I Found a Rock on the Beach and Wondered | Owlcation
Their composition is similar, but due to metamorphism (application of pressure and heat), in a gneiss the minerals are arranged in plates, giving. Granite Gneiss CONTACT RELATIONSHIPS WITH INTRUDED ROCKS. CONTACT Granite and comparable analyses of average granodiorite and granite •. Vishnu Schist and Zoroaster Granite granite (also subsequently metamorphosed to form granite gneiss). You read the walls and layers of rock the way one might read a poem or an unfamiliar expression on a loved-one's face. . the human heart and relationships through compelling stories and essays.
The porphyritic crystals are generally white, pink or orange. Porphyry rock is mostly made up of a basalt base, but sometimes granite with jagged, rectangular crystals. You can clearly see this in the sample I have provided above which was found on our Lake Michigan beach.
Here's how it happens: As the feldspar begins to crystalize, the process is disturbed when the molten rock is quickly erupted, freezing the well-formed feldspar crystals in place while the rest of the rock quickly cools and fills in around them. Gabbro Gabbro Rock - Lake Michigan Beach Stone Gabbro is igneous rock which cooled slowly intrusive deep below the Earth's surface which caused the minerals to crystallize. Gabbro can also be gray and dark green.
A lesser amount of light-colored mineral grains may also be present. Unlike many other igneous rocks, gabbro usually contains very little quartz, although the sample I collected has a quartz vein running all the way around it.
Gabbro has the same mineral composition as basalt olivine and pyroxene with smaller amounts of feldspar and mica. But whether basalt or gabbro forms, depends upon the cooling rate of the magma, not the composition of the magma. While gabbro is coarse grained which cooled slowly intrusivebasalt is fine grained because it cooled quickly extrusive. Diorite Diorte - Lake Michigan Beach Stone Diorite is another one of several coarse grain igneous stones that can easily be confused with granite.
The chemical composition is intermediate between gabbro and granite. The best way to tell diorite from granite is the salt and pepper appearance; and to tell diorite from gabbro is by the darker color of gabbro. Diorite is composed with an almost equal mixture of the light colored mineral, sodium-rich plagioclase a different type of feldspar mineral and dark colored minerals such as amphibole, hornblende, or biotite mica. They are some of the most attractive stones, like gneiss for instance, which I only occasionally find on the beach.
Gneiss pronounced "nice" usually forms at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallize, enlarge, flatten, and reorganize into parallel bands under intense heat and pressure which make the rock and its minerals more stable. While the chemical composition of the rock may not have changed, the physical structure of the rock will look completely different from the original parent rock. The bands in gneiss are often broken, can be folded foliated and can be different widths.
Individual bands are usually mm in thickness. Layers larger than that imply that partial melting or the introduction of new material probably took place. Such rocks are called "migmatites". It is not well understood how the segregation takes place.
Gneiss Cobbler - Lake Michigan Beach Stone The granular light-colored minerals in gneiss are calcium, sodium, and potassium rich minerals such as quartz, and also various types of feldspar. The alternating dark-colored layers consist of iron magnesium rich minerals including biotite, chlorite, garnet, graphite and hornblende.
The texture is medium to course, coarser grained than schist but as with the other rock types, the gneiss we find on our beaches has been grounded down somewhat smooth. What is the difference between Gneiss and Granite? There is also diorite gneiss, biotite gneiss, garnet gneiss, and a few more. Schist Schist - Lake Michigan Beach Stone Schist is medium grade metamorphic rock, formed by the metamorphosis of mud stone and shale or some types of igneous rock such as slate.
The result of high temperatures and pressures are the coarser mica minerals biotite, chlorite, muscovite forming larger crystals. These larger crystals reflect light so that schist often has a high luster. Due to the more extreme formation conditions, schist often shows complex folding patterns. Schist shows a tendency to split into sheets. The mica plates are arranged roughly parallel to each other, which is why the rock shows this tendency.
There are many varieties of schist and they are named for the dominant mineral comprising the rock, e. I find these somewhat regularly on the shoreline. The lustery shiny samples are much less common of which I've found only one.
Sand Stone Sand Stone Boulder - Lake Michigan Beach Stone Sand Stone is a sedimentary rock that forms when small quartz sand grains cement together from high pressure while silica, calcium carbonate calcite or quartz precipitates and acts like a glue around the grains.
These minerals are deposited in the spaces between the sand grains by water. Over the course of thousands or even millions of years the minerals fill up all of the spaces. You can see the tiny particles in the rock as if you were holding sand in your hand. When you're at the beach, try examining the sand very closely for all the tiny quartz crystals and different colors of other minerals contained in it, including feldspars, micas, calcite and clays.
Sandstone Cobbler - Lake Michigan Beach Stones Depending on the minerals, sandstone can be white, yellow, pink and almost any color depending on the impurities within the minerals. For example, red sandstone results from iron oxide in the rock and often causes bands of color. Sandstone rocks form in rivers, deserts, oceans or lakes. It's typically mottled with various pinks, whites and browns exhibiting either many streaks or spherical spots caused by leaching and bleaching.
It forms a wide belt through Northern and Upper Michigan and was quarried rather extensively at one time used for building material to build the cities of Northern Michigan and elsewhere in the Great Lakes region.
As with many stones formed elsewhere in Michigan, the big lake brings them southward to where I find them is lesser amounts. Estimates for the age of the Jacobsville Formation range is from the late Mesoproterozoic Era about 1.
Silt Stone Silt Stone Cobbler- Lake Michigan Beach Stone After some stubborn digging around, I finally believe I understand the difference between sand stone, silt stone, mud stone, clay stone and shale.
