Types of Rocks Found In Lake Michigan: A Guide To the 8 Most Common You’ll Spot

Lake Michigan holds a treasure of rocks from the base of the earth’s crust.

Together with years of mineral wash, pressure, and high temperatures, you can find many blends of colors and patterns.

Types of Rocks Found In Lake Michigan: A Guide

Agate

Agate stone is made up of mainly quartz and chalcedony.

It is common to find agate along the beaches and water edges of Lake Michigan.

They can be various colors depending on the history of the agate.

Thought to be formed within volcanic and metamorphic rocks, an agate species can range from blue to brown and orange and have bands running throughout.

They will have a waxy feel.

Some have pit marks from forming inside an igneous rock that is surrounded by softer rock.

Basalt

Basalt rock comes from volcanic rock and is the base of the earth’s crust.

Because it is formed from molten rock and is quick to cool when exposed to the surface, basalt is dense and finely grained.

Lake Michigan beach stone is the final product of this area’s basalt.

In soft washed colors of gray, brown, rust red, and black, they can be found along the beaches as smooth, and sometimes striped oval and round rocks.

The different types are explained further:

Amygdaloidal Basalt – Trapped bubbles give amygdaloidal basalt a spotty crystalline texture. The variety of minerals trapped will often leave dark green or dark brown crystals obvious to the eye.

Ophitic Basalt – Bumps and irregular knobs of dissolved gas create small craters in ophitic basalt of varying colors and shapes. 

Basalt Porphyry – Crystals of plagioclase can be seen in basalt porphyry due to the different phases of the volcanic eruption. These rocks are lovely and found throughout Southwest Michigan on shorelines.

Vesicular Basalt – Vesicular basalt is absent of any minerals or fillers in the pitted spaces left by the expansion of bubbled gas when trapped within active lava.

Septarian Brown Stones

These stones are formed from the ocean floor up to 50 million years ago.

It is believed that mud and clay became dried out and cracked, leaving spaces for calcite to enter.

Only certain parts of Lake Michigan hold these “turtle shell” looking stones of brown.

The distinct color is formed from the rich iron content found within the soil and the mysterious light brown lines come from the calcite.

Also called Brown Septarian Mudstone, some stones will become dislodged from the calcite and make different-sized smooth stones.

Limestone

Coral, clams, and mollusks are common marine organisms that make up the sedimentary rock known as limestone. 

This rock has several shapes and forms, depending on the minerals and fossils encountered. Here are a few:

Crinoidal Limestone – Crinoid fossils were plant-like organisms that collected microorganisms from the ocean water. You are able to see pieces of the crinoid branches embedded in these bumpy rocks. 

 Fossiliferous Limestone – This is a Lake Michigan Beach Stone that has clear signs of fossils embedded in the stones. Depending on the mineral content, they can be white, pink, red, reddish-brown, gray, or black. Because Lake Michigan is abundant in iron, most of these stones are reddish-brown.

Tuffa Limestone – Tuffa Limestone is very porous due to the shoreline water’s continuous precipitation of calcium carbonate. It is rough and pitted and grayish.

Compact Limestone – These stones have been compacted into tight units of leftover marine organisms and calcium carbonate. Compact limestone can be flat, round, or oval-shaped with a very smooth texture. Colors range from white, pink, red, gray, or black.

Granite

If you believe you have found bird eggs on Lake Michigan Beach, look again.

The lakefronts are filled with egg-shaped stones made of granite.

A mixture of quartz, feldspar, mica, and amphibole hornblende has caused the spotting throughout the granite.

Thousands of years of washes with crystallized minerals make these rocks intriguing.

The glacier melting of Canada make Lake Michigan one of the most endowed areas for showing off these different types of granite.

The dazzling colors of granite depend on the type and amount of mineral content that have been woven into the stone.

These include:

     — Biotite mica – black or dark brown

     — Muscovite mica – metallic gold or yellow

     — Amphibole hornblende – black or dark green

     — Potassium feldspar – salmon pink

     — Plagioclase feldspar – off-white

     — Quartz – white

One more example of granite is referred to porphyritic granite that has larger jagged crystals of white, pink, or orange.

Mudstone or Clay stone

The southwest region of Lake Michigan has mudstone or clay stone scattered throughout the beaches.

They are difficult to separate from sandstone, siltstone, and shale.

It takes millions of years for minerals to fill in the spaces and leave perfectly smooth surfaces.

The location and the natural brown color are usually signs of mudstone over shale.

Geodes

Starting their life as a hollow volcanic rock, they soon fill with gas bubbles.

However, a geode can begin as a tree root or mud deposit that has a hollow space within. 

An outer shell forms around the structure and allows minerals to seep inside while continuing to protect the outside.

The result is a beautiful crystal irregular shaped stone. 

Geodes are not common in the Lake Michigan region, but make for an incredible find when discovered.

Sandstone

Sandstone cobblers along the beaches of Lake Michigan sparkle in the sunlight from the variety of small quartz sands that have filled in holes and cracks over the years.

Do not be surprised to notice tiny particles of silica, calcite, quartz, or impurities wrapped around the sedimentary rock.

Together, they form a type of cement under high pressure to keep the stone intact.  

Jacobstone Redstone Sandstone can be found to the North and throughout Upper Michigan.

It is distinguished by the high iron content that leaves a reddish tint to the stones.

Sandstone cobblers range from off-white, pink, green, and red, depending on the location and types of minerals exposed to the rocks.

Lake Michigan has a wide assortment of rocks for an avid collector.

The lake edges within the state parks are protected from the public removing stones.

Check with local authorities before setting out on your rock-hunting adventure.

types of rocks found in lake michigan