Why does Davis water taste bad? Sedimentary rocks also host economic minerals such as gold and diamonds, which are eroded from other rocks and concentrated to specific areas during sediment transport.
Environmental geology. They host the biosphere, and they are most of the rocks we interact with directly and indirectly. Our actions as humans have an extremely strong effect on sedimentation and erosion. Water expands when it freezes, causing the cracks to widen and grow. Also, water absorbed into the rock makes them swell hydration , enhancing cracking.
You can see this kind of weathering very well in the Lagunas as you drive East on Interstate 8. Rocks also expand and contract as they heat up in the sun and cool down at night.
Different minerals expand and contract different amounts in response to heating and cooling causing stresses along mineral boundaries.
Water not only is an agent of mechanical weathering, but also aids and abets what is known as chemical weathering. Rain water is slightly acidic sometimes a lot more than slightly - as in acid rain. Acid dissolves many minerals; of particular importance in crustal rocks, acid attacks carbonate a common cement in sedimentary rocks and feldspar, a principal ingredient of granite, one of the most important types of crustal rock.
In addition to dissolving, some minerals alter from one form to another. For example, iron-bearing minerals literally "rust" in a process known as oxidation whereby oxygen binds with the iron to form iron-oxides rust. Silicate minerals rich in potassium and aluminum such as potassium feldspar react with water to form clay in a process known as hydrolisis.
Living things contribute significantly to the weathering process. Roots and burrowing animals physically disrupt rocks. Plants and bacteria also contribute to chemical weathering of rocks by altering the acidity of the ground water.
The ultimate product of biologic agents on rock is soil. Because of their different crystal structures see lecture and chapter on Mineralogy , different minerals are more or less resistant to weathering.
Carbonates will fizz at the drop of a hat actually at the drop of acidic solutions. Quartz remains when all else is gone and that is why it is so abundant at the beach! A quick and dirty guide to mineral stability is to consider how complex the structure is. Minerals made up of individual silica tetrahedra such as olivine weather more readily than framework silicates such as quartz. Minerals bound by loose ionic bonds like halite table salt , dissolve very rapidly.
Also, in igneous minerals, the higher the temperature of formation, the lower the stability at normal Earth conditions. Remember that basalt silica poor has a much higher melting temperature than granite silica rich and the mafic minerals in basalt alter much more quickly than those in granite during weathering. Within granite itself, feldspar is the first go since it was the first formed and quartz, like diamonds, are forever. The results of all this weathering is a pile of loose rock fragments clastic material and a lot of dissolved ions: salt, bicarbonate, potassium, etc.
The clastics wash or blow away, or are carried by glaciers from the source region to some catchment area - a sedimentary basin. While particulate matter gets transported, it weathers further.
The particles become smaller, rounder, and less stable minerals continue to dissolve. The dissolved ions are carried in water until they precipitate out, either on land as, for example the spectacular formations in caves known as speleothems , or in the ocean. Chemical sedimentary rocks can be found in many places, from the ocean to deserts to caves. For instance, most limestone forms at the bottom of the ocean from the precipitation of calcium carbonate and the remains of marine animals with shells.
If limestone is found on land, it can be assumed that the area used to be under water. Cave formations are also sedimentary rocks, but they are produced very differently. Stalagmites and stalactites form when water passes through bedrock and picks up calcium and carbonate ions. When the chemical-rich water makes its way into a cave, the water evaporates and leaves behind calcium carbonate on the ceiling, forming a stalactite , or on the floor of the cave, creating a stalagmite. The water drips, but the mineral remains like an icicle.
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Mountain building and volcanic activity were then reactivated followed by yet another rise in sea level and deposition of more marine sediment.
In fact, the rocks of the Grand Canyon record no fewer than four more repetitive episodes of structural uplift and erosion followed by deposition of more sediment. As sea level encroached on the land a nearby mountain shed sediment into the region, and this process continues today.
The Colorado River is now cutting an extraordinary unconformity surface which will eventually be covered with sediments once again. I sometimes wonder what the Grand Canyon will look like when that happens. Will it be covered by an ocean? Will the sediments be shed from mountains yet to be formed? No one yet knows, but one thing is for certain: The answers will be recorded in the sedimentary rock.
