acadia National Park, according to evidence from beach cores collected by a UMaine graduate student. In the last few thousand years, the beach has been migrating landward as sea levels have slowly risen, according to work by Alison Brandes, a master's degree candidate in the Department of Geological Sciences.
Brandes worked with park staff, and Joseph Kelley and Daniel Belknap, professors in the department, to collect sand cores as deep as 10 feet below the surface of the beach. In addition to studying geologic history, Brandes found that the concentration of shell fragments across the surface of the beach varies dramatically from about 70 percent within the tidal zone to about 30 percent by the dunes behind the beach.
Brandes presented a paper on her findings March 13 at the Northeast regional meeting of the Geological Society of America.
Research in 1992 by Walter Barnhardt, then a UMaine graduate student, revealed high concentration of shell, or carbonate material, at Sand Beach. His and other studies raised questions about the abundance of these shell beaches in cool-water settings.
"My goal was to continue that work, to understand the history of Sand Beach, why it is located there and how stable it is. Normally, carbonate tends to dissolve in conditions of cold temperatures, high acidity like acid rain, and low salinity," says Brandes.
In March 1999, Brandes extracted four cores from the high dunes behind the beach. She also measured the beach profile at low tide once every two weeks, collected samples of surface sand and used ground-penetrating radar to look for the boundaries between underground layers of sand, gravel and clay.
"I found a sequence of glacial material at the bottom followed by intertidal material from flats and dune material on top. The glacial material is about 2 meters below the surface at the back of the dune. It had been assumed to be there, but this is the first time that anyone has taken cores to document the underlying stratigraphy."
Brandes also found a sharp drop in the concentration of carbonate in sand between the low and high tide lines. "I think that shells are picked up by the wind. They're more aerodynamic than the land-derived material, more like little airplane wings, and they get blown toward the dunes," she says.
Since wind tends not to pick up wet material, high carbonate concentrations remain in the wet sand within the tidal zone, while shell material is removed from the dry part of the beach. Thus the carbonate concentration in that area is lower.
So-called carbonate beaches are relatively rare in temperate climates. At least two others have been identified in Maine on Little Cranberry Island and at Georgetown.
In addition to her master's, Brandes is pursuing a teaching certificate and hopes to teach high school earth science.