Wednesday, November 2, 2016

Carbonate Sedimentary Rocks

The carbonate sedimentary rocks are those rocks composed primarily of calcite. This includes all of the limestones. Limestone are chemical sedimentary rocks because the calcite was carried to the ocean or lake or sea in solution. The calcite then precipitated directly from the water to for the sediment or crystals that make up the rock. These sediments are NOT transported to the basin as particles like is the case with the siliciclastics. These sediments are created in the basin in which they are found. For example: a bioclastic limestone is composed of skeletal grains formed of calcite. This calcite was precipitated by the organism. Later, the organism died and its skeletal remains became part of the ocean bottom. That sediment was buried under more skeletal debris, compacted, and cemented together to form the bioclastic limestone we later see.

Carbonates have two possible origins (biogenic or chemical inorganic) and two possible textures (clastic or crystalline:
  1. biogenic origin - the grains that make up the rock were created by an organism and deposited on the basin floor when the organism dies. Most of these rocks have a clastic texture. The exception to this rule are reef rocks and stromatolites (calcite deposited by cyanobacteria).
  2. chemical inorganic origin - the grains that make up the rock were created inorganically, that is, without the help of an organism. The texture of such rocks is often crystalline with the exception of oolitic limestone. The ooids are created when a grain of something (sand, shell fragment, ooid fragment, etc.) rolls around on the bottom of a warm shallow sea and calcite precipitates on that grain as thin layers in a similar manner as pearls are formed. These ooids are later buried and cemented together to form a rigid framework = clastic texture.
Photomicrograph of modern ooids from a beach on Joulter's Cay, The Bahamas. Scale bar for size. Photo by Mark A. Wilson (Department of Geology, The College of Wooster)

Ooids on the surface of a limestone; Carmel Formation (Middle Jurassic) of southern Utah, USA. Scale bar for size. Photo by Mark A. Wilson (Department of Geology, The College of Wooster)

Photomicrograph of  thin slice of calcitic ooids from the Carmel Formation, Middle Jurassic, of southern Utah, USA. Scale bar for size. Photo by Mark A. Wilson (Department of Geology, The College of Wooster). <--parrot fish article
     In my opinion, it is best to study the siliciclastics together and the carbonates together to start. Then try to tell the siliciclastics apart from the limestones.

    Chert is off on its own because it is neither a siliciclastic nor a carbonate. Chert is formed by the recrystallization of siliceous skeletal grains such as radiolarian tests, diatom tests, and sponge spicules; or, through the replacement of previously existing material. Limestones are often replaced by chert. Petrified wood is chert. The cellulose in the wood is replaced, molecule by molecule, by chert. Therefore, it can be difficult to tell the origin of chert without field relations. Because all chert is recrystallized, either directly from pore waters or by the replacement of some other substance, it is considered to be chemical inorganic.  Note the conchoidal fracture that chert displays. This fracture pattern is what allows chert (aka flint) to be napped to form arrowheads, spearheads, scrapers, etc.

    Stone tools made from chert.

    Study hard, get to know the rocks, and you will do well on the test. Good Luck!!

    Siliciclastic Sedimentary Rocks

    Sedimentary rocks are the most interesting rocks in my opinion. I am a sedimentary geologist/petrologist by training meaning that I like learning about both the processes that affect sediments, the sediments themselves, and the sedimentary rocks those sediments become. I find that it is the most amazing thing to be able to look at a sedimentary rocks and glean information out of it that will indicate the depositional environment in which it formed. With that information, a picture develops as to what the landscape looked like, what the ecosystem looked like, and how long ago the sediment was deposited. As we look at sedimentary rocks, keep the question: 'How did it come to be that way?' in your mind. It will enable you to be able to see the data necessary to interpret the depositional environment. In this class, since it is an introductory class, we will be only making very basic interpretations. If you go on in geology, you will take classes that will allow you to make much more in depth interpretations and do the analyses necessary to make those interpretations.

