Geology of the Apostle Islands
The Apostle Islands are built out of a porous red sandstone rock. Sandstone is
made out of sand that has been cemented together over time. The sand was deposited
in this area during the late Proterozoic period in Earth’s history about 700 million years
ago. The sand is evidence of the large sea’s that covered most of the earth during that
era, and the beaches and dunes that formed along its edges. Persistent battering of
continents by the wind and waves gradually wore away mountains; Boulders and
pebbles turned into sand and other sediments that were then carried downstream.
Rivers and ocean currents redistributed the sand and sediments and deposited them
The thousands of feet of sand were compressed by a succession of glaciations
over the last 40 million years. In the last glacial maximum period, about 12,000 years
ago, the Great Lakes were covered by an ice sheet two miles high. Take the Sears
tower in Chicago and stack 8 of them on top of each other and that’s how tall the ice
sheet was. As you can imagine all of that ice was very heavy and as a result
compressed the land underneath. As the glaciers receded they left behind the scour
marks of their weight and power that today form the Great Lakes. The series of small
kettle lakes surrounded by hilly moraines found throughout Michigan, Wisconsin and
Minnesota are also evidence of the glaciers retreat into Canada and their eventual
disappearance as the climate warmed.
Once the glaciers fully retreated from the landscape the land that was
compressed by the weight of the glaciers began to slowly rebound. The Apostle Islands
formed when the compressed sandstone began to rise, and gradually got eroded away by the wind and waves of Lake Superior. Oak Island, the tallest of the 22 islands, shows
how far the landscape has risen since relieved of the compressive force of the glaciers.
Over the past few thousand years the water levels in Lake Superior have varied,
and the sea caves were carved out as the water and force of waves broke down the soft
sandstone layers. The rise and fall of the lake levels explains why you can also see
caves below the surface of the water. Those were carved out when the lake levels were
lower than at present day. When you look at the caves you can clearly see different
layers of sandstone, accented by the different colors. The colors are a result of the
different minerals present in the rock, the most prominent being iron which tints the rock
red. Black streaking on the surface of the sandstone as well as within the stone comes
from trace amounts of manganese that turns black when mixed with oxygen in the air
and wind. You can think of it as a different kind of ‘rust’. If you look closely you will be
able to see the individual sand grains and the rock with be rough to the touch much like
sandpaper. Today the wind and waves still play an important role in the formation of the
caves and are the primary cause of erosion along the coastline. Water that seeps into
the cracks in the rock freezes in the winter, as water turns to ice it expands, further
expanding the crack and creating instability in the rocks during the spring thaw.
Sometimes the cracks in the rock become so large that large pieces of the coastline will
break off in mass wasting events. This mass wasting makes it easy to find sizable pieces of sandstone on the beaches that can be studied up close. Sandstone is rough to the touch, like sandpaper, and if you look closely you can see the individual sand grainscontained within.
Lake Superior Agates
The beaches of Lake Superior are a rock hunters dream. Every major rock type
can be found on the beaches around the Chequamegon Bay region. Metamorphic rocks
that have been crushed and heated deep within the earth have been recrystallize into
schist and slate. Sedimentary rocks show us the sand and pebbles that were tumbled by ancient seas are now cemented together to make sandstone and conglomerates that
look much like concrete. Igneous rocks are also found here. Igneous rocks are created
from tectonic and volcanic activity. There is a very old fault line that runs underneath
Lake Superior. During a period of uplift a section of the fault thrust toward the surface,
creating what is now Isle Royale, Lake Superior’s biggest island. Basalt is a remnant of
the old volcanic rift that runs under Lake Superior and is responsible for the formation of
Isle Royale long before any glacial advances. Basalt is the parent rock of the most
coveted Lake Superior Agate.
Agates form all over the world and are in no way a unique formation to this
region. When a volcano erupts, magma is often ejected along with a mixture of gasses.
While most of the volcanic gases escape as the lava moves downhill sometimes the lava cools before all of the gas bubbles escape. This can happen when lava erupts into a body of water or when a volcano explodes in a very cold climate. Those gas bubbles are now forever trapped in the basalt rock and leave behind hollow spaces in the rock.
That’s what often gives volcanic rocks their rough texture or the look of Swiss cheese
when we see them at the surface. Volcanic basalt and scoria are frequently used in
landscaping so it’s possible to find a volcanic rock far from any fault zones or volcanoes. The spaces in the basalt left by the volcanic gases are the birthplace of all agates. Agates are formed inside of volcanic rock after it has cooled and is sitting exposed to the
water table. When groundwater percolates through bedrock it picks up trace minerals and allows for chemical movement through rock layers. When silica rich ground water moves through basalt, it fills up the vacant spaces left by the volcanic gas. The water then deposits the minerals it’s carrying around the walls of the empty air pocket. As you can imagine a lot of different minerals can fill those empty spaces and not all of them will make agates. Agates are specifically made out of a micro-crystalline form of Quartz
(SiO 2 ) that gets deposited slowly, one layer at a time from the outside in, until the whole air pocket has been filled with tiny layers of quartz. Agates form so slowly and are made of such small building blocks that it makes the resulting agate very dense and more resistant to weathering and erosion than its surrounding parent rock, basalt. Over time, when the basalt is moved closer to the surface and begins to erode away, the agates will pop out of the basalt. Whole agates don’t look like much, its only when they are broken or cut open that their true beauty is revealed. The colors of an agate can tell you a lot about where it formed, and what other minerals were present in the ground water that that time. The colors of different agates are a direct result of the other trace elements that found its way into the air pocket with the quarts. Lake Superior Agates are mostly red-banded agates which is a sign of the presence of iron. The white banding is ‘pure’ quartz with less iron. Lake Superior’s diverse landscape has lead to an even more diverse set of agates. Regions along the lake have different and unique agate formations and a lot of books that have been written that go into detail about their intricate differences.
Written By Barbara Richter