The History of Maine Part 10: Sea Level and the Coastline

Note: This program first aired on May 25, 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 looking at some of the landscape features the glaciers left as they retreated 13,000 years ago here in Maine. That period of deglaciation had another profound impact on the Maine landscape, particularly the one most people know Maine for: the coast.

When we look at the location of the coast line, we are really looking at the edge of the water relative to the elevation of the landscape. If the landform is low, the water can move in, if the land form is high, the water is pushed out. There are two factors here that can change, the amount or volume of the water (the more water there is, the more three dimensional space it takes up, the higher it rises relative to the land, if all things are equal), and the level of the land (this is trickier to get your head around, but just remember that the crust of the Earth is floating, just like a ship at sea—the heavier the boat, the lower it floats. Solid crust floats on the more plastic layers of the Earth below.)

When we look at these two things relative to glaciation, we see the complicated and variable history of Maine’s coastline in the past 13,000 years. When glaciers form, they form from water, generally liquid water that evaporates into the atmosphere and then falls back to Earth in solid form. Where does all that liquid come from? The ocean, so overall we see a drop in global sea level when large continental glaciers form, because so much liquid water is taken up out of the ocean. At the height of the last glacial advance, it is thought that sea levels in our neck of the woods were 300 to 400 feet lower than today as a result of this (a level that would have easily exposed the continental shelf, had it not been covered by ice).

The amount of water isn’t the only thing that can change however, remember, the actual relative level of the land can as well. Remember the ship I mentioned, when it is heavy is floats lower in the water, when it is empty it floats at a higher level. The crust that underlies the state of Maine is like that ship. And what could make the crust heavier than normal? How about 10,000 feet of solid ice? That would do it. The continental glacier was so large and massive, it caused the crust to become less buoyant and float a bit lower on the underlying mantle. The crust was literally depressed into the Earth. As a side note, this is happening today in Antarctica, much of that continent is actually below sea level as a result of the weight of its ice cap.

So that is how the stage was set when the last ice age began to end here in Maine, about 13,000ish years ago. Just like a trampoline with some one sitting on it, the Earth’s crust is deformed by the weight of the glacier, and there was less water in the ocean. That less water in the ocean bit was changing fast though, as the glacier was melting quite rapidly. All of that melt water returned to the ocean, which rose quite rapidly in response. And as the glacial melted, it relived the pressure on the underlying crust, and just like a trampoline when you bounce off it, the crust rebounded. These two events didn’t happen simultaneously though, the glacier melted a great deal faster than the Earth rebounded, which meant, the ocean water easily covered the newly exposed but still depressed landscape of Maine. This event is called the Marine Incursion and at its maximum the coastline was as far inland as Medway, near Baxter State Park. Soon though the Earth’s rebound caught up with the rising ocean, and then some (again, think of our trampoline; a bouncer gets pushed up above the equilibrium level before coming back down). At the height of the crust’s rebound, sea level was much lower than it is today, as the land rose up above the level of the ocean. During this period, about 11,000 years ago, much of the Gulf of Maine, in particular Georges Bank on the edge of the continental shelf, was dry land. Terrestrial fossils, including trees and mammoth tusks have been recovered from Georges Bank, and the Gulf of Maine was a shallow inland sea cut off from the Atlantic Ocean. Soon thereafter though, the rebound eased and even subsided a bit, and the rising ocean caught up. The present day coastline was more or less established in the last few thousand years, as the rapid changes that resulted from deglaciation ended and temporary equilibrium was restored.

Maine has what is called a drowned coastline, and I hope now you can see why. Next week we will look at the third major force that shapes the Maine landscape, us.

References:
D. W. Caldwell Roadside Geology of Maine 1998

Harry Thurston The Atlantic Coast: A Natural History 2011

David L. Kendall Glacier and Granite 1987

The Maine Geological Survey makes its surficial geology map available on line, in a printable 11x17 inch format. It has lots of good info on the results of the last glacial advance and retreat, and it’s free!
http://www.maine.gov/doc/nrimc/mgs/pubs/online/surficial/surficial11x17.pdf

Super nerds unite: check out the Maine Ice Age Trail, and you too can tour Hancock and Washington counties looking at gravel pits (I’ve done it, its awesome). http://iceagetrail.umaine.edu/ also includes an excellent overview of the last ice age in Maine http://iceagetrail.umaine.edu/content/iceageinmaine/iceageinmaine.php

The History of Maine Part 9: The Glacial Landscape

Note: This program first aired May 11, 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 talking about the patterns of glaciation in this past ice age.