At each step the particles become smaller with shale having the finest grain. Silt Stone All the clastic sedimentary rocks mentioned above are cemented very much the same way in which sand stone is pressed together.
Silica, calcite, and iron oxides are the most common cementing minerals for silt stone. These minerals are deposited in the spaces between the silt grains by water. Over the course of thousands or millions of years, the minerals fill up all of the spaces resulting in solid rock.
Silt accumulates in sedimentary basins throughout the world. It occurs where current, wave, or wind energy cause sand and mud to accumulate. Silt stone is very similar in appearance to sandstone, but with a much finer texture. It has a gritty texture to it and is more difficult to distinguish the mineral particles. When handling silt stone, a residue the color of the stone can rub off on you hand. Silt stone is usually gray, brown, or reddish brown. Silt stone can also be white, yellow, green, red, purple, orange, black, and other colors.
The colors are a response to the composition of the grains, the composition of the cement, or stains from subsurface waters. But I will mention that we especially find the brown mud stones on the beach. They are the same type of stone that form the septarian brown stones. Both the mud stones and clay stones wipe off a residue when you're handling them from their fine grained texture.
The last stone in the chain of the clastic stones for the finest ground down grains, is slate, which we find very little of. This could be because slate breaks apart at parallel stratifications and perhaps because of the extreme ice, wind and wave action of Lake Michigan, they get demolished, but I can only guess.
Geodes Lake Michigan Beach Geode Geodes are one of the less common finds along the beaches, but it's very exciting when you do find one. They begin their formation as hollow rocks. As a volcanic rock, geodes start out as bubbles, but can also form in areas other than volcanoes.
In sedimentary rocks, geodes can start out as animal burrows, tree roots or mud deposits which over time forms the hollow cavity within the rock while the outer edges harden and form a sphere. Mineral rich ground water infiltrates the cavity and many years in the making crystallize in various colors depending on the mineral content such as quartz and amethyst.
They are only as big as a penny and have a smooth, waxy texture. In order to spot these on the beach in Southwestern Michigan you have to look very closely along the shoreline where gravel is abundant, but I have found quite a few and they're usually tiny like these samples.
Michigan's northern regions and upper peninsula are excellent places for finding agates. Chalcedony and Agates Explained. Agate - Southwestern Michigan Beach Stone Rock and minerals can be very complicated, but fascinating to study. For a bit of geochemistry about chalcedony and agates, it only makes sense to begin with the microchrystalline quartz, chalcedony.
Chalcedony forms where water is rich in dissolved silica and flows through weathering rock. When the solution is highly concentrated, a silica gel can form on the walls of the rock cavities. That gel will slowly crystallize to form microcrystalline quartz very small crystals of quartz in other words, chalcedony. So why begin with chalcedony? Because agate as well as jasper are both varieties of chalcedony. They are considered gemstones. Chalcedony can be banded, have plume fluffy inclusionbranching patterns, a delicately mottled surface of leafy green, honey brown and creamy white; or also have a mossy and other colorful structures within.
It's often blue but can be almost any color. It's always translucent, never opaque or transparent. It feels waxy, greasy or silky. Agate - Lake Michigan Beach Stones Both agate and jasper form somewhat differently from chalcedony giving them their unique properties.
Many agates form in areas of volcanic activity and crystallize slowly in the cavities of igneous rock or limestone.
The Granite Pail: The Selected Poems by Lorine Niedecker
Jasper forms when fine materials are cemented by silica to a solid mass. These fine particles give jasper its color and opacity. Agate is generally translucent to semitransparent and most often banded. Observing bands in a specimen of chalcedony is a very good clue that you have an agate. However, some agates do not have obvious bands. In the early days, artistry in stone wall building had to wait. The first priority was survival, which meant clearing land to grow crops and raise livestock.
At Harvard Forest — a 1,hectare forest laboratory and classroom established by Harvard University in in Petersham, Mass. European settlement, and the beginning of deforestation, largely occurred in the 18th century. By the midth century, 60 to 80 percent of the land had been cleared.
I Found a Rock on the Beach and Wondered
After farming began to decline, abandoned pastures and fields rapidly developed into white pine forests, which obscured the stone walls. The pines were logged and succeeded by the mixed hardwoods seen today. The types of stones and their abundance may have been familiar to those early farmers, who were mainly from the British Isles, Thorson says, because rock in New England is similar to rock in England and Scotland. England and New England have similar natural landscapes because both lands have a similar geologic history.
Millions of years ago, England and New England were formed within the same mountain range near the center of Pangaea. Britain had long been deforested, with its subterranean stones brought to the surface, so its stone walls had been constructed hundreds, if not thousands, of years earlier.
Simultaneously, a post-Revolutionary War baby boom provided an abundance of young hands to help move them. During this period, thousands of stone walls were built and thousands more were improved. Constructing the walls was labor intensive. For comparison, modern masons typically lay about 6 meters of stone wall per day, Thorson says. That job usually had been done stone by stone, and load by load, by the previous generation. Shortly after the Revolutionary War, however, that began to change.
He wrote about this stone wall, on his farm in Derry, N. The white pines are going to shoot up. When I see a stone which it must have taken many yoke of oxen to move, lying in a bank wall I am curiously surprised, because it suggests an energy and force of which we have no memorials.
People refurbished rural stone walls on properties that had been abandoned generations earlier. A March study in the Journal of Archaeological Science offers a fascinating glimpse of what lies beneath the forests that now envelop many New England farms abandoned in the latter half of the 19th century.
Using a laser mapping technique called lidar that can see landscapes even through dense forest cover, University of Connecticut geographers Katharine Johnson and William Ouimet conducted aerial surveys of the heavily forested areas of three southern New England towns. Reading the walls Credit: