Tuesday, September 26, 2017

Teaching students how to learn - tutoring https://www.youtube.com/watch?v=oxdeFvTrP9k
Metacognition is the key to acing chemistry - https://www.youtube.com/watch?v=yGBfd7LeGMM

Plate Tectonic Mechanism

The mechanism for plate tectonics is generally accepted to be a combination of convection currents in the mantle coupled with ridge push and slab pull. The poorly understood part includes 1) to what depth are the convection cells active, 2) are there several layers of convection cells or only one, 3) is the lithosphere the upper part of the convection cell or a passive participant, 4) to what degree to slab-pull and ridge-push move the plates, among others. Below are two images showing how the convection cells in the upper mantle may be configured.

Figure 3: Plate Tectonic mechanism showing that the oceanic plate shown in the diagram is a passive participant in the convection cell. It is moved through convection traction as well as slab pull and ridge push (Earle, Steven, BC Open Textbook)

Figure 4: Plate Tectonic mechanism showing that the oceanic plate shown in the diagram is an active participant in the convection cell. The lithosphere forms the top of the convection cell. Ridge push and slab pull are also playing an important role. (Earle, Seven, BC Open Textbook)

The Silicate Minerals - Building blocks of rocks

As we discussed in class, a mineral is a naturally occurring, inorganic, crystalline solid with diagnostic physical properties and a definite chemical composition. Rocks are typically composed of minerals although there are exceptions. As is always the case in nature, our definitions often do not cover all of the variables that are seen in nature. Some materials formed through geological processes are not composed of minerals yet are still considered rocks. Examples include obsidain and coal. We will be concentrating on rock-forming minerals as well as a few of the common accessory minerals found in the rocks we will be learning about. The mineral quartz (SiO2) is and example of a very common mineral. It is found in all rock types and in all parts of the world. It occurs as sand grains in sedimentary rocks, as crystals in both igneous and metamorphic rocks, and in veins that cut through all rock types, sometimes bearing gold or other precious metals. It is so common on Earth's surface that until the late 1700s it was referred to simply as "rock crystal." Today, quartz is what most people picture when they think of the word "crystal."
Quartz falls into a group of minerals called the silicates, all of which contain the elements silicon and oxygen in some proportion. Silicates are by far the most common minerals in Earth's crust and mantle, making up 95% of the crust and 97% of the mantle by most estimates. Silicates have a wide variety of physical properties, despite the fact that they often have very similar chemical formulas. At first glance, for example, the formulas for quartz (SiO2) and olivine ((Fe,Mg)2SiO4) appear fairly similar; these seemingly minor differences, however, reflect very different underlying crystal structures and, therefore, very different physical properties. Among other differences, quartz melts at about 600° C while olivine remains solid to temperatures of nearly twice that; quartz is generally clear and colorless, whereas olivine received its name from its olive green color.

Minerals and their Physical Properties

This week our lab is on the identification of minerals using their physical properties (lab handout, mineral chart, flow chart.) Minerals are an important natural resource. Everything we use that is not grown, is derived from the Earth. These resources include rocks, minerals, and hydrocarbons to name a few. Therefore, understanding where to find these resources is rather important. Common mineral uses include (from http://www.mii.org/commonminerals.html):
  1. Gypsum Processed and used as prefabricated wallboard or as industrial or building plaster, used in cement manufacture, agriculture and other uses.
  2. Feldspar: A rock-forming mineral, industrially important in glass and ceramic industries, pottery and enamelware, soaps, abrasives, bond for abrasive wheels, cements and concretes, insulating compositions, fertilizer, poultry grit, tarred roofing materials, and as a sizing (or filler) in textiles and paper. Albite is a feldspar mineral and is a sodium aluminum silicate. This form of feldspar is used as a glaze in ceramics.
  3. Fluorite (fluorspar): Used in production of hydrofluoric acid, which is used in the electroplating, stainless steel, refrigerant, and plastics industries, in production of aluminum fluoride, which is used in aluminum smelting, as a flux in ceramics and glass, and in steel furnaces, and in emery wheels, optics, and welding rods.
  4. Halite (Sodium chloride--Salt): Used in human and animal diet, food seasoning and food preservation, used to prepare sodium hydroxide, soda ash, caustic soda, hydrochloric acid, chlorine, metallic sodium, used in ceramic glazes, metallurgy, curing of hides, mineral waters, soap manufacture, home water softeners, highway deicing, photography, herbicide, fire extinguishing, nuclear reactors, mouthwash, medicine (heat exhaustion), in scientific equipment for optical parts. Single crystals used for spectroscopy, ultraviolet and infrared transmission.
  5. Kaolinite: Also known as "china clay" is a white, aluminosilicate widely used in paints, refractories, plastics, sanitary wares, fiberglass, adhesives, ceramics, and rubber products.

The Layered Earth

Everything we see in geology can be directly or indirectly related to Plate Tectonics. If we played the six degrees of separation game, called it Six Degrees of Plate Tectonics, we would rarely go over three degrees before the answer came back somehow relating to Plate Tectonics. It truly is a unifying theory!

The reason that the earth's plates move is in large part due to the fact that our Earth is layered. It wasn't always layered. When debris first accumulated making the protoEarth, it was a fairly homogeneous rock orbiting the sun. Once it reached a critical mass, however, the gravitational energy accumulated caused the entire earth to melt into a fiery ball of liquid hot magma! This event is called the Iron Catastrophe. Catastrophe in this sense does not refer to a tragic event but rather to the classical, Greek-derived definition which is the culminating act in a drama. The Iron Catastrophe lasted between 100 m.y. and 500 m.y. and was the culminating act in the Earth's drama. It created the layered Earth we have now: a core made of iron and nickel, a rocky mantle that is high in iron and magnesium and poor in silica, and a rocky crust that is rich in silica and other lighter elements such as calcium, carbon, nitrogen, oxygen, sodium, aluminum, etc.

Three centuries ago, the English scientist Isaac Newton calculated, from his studies of planets and the force of gravity, that the average density of the Earth is twice that of surface rocks and therefore that the Earth's interior must be composed of much denser material. Our knowledge of what's inside the Earth has improved immensely since Newton's time, but his estimate of the density remains essentially unchanged. Our current information comes from studies of the paths and characteristics of earthquakes waves travelling through the Earth, as well as from laboratory experiments on surface minerals and rocks at high pressure and temperature. Other important data on the Earth's interior come from geological observation of the Solar System, its gravity and magnetic fields, and the flow of heat form inside the earth.

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).

https://ore.exeter.ac.uk/repository/handle/10871/17081 <--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.