Beach Stones for Rock Lovers
This is the second article in a series about beach stones often found while walking the beaches of Lake Michigan, particularly in Southwestern Michigan USA, which includes interesting facts, beautiful photos and curious information how they were formed. In the first article I featured various forms of basalt, septarian brown stones, limestone, granite, gabbro, diorite, gneiss, schist, sandstone, silt stone, mud stone, geodes, chalcedony and agate (link provided below). This article features various forms of syenite, rhyolite, pumice, dolomite, quartz, quartz veining, wishing stones, heart stones, quartzite, Presque Island serpentinite, diabase, pegmatite, conglomerate and banded metamorphic rocks, in that order! As with all beach stones, they are polished smooth from the sand, wind and wave action by the fresh waters of our inland ocean, Lake Michigan.
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- I Found A Rock On The Beach And Wondered
The first in a series about Lake Michigan beach stones with fascinating facts and beautiful photos featuring basalt, septarian brown stone, granite, gneiss, diorite, gabbro, limestone, sandstone, silt stone, schist and chalcedony and agate.
Syenite is a medium to coarse grain igneous rock similar to granite which solidifies slowly in the crust in a similar manner. Whereas quartz is an important mineral in the granite, it's lacking in syenite. Careful examination will show that syenite is composed of long prisms of the dark mineral hornblende (rather that the scaly biotite mica) and feldspar which is the chief component of the rock.
The pink coloring of syenite is due to the presence of alkali feldspar, which predominates in syenite, but this rock type is found in a wide variety of colors distinguished according to the minerals contained within. They include augite syenite, hornblende syenite, mica syenite, and nepheline syenite.
I just love finding unique rocks along the beach like these pink and pinkish-orange polk-a-dot syenite rocks! It's gratifying knowing a thing or two about all the rocks, but especially the less common!
Syenite is not a common rock in Michigan comparatively. They are occasionally substituted for granite as building stones.
Rhyolite is a felsic (silica-rich) volcanic igneous rock composed with the same mineral content as granite, only while in its molten rock form, unlike granite, it cools fast (extrusive type) near or over the surface of the crust. When these magmas erupt, a rock with two grain sizes can form. The larger crystals that form just beneath the surface which cool slower are called phenocrysts, and the small undetectable crystals that form at or above the surface which cool quicker are called ground mass with a micro-crystalline matrix.
To review "porphorytic" or "porphyry" - an igneous rock with one mineral (called the phenocryst) exhibiting a grain size larger than the remainder of the minerals (called the groundmass). Refer to my first article about Lake Michigan beach stones as to how they form.
As I mentioned in my first rock article, the igneous volcanic rock of Michigan is very old. If you hold in your hand a sample of rhyolite, along with some other types, you're holding something that likely formed a billion years ago or more . . . amazing.
Rhyolite rocks are often colored red to brown, pinkish to gray with fine grain, sometimes exhibiting banding caused by hot rock that is still flowing before its cooled and hardened.
Rhyolite usually forms in continental volcanic eruptions and is rarely produced at oceanic eruptions. Rhyolites are known from all parts of the Earth and from all geologic ages.
Rhyolite Pumice - Due to the high silica content, rhyolite lava is very viscous . . . it flows slowly, like tooth paste squeezed out of a tube, and tends to pile up to form lava domes. The thicker viscosity traps gas bubbles more easily and if rhyolite magma is gas rich, it can erupt explosively, forming a frothy solidified magma called pumice (a very lightweight, light-colored, vesicular (pitted) form of rhyolite) which includes ash deposits.
Eruptions of granite magma can produce rhyolite, pumice, obsidian, or tuff. These rocks have similar compositions but different cooling conditions. Explosive eruptions produce tuff or pumice. Effusive (slow) eruptions produce rhyolite or obsidian if the lava cools rapidly. These different rock types can all be found in the products of a single eruption.
There is considerable amount of confusion over the name of this rock. The problem is that dolomite is both a mineral and a rock type. Dolomite rock is a very common sedimentary rock derived from limestone with a high percentage of the mineral dolomite. The two types are often indistinguishable in the field and geologists usually carry diluted hydrochloric acid to test the rocks. Limestone is strongly effervescent in acid, while dolomite reacts very weakly.
As with limestone, dolomite originates in warm, shallow, marine environments where calcium carbonate accumulates from shell, algae or coral fragments. They are widespread in the Cambrian Period throughout the world. Limestone and dolomite also share the same color ranges of white to light gray, yellowish, green, purplish and black are possible. They also can both exhibit fossils.
Dolomite stone forms in several ways, but the main type is a former limestone that was precipitated by calcium magnesium carbonate (mineral dolomite) through the action of magnesium-bearing water percolating the limestone or limy mud and replacing the limestone calcium carbonate minerals of aragonite or calcite.