As I said last week, each time an ice sheet advances, it essentially wipes out the evidence left by the ice sheet before it, so while there were several cycles of advance and retreat here in Maine, we really only see evidence from the last one.

The ice sheet originated in northern Canada, and spread under its own weight as far south as Long Island New York. It reached its maximum extent some time between 24,000, and 28,000 years ago, mere nanoseconds on the geologic timescale. As a glacier moves over the landscape, it scours the earth, scraping away all the soil and loose debris down to the bed rock. Much of that material gets incorporated into the base of the glacier, frozen into the ice, and acts as giant grit sand paper, further scouring and smoothing the bedrock below. So the first effect of the glacier on Maine’s landscape is one of smoothing, rounding and eroding. The repeated scourings that resulted from the cycles of advance and retreat have given us the rolling landscape we see here today.

By picking up all that surficial material, the glacier is also a very effective earth mover. Rocks originating in bedrock in one part of the state, can be found on the tops of mountains on the other side of the state, confusing many a geologist in the days before the ice age was understood. We can think of the second effect of the glacier as being that of a redistributor of the surface sediments and materials of a Maine.

When the glacier melted, it dropped all of that material in place, so most of Maine is covered with a layer of what is called glacial till, essentially gravel, sand and rocks, broken up to varying degrees and transported in the glacial ice. A glacier is in retreat when the leading edge is melting faster than snow is accumulating at the center. The leading edge melts back and leaves all that till behind. A glacier advances when the snow accumulation rate is higher than the melting rate at the leading edge. Sometimes however, the rate of melting equals the rate of snow accumulation, and the glacier appears to stand still. It isn’t really stationary, as it is still advancing, but the leading edge is melting at the same rate, so the edge of the glacier is stationary relative to the land around it. When this happens the glacier is still melting and dumping till, but it isn’t moving back when it does this, meaning, a big ridge of till piles up at the foot of the glacier. That pile of till is called a moraine, and they are very common on the Maine landscape. My house is built into the end of one. Moraines tend to be hills or ridges full of unsorted till, meaning you find all sizes of materials in them, in no logical pattern. Giant boulders coexist with fine sand or gravel. They were simply dropped where the ice melted, with virtually no sorting of any kind.

Another thing that happens when a glacier melts is the formation of rivers of melt water within and underneath the ice. All that water has to go some were, and channels form in cracks in the ice, that follow gravity and eventually make it to the ground, melting their way out to the snout of the glacier. As the ice melts, it releases its sediment burden, but in this case, the sediment is released into moving water. The energy of the moving water provides a mechanism to sort the sediment; it takes more energy to move a large boulder than it does to move a small grain of sand. Deposits from this melt water are called eskers, and they are characterized by well sorted sediment, making them a favorite for gravel pit operators. They also tend to be quite prominent and run for long distances on top of the surrounding landscape. Many roads in Maine run along the tops of eskers as a result.

We’ll finish today with everyone’s favorite depositional feature of our glacial landscape, glacial erratics. These are large boulders dropped by the glacier as it melted, that stand out prominently on the landscape, instead of being buried in glacial till. In some cases they were deposited as part of glacial till and either by the luck of the draw or erosion of the surrounding sediment, ended up on  the surface. In other cases though they were deposited by icebergs. Yes, at many points in the retreat of the last glacier in Maine, the sea came right up to the melting edge of the glacier (much like some of the glaciers in western Greenland today). The melting edge of the glacier could actually float on the sea water, and chunks of it would break off. These chunks or ice bergs still held the rocks, boulders and gravel that makes up glacial till, and as they melted the rocks rained down onto the bottom of the sea. Large boulders deposited this way are called drop stones. 

Those are some of the depositional features we see around us here in Maine that resulted from the retreat of the last glacier not so very long ago. We’ll leave off there today, but join us in the coming weeks as we finish up the glacier chapter of the story of Maine’s landscape history.

References

D. W. Caldwell, Roadside Geology of Maine

David L. Kendall Glaciers and Granite: A guide to Maine’s landscape and geology, 1987 North Country Press, Unity 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!

The Maine Geological Survey makes its surficial geology map available on line, in a printable 11x17 inch format. It has lots of good info on the results of the last glacial advance and retreat, and it’s free!
http://www.maine.gov/doc/nrimc/mgs/pubs/online/surficial/surficial11x17.pdf

The History of Maine Part 8: Ice Age Patterns

Note: This program first aired May 4, 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 talking about the basic mechanics of continental glaciers, from their formation to their plastic-y deformation and movement.