This is a good video, I like this one quite a bit; it shows us a lot of different things that we can't really see either in the classroom or here in the studio, but you know, I wish they would have should us some photomicrographs of some of these sedimentary rocks so that we could get a sense of the texture, and the cement, and how these things are really put together.
It does show us quite a few examples of sedimentary rock structures and depositional environments, which I think, I hope anyway, adds to the presentation. Well, what I'd like to do is summarize some of the salient points of the sedimentary rocks because this video did cover it extremely well, but here's the main idea here: that sedimentary rocks remember the conditions under which they were deposited, and if we can learn to get their memories, we can find out the conditions that they were deposited and learn things about the history of the Earth, so you see, sedimentary rocks are formed from sediments, and they preserve the features of those sediments.
The process by which sediments are converted into sedimentary rocks is simply called "lithification. It consists basically of two interrelated processes: compaction and cementation. The process of lithification is also accompanied by a rise in temperature but not enough temperature to cause metamorphism or to significantly alter the compositions of the minerals.
The process of "compaction" means just what it sounds like. The rocks are weighted down by overlying layers of sediment, and the pore spaces are reduced either by the pressure of the overlying sediments or by the pressure of Earth movements. The thickness of the deposit is reduced as the porosity decreases.
Clay, for example, at a depth of one kilometer is reduced to about 60 percent of its original volume. The process of compaction also forces out fluids like water, and gas, and oil, which may migrate through the porous rocks to collect someplace else where we can later exploit them by digging wells.
Okay, the process of compaction also distorts the grains and may cause them to partially dissolve at their edges. Okay, the other process then is "cementation. The pore contains fluids, mostly water, which are rich in various atoms, usually the products of chemical weathering. A change in conditions, whether it's a change in temperature, or a change in pressure, or the addition of new atoms, can cause these atoms to precipitate as new minerals, and in the process lose these detrital or clastic particles together.
The most common cements are calcite and orbs. There are also smaller amounts of iron oxides, and clay minerals, and gypsum, and dolomite, and other kinds of minerals, but the most common are calcite and orbs. The process of crystallization of these new minerals and cementation basically occur jointly or at the same time, but certain chemical deposits are crystallized directly; others are recrystallized as the grains dissolve.
It's also important to note that biological processes of various kinds can also aid in this precipitation from solution. Certain sea creatures like corals and foraminifera, for example, use calcium carbonate dissolved in sea water to build their shells, and reef structures, and so forth. Sometimes new minerals are formed from atoms or ions in solution as these atoms and ions are added, so with this in mind, we might suspect that there are basically two major types of sediments, and therefore, also sedimentary rocks.
There are, on one hand, the fragments of mechanical weathering, the so called "detrital" or "clastic" rocks, which are the fragments of the original rock or their weathering products. The quartz is the most stable mineral, both under chemical and mechanical weathering, and it's not surprising that quartz then is the most common mineral that we find in clastic sedimentary rock. It accumulates from the weathering of granite, which granite, of course, makes up a large percentage of the granitic crust of the continents, so quartz is very common and, in fact, makes up a large percentage of various clastic deposits.
Clay minerals are also common because the clay minerals are themselves products of chemical weathering which can be transported to new locations.
The rock may also contain feldspar and other easily weathered materials if it's immature and is close to the source. Calcite crystals and grains are common here on tropical beaches, broken off fragments from shells and the reef, which are deposited on the beach, and most sediments of the clastic type also contains small grains of rarer minerals, which represent the minor or accessory minerals in the various rocks.
The chemical part represent those products of chemical weathering, which are dissolved in water and transported back in that fashion. These consist of mostly silica and carbonates. You may remember from weathering that silica and carbonates are two of the weathering products, may include some sulfates, and chlorides, and other metallic ions. Okay, though most sedimentary rocks, then, are mixtures somehow of the two different types, and we can classify these as "detrital," which means that the rocks are mostly "detrital" or clastic fragments with the chemical portion cementing them together.
