The History of Maine Part 7: How Glaciers Work

Note: This program first aired on April 20, 2013.

We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week shifting focus in our exploration of Maine’s landscape history, by moving from tectonic forces to the power of ice. Today we take some time to look at the basic mechanics of an ice sheet.

Approximately 2 million years ago, the North American continent, in essentially the same global position it is in now, entered an ice age. When we say ice age, we mean that ice covered a significant percentage of the Earth’s surface. In the case of this past ice age, at its peak, ice covered about 32% of the land surface and 30% of the oceans, significantly more than is covered today. While the cause of an initiation of an ice age is still under scientific investigation, when climatic conditions are just right, glaciers will form and behave in a well documented manner.

The climatic conditions that support the growth of glaciers are high winter snow fall, combined with cool summer temperatures. You see, glaciers, whether they form in the mountains or in the middle of a continental land mass, are formed from snow and only snow. The climate must be cool enough, at least regionally, that precipitation falls as snow, in at least the winter. The summers, if we can call them that, must be cool enough that all that snow doesn’t go away. At its simplest, a glacier starts as a multi year accumulation of snow.

The glaciers that define an ice age are continental glaciers, meaning they form large sheets of ice that cover virtually everything on a continent. Think of the ice coverage of the continent of Antarctica today. They form when it snows in the winter, and that snow doesn’t melt in the summer, over and over again, on a very large scale. After a few years, that multi year snow transforms physically to something called firn (f I r n). As snow ages it undergoes metamorphism, the snow crystals, or flakes, break down and become more rounded, and start to bond together-that’s firn, an intermediate stage between fresh snow and ice. More snow piles on top, and the weight of that snow presses down on the older snow below and accelerates this process.

As the firn snow gets more and more compressed, by the weight of the accumulating snow on top of it, the air pockets trapped by the original snow fall (think fluffy powder snow) get more and more compressed, and are slowly forced out of the compacting snow pack. It may take up to one hundred years, but this compaction gradually changes the original snow (fluffy, white!) into a solid blue material we would all look at and recognize as ice.

We think of ice as hard and solid, especially when we are first learning to ice skate, but in reality, ice is more malleable, and nowhere do we see this more clearly than in glaciers. Just like the atmosphere has mass and weighs upon us here on the surface of the earth (otherwise known as the bottom of the atmosphere), and the water of the ocean has mass weighs down on the bottom of the sea, the snow that accumulates on a glacier has mass. As that snow accumulates it gets heavier, it piles higher, it literally builds up. Once the ice gets thick enough, once enough snow has accumulated and weighs down on the snow underneath it, the ice that is formed starts to deform;  in geological terminology we say the ice has become plastic.

Gravity doesn’t like it when some things (any things) are higher than other things. Gravity wants everything to be in equilibrium, in other words, at the same level. When the things that are higher than other things are solid, like mountains, gravity can’t do anything about it except wait for erosion. But if the things that are higher than their surroundings are fluid, or plastic, they are capable of flow, and will yield (albeit slowly, in the case of a glacier) to the power of gravity. Which is to say, when a continental glacier gets big enough, and thick enough, it will start to flow outward in all directions, sliding on its base, where it is in contact with the land below, and internally deforming (or squishing) in between the surface and base. The glacier will continue to spread as long as snow keeps falling on the interior, and more snow accumulates than melts each year. A glacier is said to be in retreat, when it is melting faster than it is forming. In this way the size of the glacier is directly related to climate, which is the primary reason that so many climate scientists, including several world class ones here in Maine, study the dynamics of the world’s remaining ice sheets in their pursuit of the keys to climate change.

We’ll leave it off there for today, but join us in the coming weeks as we look into the details of the past 2 million years of glacial advance and retreat, and what that has meant for the landscape we see around us today.

References:

Caldwell, D. W. Roadside Geology of Maine

http://nsidc.org/cryosphere/glaciers/life-glacier.html
The National Snow and Ice Data Center (yes there is such a thing!) All About Glaciers!

When seen from above, it is much easier to see a glacier's fluid nature. From NASA’s Earth Observatory website (a must visit—they have a weekly email list serve for serious nerds, of which I am one). http://earthobservatory.nasa.gov/IOTD/view.php?id=4710

The History of Maine Part 6: The Ice Age

Note: This program first aired on April 13, 2013.

We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week about 200 million years ago, with the opening of the Atlantic Ocean. Europe and North America split apart and started moving away from each other, at about the same rate as our fingernails grow.

With the exception of a hot spot that New England drifted over during this time, there was little volcanic or tectonic activity between then and now. North America slowly drifted poleward from the equator, and the climate of Maine changed accordingly. The Appalachian Mountains eroded, sending a huge sediment load oceanward, forming the coastal plane we see today on the east coast south of Long Island. The continents, freed from the bondage of Pangea, moved slowly over the surface of the Earth, coming closer and closer to their present day positions. And it is this continental movement and positioning that may have set in motion what happened next.

Up until this point, we have been referring to a timescale of 10s to 100’s of millions of years; big and fairly imprecise chunks of time, inferred from a geologic record of highly metamorphosed rocks, thousands of feet of sediment, and broad brush continuities on a global scale. For this next part of the story, we need to hone our gaze and zoom in quite a bit, this next chapter covers only 3 million years at best. We can call this chapter: The Ice Age, and technically, because there are still ice caps on Greenland and Antarctica, we’re still in it.