The most recent ice age on Earth began about two and a half million years ago, and marked the beginning of what geologists refer to as the Quaternary Period. This ice age was the culmination of a gradual cooling trend in Earth’s overall climate that began about 75 million years ago, while dinosaurs still roamed the Earth. On the geologic time scale, two and a half million years is chump change, and when compared to the time required for opening and closing of an ocean or the subduction of a plate, it is hard to believe anything significant could happen to a landscape in such a short time. But believe it. The effects of the ice age absolutely define the hardscrabble nature of Maine’s land forms and biotic communities today.

The ice age that began two and a half million years ago* is actually a series of repeating expansions and retreats of continental glaciers, which originate at high latitudes, and spread into temperate or mid latitudes at their furthest reach. The pattern has been generalized as 100,000 year cycles, about 60-90 thousand years of glaciation, followed by a warmer period lasting 10 to as much as 40 thousand years, but there is certainly variability. These warmer periods are called interglacials, and we are in one right now—if we weren’t, Maine winters and summers would be a whole lot colder than we currently experience.

It is difficult to say with certainty just how many times the continental glacier that originated in northern Canada advanced over the landscape of Maine, as each glacial advance essentially obliterates all evidence left by the glacial advance before it. Billions and billions of pounds of one to two mile thick ice quite literally wipes the slate clean every where it goes. But we do have a very clear picture of the global climate during this time, which gives us a good sense of cooler periods and warmer periods, which are relatively safe to correlate with periods of glacial advance and retreat in the northern hemisphere. The climate record is recorded in the ice sheets that persist today (in Antarctica, in Alaska, in Greenland). Scientists drill into these ice sheets and retract a core (essentially a long skinny tube) of ice. Remembering that glaciers are formed from repeated snow fall, researchers can actually discern the layers of snow, compressed but still distinct, and like scientists looking at tree rings, actually reconstruct the climate history with amazing resolution. Tiny bubbles of gas from the fossil atmosphere and dust, ash and other atmospheric particles are clues that aid this research.

While all this glacial activity was going on, and the Earth was over all a bit cooler and probably a bit drier as well, something else was happening, something we are pretty interested in. Importantly for us, this past ice age coincides with the period of hominid evolution, us. While it is impossible to say that humans evolved because of the ice age, it is true that we evolved in a relatively uncommon climatic regime. In the big picture, Earth has been much much warmer than it is now, humans evolved during a cool spell. While it is hard to predict with any certainty just how hot it will get as a result of human influenced climate change, the possibility that keeps climate scientists up at night is that it will get warmer than humans, ANY humans have ever experienced. That is the kind of climate shift that marks the end of the evolutionary line for many a species. We see it over and over again in the fossil record.  That is why many people define climate change as an existential threat, it is estimated that 99% of every species that has lived on Earth is extinct, primarily due to the fact that they were not able to evolve fast enough to adapt to rapidly changing climate regimes. Ice ages come and ice ages go, changing the shape of the land along the way, but probably more importantly, ice ages are a symptom of instability in the global thermostat, with significant repercussions for everything that lives on Earth.

We digressed a bit today, but join us next week as we get back on track and look at the direct impact of the glaciers on the landscape of Maine.

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!

Super nerds unite: check out the Maine Ice Age Trail, and you too can tour Hancock and Washington counties looking at gravel pits (I’ve done it, its awesome). http://iceagetrail.umaine.edu/ also includes an excellent overview of the last ice age in Maine http://iceagetrail.umaine.edu/content/iceageinmaine/iceageinmaine.php

Nice bigger picture  material from PBS (written quite a while ago by a UMaine researcher):
http://www.pbs.org/wgbh/nova/earth/cause-ice-age.html

Some scholarly business, though there has been a flurry of research in the past two decades on ice sheets, so this is likely a bit dated: Richmond, G.M. and D.S. Fullerton, 1986, Summation of Quaternary glaciations in the United States of America. Quaternary Science Reviews. vol. 5, pp. 183-196

On Extinction—From the Bristol University late Triassic Website: http://palaeo.gly.bris.ac.uk/palaeofiles/triassic/extinction.htm

Explore UMaine’s Climate Change Institute http://climatechange.umaine.edu/ especially this link to the basics of ice core research:
http://climatechange.umaine.edu/icecores/IceCore/Ice_Core_101.html

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!)

The History of Maine: Part 4 The Carboniferous/Permian Interlude

Note: This program first aired March 23, 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 360 million years ago. The Iapetus Ocean (the predecessor to today’s Atlantic Ocean) had just finished closing, and was gone for good. The continental land mass that today we call Europe had just collided with North America, crushing coastal sediments and causing volcanic activity at and around the edges of the two plates. Geologists refer to this collision as the Acadian Orogeny, and aspects of it did indeed give us the mountains of Acadia National Park.

