Like reading a bookThis wonderful image captures two distinct geologic events, separated in time by

Like reading a bookThis wonderful image captures two distinct geologic events, separated in time by over 700 million years.This is a lake in Nunavut, in the Canadian Arctic. Near the top of this image you can see a single, thick black layer; that layer is a 723 million year old basaltic sill.The rock was once magma rising through the Earth’s crust. At some point, it hit a level where even though it was a liquid, the rocks above it were less dense, so it couldn’t punch through to the surface. The magma instead stayed at that level and spread out as a sheet, a single layer of magma forcing its way into the stratigraphy, which we call a sill.The magma was produced at an interesting time. 700 million years ago, a number of different magmatic events hit the area that is today Arctic Canada and Greenland, a pulse known as the Franklin igneous event. At the time, Canada was part of a supercontinent known as Rodinia – the predecessor to the better-known Pangaea supercontinent. These sills might have been part of a big pulse of magma which forced its way into the crust, in the process causing the supercontinent to break apart.If you look at the lakeshore, you see a different geologic process. The rocks have dozens of linear marks on them; each cut by this lake.For much of the last 100,000 years, kilometers of ice sat on top of this site. That ice’s weight was so great that it pushed the Earth’s crust downwards, pushing the mantle out of the way like toothpaste. Once the ice sheets melted, that weight was removed and the mantle began flowing back in.As the mantle flowed in, the crust above began to rebound, gradually pushing upwards at a rate of centimeters per year. When the rocks are at the lakeshore, the lake erodes a notch. A few years later, the continuing rise of the rocks moves them up above the water level, stranding the notch and leaving this record of parallel lines.-JBBImage credit: Mike Beauregard (creative commons license)http://www.flickr.com/photos/31856336@N03/9404384095Ages:http://www.sciencedirect.com/science/article/pii/S0301926803002869 -- source link

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