Isostasy and Continental Rebound
One of the many changes that occurs over very long time periods in Earth's history is the glaciation (and subsequent glacial melt) of the continents. In oceans, ice simply floats on top of the water, not impacting the water level at all. But on continents, large masses of ice develop and actually weigh down the otherwise buoyant continental crust. As the continental glaciers recede, the crust slowly rebounds. It's the much, much slower (over tens to hundreds of thousands of years) version of holding a cork underwater and then watching it bob back up to floating on the surface once you let it go. Like the cork, continents are trying to 'bob' back up to float on the molten magma below. The need to reach equilibrium is known as isostasy.
It's hard to imagine that ice could be heavy enough to physically weigh down something as big as a continent, but we know this happens because today both Greenland and Antarctica are weighed down by their respective ice sheets. The Antarctic ice sheet covers over 14 million square kilometers and is 2 kilometers thick at its deepest point. Scientists estimate it contains about 30 million cubic kilometers of frozen water. Some basic math tells us that:
It's hard to imagine that ice could be heavy enough to physically weigh down something as big as a continent, but we know this happens because today both Greenland and Antarctica are weighed down by their respective ice sheets. The Antarctic ice sheet covers over 14 million square kilometers and is 2 kilometers thick at its deepest point. Scientists estimate it contains about 30 million cubic kilometers of frozen water. Some basic math tells us that:
- 30 mill cu km X 264 billion gallons/cu km = 7.92 trillion gallons of water
- One gallon of water at freezing point = about 8.36 pounds
- So the Antarctic ice sheet weighs (7.92 x 8.36 = ) about 66 trillion pounds (give or take a few).
No wonder the western portion of the Antarctic continental land mass is actually almost 2.5 km BELOW sea level. That's a lot of ice.
Log in to BBLearn and go to Module 2. Click on the Study Area icon, and in the Study Area, under Interactive Animations for Chapter 13, select 'Isostasy.' Using the animation, answer the following questions:
1. As the ice sheet grows and forces the crust down into the mantle, pushing it away from the area of subsiding crust, what happens to the portions of the crust outside of the ice sheet, where the mantle is flowing to?
2. According to the animation, during the last ice age, what was the maximum depression (in meters) of the continental crust?
3. As the ice melts at the end of an ice age, what happens to the areas that were uplifted during the ice age?
4. Are continents today still undergoing isostatic adjustment from the end of the last ice age or are they now stabilized, according to the animation?
5. Ice sheets tend to be thickest (and heaviest) in the middle of continents, and uplift generally occurs on the edge of continents during an ice age. Today, we worry about sea level rise associated with climate change, but sea level rise is also associated with isostatic adjustment and subsidence (as the crust subsides, the ocean level rises).
Given our warming Earth, do you think that the relatively slow isostatic adjustments going on from the end of the last ice age will make sea level rise even more at the edges of the continents, or will act as a counterbalance to sea level rise associated with more recent and rapid climate change?
Log in to BBLearn and go to Module 2. Click on the Study Area icon, and in the Study Area, under Interactive Animations for Chapter 13, select 'Isostasy.' Using the animation, answer the following questions:
1. As the ice sheet grows and forces the crust down into the mantle, pushing it away from the area of subsiding crust, what happens to the portions of the crust outside of the ice sheet, where the mantle is flowing to?
2. According to the animation, during the last ice age, what was the maximum depression (in meters) of the continental crust?
3. As the ice melts at the end of an ice age, what happens to the areas that were uplifted during the ice age?
4. Are continents today still undergoing isostatic adjustment from the end of the last ice age or are they now stabilized, according to the animation?
5. Ice sheets tend to be thickest (and heaviest) in the middle of continents, and uplift generally occurs on the edge of continents during an ice age. Today, we worry about sea level rise associated with climate change, but sea level rise is also associated with isostatic adjustment and subsidence (as the crust subsides, the ocean level rises).
Given our warming Earth, do you think that the relatively slow isostatic adjustments going on from the end of the last ice age will make sea level rise even more at the edges of the continents, or will act as a counterbalance to sea level rise associated with more recent and rapid climate change?