On the other hand, we may have simply chemical sedimentary rocks, which is only the chemical part, which means that the crystallization happened in a place where there didn't happen to be grains to cement together, so then we can classify sedimentary rocks based upon composition and texture.
We classify all rocks on composition and texture, and in the case of composition, we're looking simply at the composition of the minerals themselves. If it's the grains we're concerned with, then we want to identify the grains, and if it's the cement we're concerned with, we want to identify the cement; in fact, in many sedimentary rocks, we name the rock according to both the composition of the grains and of the cement.
We might say there's a calcareous quartz sandstone, for example, which means that the grains are quartz and the cement is calcite.
I don't want to get into all the possibilities for the names but recognize here that you have several different kinds of materials that can be deposited as grains, several different kinds of materials that can be deposited as cement, and any mixture or any combination of these things is possible in a wide range of sizes. Okay, as far as the composition then of the chemical part, we expect to find the chemical part of the sedimentary rocks composed out of those substances which are dissolved in water as the result of chemical weathering; in other words, mostly quartz or other silica minerals and also calcite.
Let's consider the texture for a minute. By "texture" in igneous rocks we mean not only the size of the grains, but also their shape, and their relationship to one another. In sedimentary rocks we're mostly concerned with the size of the fragments. The reason for this is simple. The size of the fragments reflects the depositional environment or at least the speed of the water that was involved in the deposition in the first place.
We usually consider sizes of clastic grains in only four categories. These are very broad categories invented by a fellow named Wentworth, and today the scale is called the "Wentworth Scale. In general, gravel is material that is greater than two millimeters in size.
Clay is material which is less than four ten thousands of a millimeter in size, so clay size material is really quite small. Sand and silt, then, fall in between there.
The ambiguity is easy to understand if you recognize that most of those things which are small clastic particles, in other words, small enough to be considered clay size particles are, indeed, clay in composition. Not all of them, but many of them. On the other hand, most clay minerals only occur in these clay size fragments, so it's not hard to understand how this ambiguity arose in the first place when geologists were first classifying this.
To accurately determine the name or the size of a particular particle requires us to either pass them through sieves, screens of various sizes or to use a microscope where we can measure the size, but we don't really need to do that for our purposes. There are several simple field tests that you can use if you have a piece of sedimentary rock in your hand.
It's a very simple classification. If you can identify the grains; in other words, if the individual fragments are large enough that you can identify what mineral they're made out of, then that material is called "gravel," the large particles.
If you can recognize that there are grains there; in other words, you can see the individual grains, but they're too small to identify their properties, then that material is "sand. If you can feel the grit of the grains, but you cannot see the grains or identify them, then that material is called "silt.
The conglomerate is usually poorly sorted and may contain smaller sized grains as well in between the larger pieces, but we still call it "conglomerate" based upon the size of the largest piece, and of course, these individual fragments are cemented by chemical precipitants.
A rock that's made out of sand sized particles, no surprise is called sandstone, and sandstone may be classified as coarse, medium, or fine depending on how big the sand grains are. The sandstone may be poorly sorted; they may be well sorted; the grains may be angular; they may be rounded; and to give a complete description of this particular sedimentary rock, you would need to specify all that, so if a geologist was talking to another geologist, he might say, "I have a quartz sandstone cemented by calcite that's fine grained; it's well sorted and consists of angular grains.
The clay sized particles form the widest variety of sedimentary rocks, and these are generally classified into a broad category called " mud rocks. They have a smooth feel kind of like talcum powder, generally gray to red in color. It's called simply "mud stone" if its massive but if it's "fissile ," meaning it splits easily, it's generally called "shale.
The shales tend to collect a lot of this material. Okay, the chemical sedimentary rocks have interlocking grains that under the microscope look similar to igneous rocks.
The difference is that these rocks are not formed by cooling of a melt; they're formed by precipitation from a liquid, a liquid solution. The minerals formed are those minerals which are stable at low temperatures and pressures: silica, calcite, iron oxides, and clay minerals.
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