Ice ages have occurred throughout Earth’s history, current thinking puts the number at 5. The causes of the ice ages are not well understood, though there are many scientists working on this question, as the understanding of what is called “climate forcing” is directly related to current investigations of climate change. Causes likely include changes in oceanic circulation due to continental drift (as the continents move the ocean basins change shape, and currents can be redirected to higher or lower latitude), changes in atmospheric composition (including key greenhouse gasses like carbon dioxide and methane), fluxuation of solar out put, changes in planetary orbits (called orbital forcing) and changes in atmospheric circulation due to tectonic uplift. Its pretty complex stuff, and the scientists working on this are trying to reconstruct all of these factors and let them run in fantastically complicated computer models, hoping that what the models predict should have happened corresponds with the known geologic record.

So for this last ice age, which stretches back about 2.5 million years to the beginning of the Quaternary Period, one event that coincided with the start of global cooling was the cutting off of the Atlantic from the Pacific when the isthmus of Panama fully formed. This changed circulation patterns in both oceans, especially the Atlantic, and it drove the current we now call the Gulf Stream further north. The Gulf Stream is a warm water current, and you may be wondering how a warm water current moving further north triggers a global ice age. This is a reasonable question, however the concept is that warm air is able to hold more moisture than cold air, and warm water evaporates more readily than cold water. Both of these factors put more moisture into the atmosphere in higher, cooler latitudes, which can then be precipitated out. That increase in precipitation could have yielded the continental glaciers that covered much of the northern hemisphere during periods of the past two and a half million years. Is that exactly what happened? We don’t know, yet. Remember, science is about noticing patterns and then trying to explain them with additional evidence. In the Western scientific tradition, we’ve been noticing and trying to explain glaciers for the past 200 years, and much of the current research is simply about trying to discern the observable patterns at higher and higher resolution. Questions about the current patterns of climate change are driving us to want to know more and more about significant climate change events in the past, but most of these questions are still wide open, which makes the research that much more exciting.

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References:

Yep, these references really hit all the key points:  same as for “The History of Maine: Part 1”. http://theworldaroundusradio.blogspot.com/2013/02/the-history-of-maine-part-1.html

Good overview of the Quaternary Period from National Geographic http://science.nationalgeographic.com/science/prehistoric-world/quaternary/

The History of Maine Part 5: The Atlantic Opens

Note: This program first aired on March 30, 2013.

We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week with the mass extinction event that marked the end of the Permian period, an era of Earth history that came to a dramatic closure about 250 million years ago. All of us animals here on Earth today are some how related to the hearty 10% or so of species that survived this cataclysmic event (plants seemed to have weathered the end Permian much better, mostly surviving into the Mesozoic).

A bit after the end of the Permian age, and possibly related to it, the crust that made up the super continent of Pangea started to weaken. Tectonic forces from the Earth’s mantle and core were awaking, or at least changing direction, and the continents that had been crushed together started to pull apart again. This tension started gradually, about 225 million years ago; by 200 million years ago the continents were separating in earnest. The rifting we are most interested is that which returned the land that would become Maine to the coast. Remember, during the duration of Pangea, Maine was most decidedly inland.

The continents that would eventually become known as North America and Europe had previously drifted towards each other as the Iapetus Ocean closed. Now, they pulled apart from each other, on essentially the same line as the Iapetus Ocean. As the continents pulled apart, a rift formed between them. At first it was likely just an inland valley, much like the East African Rift Valley today, spotted with long narrow lakes and volcanoes. As it grew wider, one end or the other of it eventually contacted the global ocean, and was inundated with sea water. Voila! An ocean was born. To see this in action today, you need look no farther than the Red Sea—an ocean flooded extension of the rift valley from East Africa.

The ocean that formed of course, is our own dear Atlantic Ocean, named for Atlas, the titan of Greek mythology, who was the son of Iapetus. So clever. And I’ve said that it formed in nearly the same location as the ancestral Iapetus. Nearly, but not exactly, and here is why. Some of the small bits of land, the micro continents, terranes and crushed coastal sediments, that were accreted on to the coast lines of North American and Europe, they stayed accreted, or stuck to the larger continental land masses, instead of getting dragged back out to sea. These bits of land added more “continent” to the continents, and gave the land masses new coastlines as the sea filled in the growing gap between North America and Europe. Of particular relevance for Maine, the microcontinent called Avalon stayed stuck to our coast line, and today makes up much of Hancock and Washington Counties, and a thin strip of coast on the west side of Penobscot Bay.

The opening of the Atlantic Ocean was the last major tectonic event to affect the land we now call Maine. The youngest rocks in the state date to this time period. When the continents were under tension and were pulling apart, many cracks formed in the crust. As these cracks formed, they were quickly filled in by hot fluid magma from deep within the crust, forming intrusions called dikes. This magma cooled into basalt, a rock type that makes up most of the crust of the bottom of the ocean. It is typified by being dark in color and fine grained. In many places along the coast, you can recognize these basalt dikes by their dramatic dark color, in an otherwise light granitic rock. After these dikes formed, there was no more igneous activity, no more volcanoes, no more rifting. At this point in history, Maine settled in for about 200 million years of erosion, which removed much of the material that was covering the mountains and hills we see around us today. These rocks started many miles below the surface, and are only on the surface today due to the constant pressure of the forces of erosion.

We’ll leave it off there for today, but join us in the coming weeks as we continue  piecing together the story of the long and fascinating history of the land that we Mainers call home. Next week we take a big leap in time, and change the scale of our gaze as well, as we begin to look at the ice age as the next “big thing” in the history of Maine.

References:

Yep, these references really hit all the key points:  same as for “The History of Maine: Part 1”. http://theworldaroundusradio.blogspot.com/2013/02/the-history-of-maine-part-1.html

 Our friends at the US Geological Survey have some nice material on plate tectonic basics, including information about just how they know how fast the plates move--http://pubs.usgs.gov/gip/dynamic/understanding.html
 (This is exactly the kind of initiative I want my tax dollars spent on! Thanks USGS!)