As you can guess, I find these medium size dolostone boulders quite often along the shoreline in key locations. They are so colorful and make a wonderful display around my garden landscape.
Did you know quartz is the single most abundant mineral on the planet? Quartz is made up of the elements silicon and oxygen, otherwise known as silica. Quartz can form large, six-sided crystals in rocks or can be found within rock cavities such as granite or also fill rock vesicles (gas bubbles) during the cooling process of molten rock.
It can be found in a large range of sizes as masses larger than a basketball or crystal points smaller than a pea. Several varieties of quartz are microcrystalline, (too small to be seen with the naked eye). These include agate, jasper, chert and chalcedony. Other well crystallized varieties are named for the different colors; amethyst contains impurities of iron and aluminum, smokey quartz is colored by aluminum, red quartz is iron stained.
Milky quartz is the most common variety of crystalline quartz. The cloudiness of milky quartz comes from microscopic inclusions of fluids, gas or both that have been encased in the crystal from the time the crystal first grew. The inclusions have spoiled the crystal for optical purposes and for the use in jewelry making gemstones.
By holding the milky quartz up to the sun, the light can be seen through the translucence of this stone! Of course, we find these in their rounded polished form due to the weathering action of the big lake!
The quartz veining in these samples certainly elicit curiosity. One can't help but marvel and wonder how. There are a few methods, but the simplest type of a quartz veining is by the filling of a crack in a rock. The crack might have formed during folding of rock in mountain-building processes, or by shattering during tectonic events, or by a decrease in pressure during the uplift of rock, or because a rock cools down and shrinks. Later, hot brines migrate through the cracks in the host rock depositing various minerals which may or may not crystallize.
A "wishing stone" is nothing more than a stone with quartz veining occurring in various host rocks such as basalt, as in the case above! But in order to be a true "wishing stone" it can only have a single vein which circles entirely around the stone without any breaks. If you're lucky enough to find one, close your eyes, make a wish, then throw the stone into the water as far as you can and your dream wish will come true. The wishing stone in the photo above is shaped like a heart which makes it even more special!
Heart Shaped Stones
Speaking of heart stones, they are one of my favorite stones to collect. I think of them as heart-warming messages from loved-ones who have passed!
Can you guess the various rock types of these heart stones based on the information I have already provided in my articles about Lake Michigan beach stones I and II?
Quartzite is a metamorphic rock composed almost entirely from sandstone. The sandstone is altered by heat, immense pressure, and chemical activity. These conditions recrystallize the sand grains and the silica cement that bind them. The result is a network of interlocking quartz grains of incredible strength. Because it is so hard and dense, quartzite has not been quarried as extensively as other stones such as limestone, sandstone and granite.
Quartzite ranks high on the Mohs Hardness Scale as one of the most physically durable and chemically resistant rocks found on Earth. When mountain ranges are worn down by weather and erosion, less-resistant rocks are destroyed, but the quartzite remains. Quartzite is also a poor soil-former. Unlike feldspars, which break down to form clay minerals, the weathering debris of quartzite is quartz.
In identifying the rock, geologists have learned to strike quartzite with a rock hammer only when necessary and to tap softly to avoid possible sparks or sharp pieces of rock flying at high velocity due to the way the rock breaks apart.
Click on the images so can see the individual grains of quartz in these Lake Michigan samples!
Quartzite can be a very attractive stone when it is colored by inclusions. It is usually white to gray in color, but some rocks are stained by iron and can be pink, as with these Lake Michigan samples, or red, or purple. Other impurities can cause quartzite to be yellow, orange, brown, green, or blue.
Presque Isle Beach Stones (Serpentinite)
These heavily veined cobble beach stones came from Presque Isle Park in Marquette, Michigan of Upper Michigan bordering Lake Superior. One source I found states it is serpentinized peridotite (serpentinite rock) of the Mona Formation, Archean in age - 2.6 billion years. Most locals simply call them Presque Isle Stones.
For those interested it this unusual rock's formation, here is a simplification for us non-geo types.
To begin, peridotite is a dense, coarse-grained igneous rock consisting mostly of the minerals olivine and pyroxene with lesser amounts of chromite, plagioclase and amphibole varying its compositions. Peridotite is ultramafic (rock containing less than 45% silica). It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron.
Peridotite is the dominant rock of the upper part of the Earth's mantle either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle.
Serpentinized is a process whereby rock (usually ultramafic as with peridotite) is changed, with the addition of water, heat and pressure into the crystal structure of the minerals found within the rock. Serpentinization of peridotite into serpentinite (the metamorphic equivalent) is a common example of this process. Way to stick with it!
