While I’m not a geologist, it’s hard not to be fascinated by the geology of the largest island on earth. It holds a great many mysteries as only 1/5th of the island is not covered by ice.
The geology is ancient; most of Greenland is dominated by crystalline rocks and is part of the Precambrian shield, going back more than 3 billion years. That’s billion with a b. Some of the oldest rocks on the planet are here in Greenland. And they’re everywhere.
This gneiss forms much of the bedrock in Greenland. It's some of the oldest rock on the planet, going back 3 billion years.
A classic example of gneiss. Gneiss is metamorphic, meaning is starts out as igneous or sedimentary and is then changed or morphed into another kind of rock.
If you’ve hiked Michigan’s Upper Peninsula, northeastern Minnesota, or even the northern-most part of my home-state, Wisconsin, then you’ve tread on parts of the Precambrian shield. A vast area of exposed metamorphic rocks like: gneiss, schist, and quartzite; formed billions of years ago from igneous rocks when the area was originally volcanic. The Precambrian shield is also known as the Canadian Shield or Laurentian Plateau.
This vein of feldspar intruded into the older gneiss. Feldspar is a common mineral.
You can see the cleavage plane in some of the larger crystals of feldspar.
The small, shiny minerals are biotite. Biotite and micas are found in many of the metamorphic rocks.
The eastern side of Greenland is geologically quite different from the rest of the island. While the basement rock is still Precambrian, it has been reshaped by the splitting of the Atlantic basin. Piled on top of the basement rock are up to 6,000 feet of sedimentary rock giving many of the mountains their ‘layered cake’ appearance. Also mixed in are huge basalt columns, like you’d see in Wyoming’s Devils Tower. The columns are evidence of volcanic activity from the spreading of the Atlantic plates.
You can see the sedimentary layers. Also a hanging glacier in the center.
The sedimentary layers go back 900 million years. Credit James Yungel/NASA.
I've tried to include some other interesting geology in the captions below. If you have kids, or are still a kid yourself, start a rock collection from all the places you visit. We have one that wraps around the house. The limit on my international luggage is 28 kg, but a couple 3-billion-year-old rocks might be making the journey.
Glaciers are not slow. They are dynamic! They are incredibly fast moving in geologic time-scales. It's easy to see how glaciers shape the landscape everywhere. You can 'see,' or more correctly, imagine them flowing.
Greenland is the remaining core of the huge ice sheet that covered most of North America, including Wisconsin and parts of Illinois, in the last 2 million years. You can still see those geologic processes in action here.
When a glacier grinds up rock it's called till.
Close up of till. This photograph would make a good jig-saw puzzle!
The till gets broken down by both the glacier and the river to form finer and finer sediments like this sand.
You can see the current river channel on the right and the different flood stages 'written' on the banks.
The boulders mark the edges of the recent floods.
These large boulders are rafted on ice or pushed along the river. This one is about 100 tons.
This boulder is far from the river. Imagine how much water and how fast it had to be moving to deposit this boulder all the way up here.
This gneiss bedrock has been worked by high speed flow which carries lots of sediment.
How long would you have to be on your hands and knees, rubbing water and sand on the rock, to form these features? Hard to imagine the time scale ... and this is a very recent feature, geologically speaking.
The incredible speed of the water carrying fine sediment can polish the rock and form a varnish on the surface.
The sediments, worked by the Russell Glacier and the Watson River, eventually wind up deposited here on this huge outwash plain. Sediments drop out of the water when the water slows down as the ground flattens out.
Folding and uplift are seen everywhere.
More folding in the metamorphic layers.
This is the fifth largest meteorite in the world and was found in northern Greenland. It's at the Danish Geologic Museum in Copenhagen -- I stopped in on my way to Greenland. In fact, this is only about half of it, as the other half was sliced off and used for various studies. It's around 4.5 billion years old!
A short movie of the Watson River as it flows through Kangerlussuaq. The river has cut a channel in the ancient bedrock, probably exploiting a fissure, and time, water, and freezing/thawing have created the channel. And the end of the shot you get a nice view of how the gneiss has been worked and carved by the sediment carried in the high velocity flow. How long would you have to scrub a spot to form a 'pocket' like those? Finally a view of the great outwash plain as the river empties into the fjord in the distance.
Thanks to Al Fleming for all the geology help in this journal!
Comments