    The majority of sedimentary rocks fall into one of two end-member groups:
    1. siliciclastics: rocks composed primarily of silicate minerals such as conglomerates, sandstones, siltstones, claystones, mudstones, and shales
    2. carbonates: rocks composed primarily of carbonate minerals such as calcite and aragonite (a polymorph of calcite meaning that they both have the same chemical formula but different crystal structures) such as bioclastic limestones and oolitic limestones
    There can be some mixing of the two types of sediment to produce mixed siliciclastic-carbonate rocks but for the most part, and for this class, they will be considered separate. There are other groups of sedimentary rocks such as the organic sedimentary rocks, those rocks composed entirely of organic material such as peat, lignite, and coal, as well as chert, which belongs to no group.

    The siliciclastic sedimentary rocks are those rocks composed of silicate minerals that were weathered out of preexisting rock, transported by wind, water, glaciers or gravity to the location in which they were deposited. These sediment are referred to as clastic sediments. The locations that accumulate sediments are called basins, they are depressions in the crust where sediment is deposited and doesn't erode away. Some basins are on land such as the Permian Basin in West Texas and the intermontane valleys of the mountainous west. The largest and ultimate basin is the ocean.

    The transported clastic sediments were then buried over time, compacted, and cemented together (lithified) to form sedimentary rocks. All siliciclastic rocks have a clastic texture and all have a detrital origin. Remember that a clastic texture is a rock texture in which the particles that make up the rock were once loose sediment that was later compacted and cemented together (lithified) to form a rigid framework. A detrital origin means that the particles that comprise the rock were weathered out of a source rock located outside of the depositional basin, transported by water, wind, glaciers or gravity to the depositional basin where the sediments were deposited.

    Siliciclastics are named based on the dominate grain size present in the rock as well as it's mineralogy. For example:
    1. a quartz sandstone is composed of sand-sized quartz grains and quartz comprises at least 90% of the rock.
    2. a conglomerate contains primarily gravel-sized grains of quartz, rock fragments, some feldspar, etc.
    Mineral composition adds to the name to help distinguish one particular rock from another such as a hematitic quartz sandstone from an arkose. Remember that all siliciclastics have a detrital origin and all detrital rocks have a clastic texture.

    Remember our definition of rock texture which refers to the size, shape, and arrangement of grains in a rock. Rock texture can tell us a significant amount about the conditions under which a sediment was deposited. Siliciclastic sedimentary rocks are named based on grain size. Refer to the table below.

    This table shows the relationship between grain size, sediment name, and rock name.
    In addition to grain size, we also look at grain shape. The grain shape we are most concerned with is the roundness, that is, how common corners and edges are on the grain. Sorting is another important textural parameter to examine. Some transporters of sediment such as wind and water are very good at sorting sediments. Gravity and glaciers, not so much. Refer to the figure below for a visual comparitor for sorting and roundness.

    A visual comparitor for sorting and roundness of sediment grains.

    Lets look at some examples of the rocks from your boxes:

    Quartz sandstone: contains at least 90% quartz, frequently the sediment is well-rounded and well-sorted. In order to have quartz concentrated to this level, the sediment must have been well-weathered and well-traveled. Therefore, quartz sandstones are considered to be mineralogically and texturally mature.

    Here is a quartz sandstone in hand sample. We can see that the color is very uniform. It appears to be composed of only one mineral. When we see a sandstone that appears to be only one color (one mineral) it is safe to bet that that mineral is quartz.

    This is a quartz sandstone under the microscope. All of the round grains are quartz sand grains. There is a bit of dust, probably clay minerals or iron oxides on the grains but by and large, it is primarily composed of quartz. This photo is courtesy of Suvrat Kher and his blog titled Rapid Uplift.

    Here is a photomicrograph of another quartz sandstone. This time, the upper polarizer has been inserted into the microscope. This allow geologists to identify minerals as they will change color when the stage is turned. Quartz turn from white to gray to black as the stage is turned. The quartz grains are the rounded grains in the photo. The extra bit of quartz outside the dust rim is the quartz cement. The cement is what holds the rock together. This photo is courtesy of Suvrat Kher and his blog titled Rapid Uplift.
    Quartz sandstones are often interpreted to be beach deposits. Beaches along passive continental margins (coastlines without a plate boundary nearby), tend to have beaches composed of quartz sand. Often times the beach is part of a barrier island such as Santa Rosa Island, FL as shown in the photo below.

    Quartz sand at Gulf Coast National Seashore on Santa Rosa Island between Pensacola Beach and Navarre Beach in Florida.

    Hematitic Sandstone: is a sandstone that contains enough hematite to stain the rock red. Typically these rocks are predominated by quartz grains.