At this point, Maine, a state known for all of its beautiful coastline, was decidedly inland. To the what will be west we had the extent of North America. To the what will be east, we had the Eurasian continent. Maine was right in the middle. And this is how it stayed, for another 100 million plus years. These years included two major geological periods, the Carboniferous and the Permian. Geologically during these time periods all of the continental land masses were coalsesing, into the familiar super continent we all know and love, Pangea. In terms of North America, the formation of Pangea was completed 300 to 250 million years ago, when the Africa (accompanied by the rest of Asia), smashed into what would become the southern United States, in what eologists call the Alleghanian Orogeny. This tectonic event didn’t do much to the geology of Maine, though there is some speculation that the impact caused some shifting along the faults that sutured Avalon to North America. The completion of the formation of Pangea though, did influence the global climate, drying things out slightly in an otherwise tropical and moist environment.

While we wait for the next major geological happening, lets take a slight detour and look a the biology of the time period while Maine was just sitting there in the middle of the super continent. The Carboniferous period ran from about 350 to 300 million years ago, and is known as the Age of Ferns. Picture a majestic forest, but instead of trees, the plants towering over your head are ferns, and horsetails and club mosses. During the Carboniferous, plants had fully migrated on to land from the oceans, but were still mainly of the vascular, non flowering, spore bearing type. Today these kinds of plants are much more diminuitive, but in the Carboniferous, they reigned supreme. Conifers, the first true seed bearing plants were just getting going in the drier areas of this warm wet world. The Carboniferous is named for the major geologic feature of the time, the massive coal beds that started as these lush living forests. Due to all this lushness, as well as the large amount of erosion and burial of organic matter, oxygen levels were higher during this time period than any other time before or since. These high oxygen levels (as high at 35% of the composition of the atmosphere) allowed for gigantism in another group of organisms, ones that today are generally much smaller than during the Carboniferous; athropods. This is the time of the 6 foot long arthropod called Arthopleura, and the dragonfly with three foot wing span. Arthropods do not actively inhale air, but simply let it diffuse into their bodies through a series of tubes called the tracheal system. The rate of gas diffusion is thought to be a limiting factor on body size, so the higher oxygen content (the biologically most important gas) would release some of that limitation, allowing for larger (much larger!) arthropods.

Amphibians were another group of animals that were starting to make a name for themselves during this time. Terrestrial tetrapods were abounding, and included a group that evolved out of the amphibians, to become our animal ancestors: the amniotes. They were named thus because of they had evolved an amniotic membrane, that allowed their eggs to be laid on land instead of in the water. This group rapidly split into two distinct lines; the synapsids included animals that eventually evolved into mammals, and the sauropsids, animals that evolved into reptiles (including birds and dinosaurs).

During the Carboniferous and the Permian, climate fluctuated from warm, wet and swampy, to warm and dry, with a bit of ice cap activity thrown in. Throughout it all, evolution was proceeding in leaps and bounds. All good things though come to an end, and at the end of the Permian period, about 250 mya, Earth experienced the biggest mass extinction ever. The End Permian eliminated as many as 90 % of all species on Earth at the time. Evolutionarily, mass extinctions are like hitting the reset button. All the species that we find on Earth today are some how related to those 10% of survivors. The cause of the End Permian is thought to be a massive volcanic eruption event, one that lasted perhaps a million years, changing the composition of green house gasses in the atmosphere and dramatically altering climate and atmospheric and ocean chemistry. The details are still hotly debated, but regardless of how it happened, the End Permian event was one for the record books.

References:

Awesome recreations of extinct creatures, including the Carboniferous Arthorpleura: http://www.windsofkansas.com/lifesize.html

Fun stuff about the life forms on Earth during the Carboniferous (check out the tab for the Permian as well!):
http://museumvictoria.com.au/melbournemuseum/discoverycentre/600-million-years/timeline/carboniferous/

Read more about the happy go lucky times of the End Permian Extinction:
http://finstofeet.com/2012/08/02/permian-apocalypse/
This blogger hasn’t referenced his material, but it isn’t too bad and provides a nice overview.

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

Maine’s own Geological Survey has a wealth of resources available online: http://www.maine.gov/doc/nrimc/mgs/explore/index.htm
This is a link to a pdf of a simplified bedrock geology map of the state of Maine, clearly showing the southwest/northeast trend of bedrock:
http://www.maine.gov/doc/nrimc/mgs/pubs/online/bedrock/bedrock11x17.pdf

 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!)