Dating back to the 18th century concretions have been known as geologic curiosity s due to the various sizes, shapes and compositions. Concretions have also been thought to be dinosaur eggs, extra-terrestrial debris, human artifacts and animal and plant fossils. Due to these curiosity s I will try and enlighten you more on these in the following by explaining the process in which they are formed and explaining some locations where they can be found in large outcrops.
The word “concretion” is derived from the Latin “con”– meaning “together” — and “cresco” — meaning “to grow.”
Concretions are hard compact accumulations of mineral matter and are found inside sedimentary rocks. Some examples of this are Sandstone and in some weathered volcanic rock. Concretions come in many different shapes and the most common of the shapes is spherical or disk shaped. Concretions are the most varied-shaped rocks of the sedimentary world. The way concretions come to be is the mineral matter concentrates around the nucleus of a host rock. The nucleus is often organic such as a tooth or leaf or shell or fossil. As the mineral matter concentrates around the nucleus it forms harder zones known as nodules.
Concretions are very odd in the sense that they very in size, shape, color and hardness. Often concretions are mistaken for bones, fossils, meteorites and other odd objects. They can be so small that it requires a magnifying glass to be visible or as large as 10 feet in diameter and weigh hundreds of pounds. Concretions can also have somewhat of regular shapes such as boxes, blocks, flat disks, pipes, cannon balls and have even been known to resemble parts of a human body such as a foot or ribs.
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Concretions are most commonly composed of calcite but sometimes can be composed of iron oxide or iron hydroxide such as goethite. But also concretions can be composed of other minerals ranging from siderite, ankerite, dolomite, pyrite, baryite and gypsum.
Concretions form in many different ways. The box shape of some ironstone concretions most often depend on the way a sandstone or shale bed breaks up due to the action of weathering into regular blocks of various sizes. The way in which this separation takes place along natural planes of weakness in a rock such as a horizontal bedding surface and vertical joints. Before this process of separation, as well as during the separation ground water soaks into the rock and circulates through the planes of weakness making the rock more porous. In these iron stone concretions water dissolves the iron compounds from the inner portion of a block and then deposits the iron compounds as insoluble iron oxide or iron hydroxide in the outer parts of the block. As the water dissolves the iron hydroxide/oxide and deposits them in the outer portion it cements together grains in the original host rock to make that specific zone harder. One way to tell that it is an ironstone concretion is that there is often brown, yellow or red concentric ironstone bands inside the block. This process only takes place as long as the rock is below the ground water level. When the water level drops is when the drying and oxidation begin to take place. After this is when the concretion may be released from the surrounding softer rock due to weathering and also because the concretion is harder and more resistant. They will be found as separate objects on the ground or in the soil and as erosion takes place it wears down the edges and corners of the concretion giving it the typical round or disk shaped appearance.
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Concretions can also be formed by building up layers of material such as a nucleus as I stated before. The ground water around them contained with dissolved iron, calcium, silicon and other chemicals will drop them as calcium carbonate, silica solids or as iron oxide when the chemical conditions change. Every time this happens it forms a thin layer and as the layers build up they may contain different concentrations of compounds which in turn shows up as different colors.
Some concretions can also be hollow containing an empty center filled with powdery clay and sand or can include a detached hard lump resembling a nut. The loose powder in the middle can be explaind by the iron oxide has bee drawn away from the middle and towards the outside contributing to the hard iron oxide shell. If there isn t any powder inside the concretion and the center is hollow it means that cracks in it have allowed the powder to escape. If when you pick up the concretion and shake you here a rattling that means that it contains a loose detached lump. This is caused by some internal shrinkage and both the nut and shell might show banding.
Though there are many different kinds of concretions there are also many different places that have such unusual concretions. The Pumpkin Patch Concretions that can be found in the Colorado Desert and in southeastern California are one example of this. These concretions are strange masses that form from the cementing of sand particles. The concretions found in those areas vary from realistic rounded, flattened or spherical shapes to more unusual and imaginative shapes. These unusual concretions also have unusual names like botryoidal (bunch of grapes), fusiform (like a spindle) and ameboid (of the microscopic creature).
The reason for the name of these concretions in this specific area is probably very easy to guess. These concretions resemble pumpkins in their size, shape and distribution . This landscape has come to be because of both precipitation and diagensis to form the concretions. But without the wind and water that are continuously eroding the surface soil these concretions wouldn t be uncoverd so you can see them.
Other outcrops of concretions can be found in Roosevelt National Park in South Dakota. These concretions are almost 10 feet in diameter and are red. The reason that these concretions are red is because of the iron-bearing minerals which are goethite and hematite which are also present in the surrounding sediment, a sandstone that eroded away at this outcrop to expose the concretions .
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Kettle Point in southern Ontario is another place where a major outcrop of concretions occur. Along 500 feet of the lake shore of Lake Huron is where these outcrops are located exactly. These concretions are often refeard to as kettles because of their resemblance to a large cooking kettle. These concretions aren t as large as the ones found in South Dakota but range in size from 1 to 5 feet in diameter. The outter shell of these concretions often have small indentations that resemble the surface of a golf ball.
I have found that concretions are a very interesting part of geology because of their different characteristics and wide range of locality. When it comes to concretions its almost like treasure hunting due to the fact that they can hold fossils and other debris that has been caught and suspended in the concretion for many years. Concretions are one of the most common items to arouse excitement in people and to be brought to museums for identification . This is because of their unusual shape and size and resemblance to something unusual and alluring. Not only are they fascinating to look for and to open but knowing how they are created and what makes them do what they do is another fascinating part of them. The size of some concretions like the ones in South Dakota is just phenominal. Just sitting back and thinking of how long of a process it was to creat such a mamouth rock. Another thing is how they have such a wide variety of origins that require geologists to hold knowledge in the fields of biology, chemistry, soil science, meteorology and geology itself. All and all concretions are oddities that like humans come in many different sizes and shapes and have personality s of their own. Unlike humans though they will be here for an eternity to come and were here an eternity before.Bibliography
1.) Concretions-DesertUSA. www.desertusa.com/mag98/oct/papr/geo_conc.html,
Bob Katz. Sept. 99.
2.) Concretions Thunder Eggs and Geodes. www.austmus.gov.au/is/sand/geods.htm,
Austrailian Museum Online.
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3.) Naturally Weird Concretions. www.rcbm1.rcbm.gov.bc.ca/index.html,
Grant Keddie. 95.