    Hematitic sandstone in hand sample. The scale bar indicates 1 cm.
    The above hematitic sandstone in thin section. It was photographed using plane polarized light. Most of the grains here are quartz. They all have a rim of hematite. The scale bar represents 1 mm. 
    This photomicrograph is taken of the same spot as show in the photomicrograph above but using crossed polarized light. It is a technique that allows for mineral identification. Quartz will appear in colors ranging from white to gray to black. The scale bar represents 1 mm

    Arkose: a sandstone with at least 25% feldspar. The typical source rock for an arkose is granite or gneiss, rocks with significant amounts of feldspar. This rocks is considered texturally and mineralogically immature since the grains have not be subjected to much weathering. The sands below illustrate what the sediment that makes up an arkose may have looked like.

    River sand from Yosemite National Park. The source rock was a 
    granodiorite which is very similar to a granite except that granites
    have more potassium feldspar (k-spar)The minerals present in this 
    sand include: quartz, plagioclase, and biotite. If this sand became a 
    sandstone, it would be called an arkose. The width of this photo is
    10 mm. Photo courtesy of Sandatlas.
    This beach sand is from the Canadian Arctic along the  shore of the 
    Coronation Gulf, Nanauut. The minerals in this sand include:  
    quartz, potassium feldspar (k-spar), andhornblende. The rock that 
    would result from the lithification of this sand is an arkose. The width of the photo is 10 mm. Photo courtesy of Sandatlas.

    This image depicts the alluvial fan in Rocky Mountain National Park. The 2013 floods moved a significant amount of sediment down the canyon and onto the alluvial fan blocking/covering roads in the process.

    The West Alluvial Fan Parking lot is now covered under feet of alluvium brought out of the mountains in 2013 due to the unprecedented amount of rain the area received.

    Wide view of the West Alluvial Fan farther down hill on the fan.

    Arkose "flat irons" of the Fountain Formation at Roxborough State Park near Denver, CO.

    A notched weathered in to some of the exposed Fountain Formation at Roxborough State Park, CO.

    Close up of the Fountain Formation rock at Roxborough State Park. the pink potassium feldspar crystals are easy to see in this photo. The larger grains are approximately 1 cm in size. 

    In thin section, the quartz, feldspar, and other igneous minerals are easily identified. Additionally, any weathering products such as hematite will coat the grains. The rounding and sorting of the grains is also easier to see than in hand sample.

    This view gives a good impression of the range of sizes of sand grains. Some are angular, some are more rounded. The degree of rounding should depend on how much time they have spent being transported in the river system. Clear grains are mostly quartz, cloudy grains are feldspar. Dark iron oxide material forms a thin coating on the grains and makes up part of the matrix or cement, giving the red-brown or purplish colour to the rock. Plane polarized light, field of view is 7 mm across.

    Between crossed polars we see the great variety of different types of material making up the grains. There is quartz (e.g. large grain at left), potassium feldspar with "tartan" twinning (top right) and fragments of various rock types including quartzite, sheared quartzite, and Lewisian Gneiss. Some of these rocks occur nearby, such as the gneiss. Others, like the quartzites, do not, and must have been brought down the rivers from much further away. Crossed polars, field of view is 7 mm across.

    Sunday, October 9, 2016

    Metacognative Learning Strategies - How to get the most out of your study time

    Metacognative learning strategies give you the tools to really learn the material in your courses not just get enough information to maybe get an A on the test. Metacognition is thinking about your thinking. Analyzing if your study methods are actually helping you reach your goals. How well you do in your classes is a direct reflection of your behavior. If what you are doing isn't getting you the grades you want, you must try something different! Dr. Saundra McGuire has created several steps that help students turn their grade around. She has had many students go from failing tests to getting A's after applying metacognition. In this video she talks about what metacognition is and how it can help students.

    Watch this video on the Study Cycle to learn about the Study Cycle that Dr. McGuire and her staff at LSU put together. If you commit to these steps, you will learn much more efficiently and your learning will be more complete.

    Bowen's Reaction Series - Relationship between ignous rocks, minerals, and silica content

    Bowen's Reaction Series
    In the early 1900's, N. L. Bowen and others at the Geophysical Laboratories in Washington D.C. began experimental studies into the order of crystallization of the common silicate minerals from a magma. The idealized progression which they determined is still accepted as the general model for
    the evolution of magmas during the cooling process.