Michigan and The Great Lakes and Great Lakes States of USA
Diabase (Dolerite - Older Term) is a dark rock that may have light colored lath shaped (flat elongated) grains. Like basalt, rhyolite and gabbro, diabase is a common Michigan volcanic rock found along the shoreline as a beach worn stone.
It is equivalent to gabbro and basalt in composition, but between them in texture. The term “microgabbro” is sometimes used to refer to such rocks, but they cooled closer to earth's surface, hardening much faster, and therefore have far fewer visible crystals than gabbro. They are classed as a separate rock because of the peculiar lath shaped crystals of "plagioclase" lime-feldspar minerals (mostly labradorite) in a ground mass of the "pyroxene" mineral, augite.
To review plagioclase minerals; any member of the series of abundant feldspar minerals usually occurring as light-colored, glassy, transparent to translucent, brittle crystals.
To review pyroxene minerals; any of a large class of rock-forming silicate minerals, usually dark-colored, generally containing calcium, magnesium, and iron and typically occurring as prismatic crystals.
Diabase minerals of lesser importance are magnetite, olivine, ilmenite, hornblende, biotite, chlorite, etc.
Note: Specimens with few visible crystals can easily be confused with basalt, and a microscope would be necessary to distinguish the two.
Why do the lighter feldspar crystals often appear fuzzy or flat stick-like in shaped in diabase rock? This is because they crystallized first, forcing the other, darker minerals to squeeze in around them, which distorted the feldspar. This is the opposite of what generally happens in rock formation; the dark minerals tend to crystallize first.
Colors can vary with diabase from gray to black, greenish black, and brown.
Pegmatite is an extreme plutonic igneous coarse-grained granite that forms during the final stage of magma’s crystallization. They are extreme because they contain exceptionally large crystals made of feldspar, quartz and mica as with granite. Many of the crystals can be from several inches to a foot or more in diameter. It is the parent rock of many gem stones including topaz, tourmaline and including rare and valuable minerals such as beryl and others.
Note: Even though my pegmatite beach stone sample has large coarse grains, Lake Michigan has molded it into a round ball demonstrating the powerful tumbling action of the wind, waves and sands of the inland ocean.
Pegmatite is seldom in large masses but is usually in veins cutting through other kinds of rock such as granite and diorite. Pegmatite should not be confused with porphyritic granite (see my first beach stone article) as the two can be distinguished by the relative size of the mineral grains. In pegmatite, the crystals are uniformly large, but in granite porphyry usually one mineral is in large crystals within the finer ground mass.
To indicate the mineral composition, pegmatite can be “granite pegmatite”, “gabbro pegmatite,” “syenite pegmatite,” and any other plutonic rock name combined with “pegmatite” are possible. My first sample reminds me of the salt and pepper grains contained in diorite, so it could possibly be "diorite pegmatite".
Diorite is primarily composed of feldspar and various dark-colored minerals, which explains its black and creamy white coloring. Granite is composed of four materials: feldspar, mica, quartz and hornblende minerals. These minerals themselves come in a variety of forms, giving granite a much larger variety than diorite. I featured both diorite and granite in my first photo essay about beach stones.
Conglomerates "Pudding Stones"
Conglomerates are sedimentary rocks with inclusions of rock pieces of various sizes and shapes cemented with sand and pebbles by dissolved minerals. Heat and pressure during long periods of geological time molds the mixture and holds it together. The pebbles and small rocks in a conglomerate are typically rounded, a feature that differentiates them from "breccias" where the larger stones in the mix are angular. They are not the most common rocks I find on the beach, but nevertheless, a regular find. They are often referred to as "Pudding Stones".
These conglomerate samples are actually man-made cemented together with tar for road construction, however, they find their way into our beaches with the tell-tale polishing from the wind, waves and sand action!
Banded Metamorphic Stones
I've always been attracted to these banded beauties and have fun arranging them in my rock garden. The samples directly above and below are metamorphic slate transformed from sedimentary shale, but haven't yet identified the others. Feel free to make a recommendation if you think you know.
To briefly review metamorphic rocks. Metamorphism involves the alteration of existing rocks by either excessive heat and pressure, or through the chemical action of permeating fluids. This alteration can cause chemical changes or structural modification to the minerals making up the rock.
Metamorphic Rock Traits
- Because their mineral grains grew together tightly during metamorphism, they're generally strong rocks.
- They're made of different mineral than other kinds of rocks and have a wide range of color and luster.
- They often show signs of stretching or squeezing, giving them a striped appearance.
Of the metamorphic rocks I have covered, slate, gneiss, and schist have a layered or banded appearance, but metamorphic quartzite does not have a layered or banded appearance.
I leave you with a beautiful photo of Lake Michigan beach stones shimmering in the shallows!