The Geology of Isle Royale

NASA Satellite Image

  • Where is Isle Royale This page provides a brief description of where Isle Royale is located and includes links to videos about the island.

  • How did Isle Royale form?  This page gives a brief description of how Isle Royale was created.

  • Step One - This page gives a description of how a tear in the crust long ago made the rock material that is now the Lake Superior basin.

  • Step Two - This page describes how glaciers formed during the ice ages..

  • Step Three - This page describes the process that made the rock layers curve downward forming the Lake Superior basin.

  • U-shaped Valleys - This page describes the distinctive U-shaped valleys that glaciers create.

  • Step Four- Glacial melting filled the depressions forming the Great Lakes

  • Parallel Ridges- Isle Royale has very distinctive ridges that give clues about the crust.


Where is Isle Royale?

Isle Royale is an island of the Great Lakes, located in the northwest of Lake Superior, and part of the state of Michigan. 

Isle Royale in Lake Superior

The island and the 450 surrounding smaller islands and waters make up Isle Royale National Park.  View the videos to experience Isle Royale (click on the links below):

Isle Royale: Michigan's Superior Wilderness

Isle Royale National Park: Impressions

How did Isle Royale form?

The science of geology  studies the structure and function of the Earth.  Geologists  study rocks and rock layers to learn how an area was formed.  Isle Royale was formed by igneous rock layers that folded as a result of tectonic plate movement.  The folded rock layers curve downward forming the Lake Superior basin  and then curve back up again to form the Keweenaw pennisula (see the diagram below).  

Cross Section of Lake Superior

North American Rift

Step One

About one billion years ago the land that is the continent of North America begin to pull apart.  This rift  allowed magma from the mantle to ooze out into the rift valley .  This is the reason there is so much igneous rock found in these areas including Isle Royale and the Lake Superior Basin.

If this process would have continued, an ocean would have formed in the middle of what is now the United States.  The plates stopped pulling apart and the magma  stopped flowing out of the Earth. 

Click on Plate Tectonics to view a video that show rifting.

Step Two - About 1 million years ago, the ice ages began.  Each ice age lasted about 50,000 years seperated by warmer periods.  This occurred 8 to 12 times. The last ice age retreated abut 14,000 years ago.  During these ice ages, glaciers  formed and slid southward creating huge depressions .  As the glaciers melted, the water filled the depressions forming the Great Lakes.

View this glacier animation to understand what causes a glacier to move. 

Also view these four animations.   

Many of the surface features of Isle Royale are caused by the scraping of the glaciers that slid across it.

diagram of ice sheet during the ice ages

Step Three - Imagine placing a baseball on a mattress.  What would happen?  The Earth's gravity would act on the baseball's mass causing it to push down on the bed.  Now imagine placing a bowling ball on a mattress.  What would happen?  Would the mattress react differently to the bowling ball than it did to the baseball? 

The glaciers that covered the Great Lakes region during the ice ages were massive.  The Earth's gravity pulled down on the glaciers.  The crust was already thinner in this region due to the forces that caused the ancient rift valley, so it could not resist the downward thrust of the glaciers.  The result were deep depressions.  This process is called isostatic depression  When the ice of the glaciers melts the depression acted like a bowl holding the water.  


Isostatic Depression

Primary Succession: The Appearance of Life- Once the glacier had melted, bare rock was exposed.  It is very difficult for an ecosystem   to become established on bare rock.  Primary succession  is the process in which an ecosystem is established in a new environment.  Lichen  grow and chemcially erode the rock forming the beginnings of soil.  After many years, very small scrubs will grow.  This will attract small animals.  As more siol ins formed, grasses start to grow, then small trees and the animals they support.  It takes several decades for the system to mature to the climax community .     

Primary Succession

U-Shaped Valley

U-Shaped Valleys

Glaciers erode the land as they move causing a u-shaped valley  or gouge.  This usually happens in the mountains so the water that melts will flow down into rivers. You should remember that the Great Lakes area is part of an ancient rift valley, so the land is flat and some what lower than the areas east and west of the rift valley. So in the case of the Great Lakes, because the land was flat, the glaciers caused a deep depression that holds water. 

Step Four - As the climate warmed after the last ice age, the water from the glaciers filled the depressions forming the Great Lakes.


Sequence of glacial melting

Isle Royale ridges

Water between the ridges

Parallel Ridges- The island has many distinctive parallel ridges.  These ridges provide clues about the structure of the crust below Lake Superior.  Scientists studied these ridges revealing much about the structure of the crust below Lake Superior. The ridges are the edges of the exposed rock layers that are angled down forming the Lake Superior basin.  The layers rise up again on the southeast side of the lake at Keneewa pennisula.  The pennisula has these distinctive parallel ridges too.  Scientist matched the layers by their type of rock and their ages to prove that they are the same layers seen at Isle Royal.  They explain that massive glaciers caused isostatic depression resulting in the Lake Superior basin. 

Would you go to Isle Royale if you had the chance?  What would you look for when you go there?

Isle Royale in winter