    The Principles that Bowen realized are as follows:
    1.  As a melt (the liquid portion of the magma) cools minerals crystallize that are in thermodynamic equilibrium with the melt (dissolution equals crystallization; if no equilibrium exists either crystallization will dominate [supersaturation], or dissolution will dominate [undersaturated]).
    2. As the melt keeps cooling and minerals keep crystallizing, the melt will change its composition.
    3. The earlier formed crystals will not be in equilibrium with this melt any more and will be dissolved again to form new minerals. In other words: these crystals react with the melt to form new crystals, therefore the name, reaction series.
    4. The common minerals of igneous rocks can be arranged into two series, a continuous reaction series of the feldspars, and a discontinuous reaction series of the ferromagnesian minerals (olivine, pyroxene, hornblende, biotite)
    5. This reaction series implies that from a single "parental magma" all the various kinds of igneous rocks can be derived by Magmatic Differentiation (see below)

    To summarize: Bowen determined that specific minerals form at specific temperatures as a magma cools. At the higher temperatures associated with mafic and intermediate magmas, the general progression can be separated into two branches (see below). The continuous branch describes the evolution of the plagioclase feldspars as they evolve from being calcium-rich to more sodium-rich. Plagioclase feldspar crystals have a core that is calcium-rich and a rim that is sodium-rich. The average composition of the calcium and sodium content in plagioclase feldspars will approximate the calcium and sodium composition of the magma.

    Igneous Rocks

    Igneous rocks are rocks formed through the crystallization of magma either on the surface as volcanic (extrustive) rocks or deep underground as plutonic (intrusive) rocks. Remembering that at one time early in our Earth's history, the Earth was molten. When the Earth cooled, the minerals crystallized into igneous rocks. Therefore, the rock cycle on Earth, begins with magma.

    Rocks are classified as igneous, metamorphic, and sedimentary based on origin. Classification schemes are designed to answer a specific question or to organize the objects of the classification scheme in groups for easier identification and understanding. Sometimes classification allows us to see patterns in data which can then lead to interpretations. Oftentimes it is just a system of organization to handle quantities of data that are two large to be examined individually. Within each rock type are further classification schemes to better understand how the rocks were formed.

    Wednesday, September 28, 2016

    How to plot 2 scattered plots on the same graph using Excel 2007

    Below is a video with instructions on how to create a scatter plot on Excel (it works the same way for Excel 2010 and 2013) with more than one set of data on the graph. These instructions are perfect for making your graphs for the Cemetery lab write up. It is best to put data that you plan to compare on the same set of axes. It makes comparison so much easier.

    You should also add a best fit line by choosing linear from the trend line options instead of polynomial as the man did in this video. Be sure to check the intercept box and have the intercept be 0.0 so that the trend line will go through the origin.

    Thursday, September 22, 2016


    Killer Volcano - The Mt. St. Helens Story

    Sunday, August 28, 2016

    Cognitive Dissonance and the Nature of Science

    Cognitive Dissonance Theory was proposed in 1957 by Leon Festinger. His theory states that cognitive dissonance is the mental stress or discomfort experienced by an individual who holds two or more contradictory beliefs, ideas or values at the same time; or, is confronted by new information that conflicts with existing believe, ideas or values. This mental stress or discomfort can be as strong as thirst or hunger. When we experience hunger or thirst, we look to relieve that stress by getting something to eat or drink. When a person experiences cognitive dissonance, they are equally as driven to reduce that dissonance and reach consonance. In addition to trying to reduce dissonance, a person may actively avoid situations and information that would likely increase dissonance.

    In science, cognitive dissonance can be a strong driver to move science forward or it can be an anchor holding science back depending on how consonance is achieved. The history of science is littered with stories of both moving forward and holding back. Whether new information will move science forward or whether it will hold it back is up to the scientist who examine that information.

