Lava in the Keweenaw

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Oh. Hello, my name is Karl Larson; I am an interpretive ranger for Keweenaw National Historical Park in Calumet, MI. Did you know one of the Earth’s largest lava flows is right here in the Keweenaw Peninsula? Indeed it is and I will talk about it as well as the Portage Lake Volcanics that occurred here as we take a trip back in time to explore the Keweenaw Peninsula. Imagine that the National Park Service has a time machine and we’ve just traveled 1.1 billion years into our past. What we discover is a much different landscape. In fact, in some places, it looks similar to this. One of the most striking features, besides the lava, is the complete absence of trees or any plant or animal life on land. The atmosphere has only a fraction of the oxygen as present day. Lava forms massive seas of molten basalt and on the horizon, there are mountains off to the south and southeast. The lava flows were very much like the ones that flow on Hawaii today. For the most part they were calm eruptions with basalt flowing over the landscape creating the rippled appearance of Pahoehoe lava. Because basalt is denser than the surrounding continental crust it started to sink under its own weight creating depressions. During lulls in eruptions, sediments eroded from the higher elevations made their way down and were deposited in these depressions. Then, eruptions started again and sandwiched the sediments. Some of these sediments later became known as conglomerate. So after about 2 million years of nearly steady eruptions, the lava stopped flowing. What we were left with is hundreds of lava layers and around 20 sedimentary layers. One particular lava layer is extremely large. In fact it is one of the world’s largest lava flows! At its thickest point it is over 1640 feet. So it must have existed as a magma sea for hundreds if not a thousand years before it finally solidified. I am standing on it right now actually. This, the Greenstone Flow, is a prominent feature of the Keweenaw landscape. It acts as kind of a backbone for the peninsula. So why should I care? Well, this lava flow and others form a great deal of our current landscape and account for most of the waterfalls in the area. They also create vantage points for stunning views such as here on the Greenstone Flow as well as on Brockway Mountain. They create a variety of habitats for local fauna and also can affect the weather by forcing lake effect clouds to release their snow load. Geology affects our lives more than people realize. It only takes a few moments of thought to make a connection of why geology matters to me and hopefully you will take a few moments and discover why it matters to you.

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3 minutes, 54 seconds

Lake Superior volcanoes? A mid-continent rift? It all happened right here on the Keweenaw Peninsula! The cold exposed bedrock in the Keweenaw had a fiery beginning some 1.1 billion years ago, but it is still affecting our lives today.


Produced by: Karl Larson

Written by: Karl Larson


Last updated: August 29, 2023

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