    For example, Lord Kelvin, the famous physicist who formulated the first and second laws of thermodynamics, attempted to calculate the age of the earth using using thermodynamics, looking at heat transfer within the earth itself and from the sun to the earth. He calculated the age of the earth to be between 20 million years and 100 million years using mathematical models that he derived. Therefore, his calculations were only as good as his models which, in hindsight, turned out to not be very good. Geologists and biologists knew that the age of the earth HAD to be much older. Their observations of processes and changes indicated that a VAST amount of time had passed to create many of the creatures and landscapes they observed. As new information about heat sources within the earth came to light, Kelvin stubbornly refused to acknowledge this new information. His understanding of mathematics and thermodynamics shaped how he saw the world which in turn clouded his judgement. Check out these articles by T. H. Heaton and M. Livio for more information on Lord Kelvin's mistake.

    A recent example that I heard about on NPR on my way to class one morning is a perfect example of cognitive dissonance moving science forward. Have a listen:

    Here is a video by Richard Norris of the research he and his cousin, James Norris, conducted. Their paper is publish in PLOS One.

    Here is a whiteboard explanation of how the rock are moving:

    Paula Messina, featured in the NPR recording, had written her PhD thesis on the movement of these rocks. Her data suggested that the most likely explanation for the rocks' movement was freak wind storms that pushed the rocks on a thin film of water creating the grooves in the soft sediment. The Norris' data flew directly in the face of her thesis! At first, she admits, she couldn't believe what they were saying, she was in a state of cognitive dissonance. She needed to see for herself that large, wind-blown sheets of ice were the driving force behind the moving rocks as shown in this cellphone video captured by John Chadbourne. The rocks are moving to the left:

    In the end, Dr. Messina reached consonance by accepting the new information and abandoning her old explanation. Science moved forward! Dr. Messina and other researchers working on the rocks of Racetrack Playa could have become increasingly dogmatic as new information came to light, strongly sticking to their explanation as a means to reach consonance. If that happened, those researchers would have either been marginalized or they may have put the breaks on the understanding of the rocks at Racetrack Playa.

    Wednesday, March 30, 2016

    Metamorphic Rocks

    The best way to study the metamorphic rocks is to separate them into two groups:

    1. Follated Rocks - slate, phyllite, schist, and gneiss
    2. Non-folliated Rocks - marble, quartzite, hornfels, and coal
    Foliation is a layered fabric in the rock developed during metamorphism. This layering MUST be developed during metamorphism. It cannot be relic sedimentary layering. The layering observed in the foliated rocks is the result of a preferred orientation of constituents in the rocks such as platey and/or elongate minerals or stretched pebbles or fossils. The platey minerals are often the micas (muscovite, biotite, chlorite) and the elongate mineral is often amphibole (hornblende). Two other minerals that could produce a foliated texture include talc and tourmaline. Key clues to identify the folitated rocks:

    1. slate: very fine-grained; dull appearance; can be red, black, gray, or green; displays good to excellent rock cleavage; foliation type: slatey cleavage
    2. phyllite: fine-grained; greasy/glossy sheen; can be black, gray, or green typically; can display good to excellent rock cleavage; the layers are often crinkled into tiny folds; there is no specific foliation type so, in this class, we will call it phyllite-type.
    3. schist: medium- to coarse-grained; sparkly or glittery; foliation type: schistocity May be mica-rich or may be amphibole-rich (hornblende-rich).
    4. gneiss: medium- to coarse-grained; dark and light banding; foliation type: gneissic banding.
    Beginning with a shale parent rock, this image shows the development of foliation in rocks metamorphosed under regional metamorphism. This continuum is called prograde metamorphism.

    Non-foliated rocks do not have a layering present that was developed during metamorphism either because there is no elongate or platy mineral present as is the case with quartzite and marble or the rock was metamorphosed under confining stress rather than differential stress. Key clues to identify the non-foliated rocks:
    1. marble: fine- to coarse-grained; white to pink; no layering present; crystals are usually visible and the cleavage planes usually reflect a lot of light giving the rock a glittery luster; hardness is less that a steel nail hardness < 5.5); often reacts with acid.
    2. quartzite: fine- to coarse-grained; white to pink; no layering present; crystals are not usually visible; conchoidal fracture is sometimes observed on the fracture planes; hardness is greater than a steel nail (hardness > 5.5)
    3. hornfels: fine-grained, dark rock with no layering; hardness > 5.5
    4. coal: black "rock" with no grains present; conchoidal fracture is common; "rock" has a low density
    Metamorphic Lab key - to be posted after the lab is completed