Chapter 2 Rocks Introduction to the world directly beneath our feet Why study rocks? Three reasons… To find and utilize natural resources Prepare for and maybe prevent natural disasters Learn about earth’s natural history Natural Understand Earth’s Resources Disasters History Coal Earthquakes All rocks tell Petroleum Volcanoes an individual Gravel Landslides story about Uranium their past Tsunami’s All metals If we look and used in Sink Holes listen closely, construction Foundation we can gain Aquifers Problems knowledge of our planet’s history Three basic rock types We classify rocks into three categories These categories are distinguished by how each rock type is formed Igneous Sedimentary Metamorphic Rocks Rocks Rocks The Three Rock Types Igneous– Rocks Born of Crystallized Fiery Molten Magma or Lava Sedimentary– Rocks Born of Layered Water or Wind Deposits – Rocks Born of a Metamorphic Change Deep Within the Earth Magma or Lava….which is it? Magma – hot molten rock that is beneath the surface of the earth Lava– hot molten rock that has reached the surface of the earth How does an Igneous Rock Form? Crystallization – the solidifying of magma either at depth or on the exposed surface of the earth To become Sedimentary--- 1. Weathering – rocks exposed at the surface undergo disintegration and decomposition from natural forces Water, wind, glaciers, waves, heat, etc… All these forces will break down rocks and transport the smaller broken down pieces These broken down pieces are now called… SEDIMENTS Sediments – the eroded particles of rock that have been transported away from the original rock 2. Transportation 3. Lithification (compaction and cementation)– literally means “conversion into a rock” Lithification is when compacted and cemented sediments get buried and formed into a new rock Hence, sedimentary rocksaddition More pressure is applied due to the of overlying layers Sediment grains are fused and cemented together as the water content is reduced Check out all these sedimentary layers Sky Walk Two things can happen to sedimentary rocks: They either get exposed at the surface as sedimentary rocks for us to find, or… They continue to get buried deeper and deeper within the earth’s crust To become Metamorphic 1. rocks get buried very, very deep 2.The intense heat and pressure of deep burial will ultimately change the rocks into something new Metamorphosis Metamorphic Rock – a rock that has been changed from its original parent rock to a new form due to intense heat and pressure And if you want to go even deeper? Rocks that reach depths where the heat is extreme enough will turn back into molten rock material... We have come full circle Time, we need more time All the processes of the rock cycle happen over incredible time spans We cannot see the entire cycle in action, it takes much too long We see the results of different stages of the cycle Evidence for the cycle is found everywhere, in every rock What Cycle are we Talking about? ROCK CYCLE – The series of geologic processes that forms igneous, sedimentary, and metamorphic rocks and then breaks them down only to form new rocks again at a later time and place Developed by James Hutton THE ROCK CYCLE ROCK CYCLE with Diagrams To Review: There are 3 main Heat & Pressure form reasons to study rocks metamorphic rocks There are 3 rock types: The Rock Cycle is not igneous, sedimentary, & just a one way street metamorphic There are varying Magma & Lava are not paths, and rocks can quite the same thing reform or skip whole Erosion must occur to sections of the cycle form sediments Geologic processes Deposition occurs to take incredible amounts form sedimentary rocks of time Lubbock Haboob http://www.washin http://youtu.be/ta gtonpost.com/blog uoBRoRlzU s/capital-weather- gang/post/dust- Jessica storm-haboob- Interview blasts-lubbock- texas- video/2011/10/18/ gIQAY37NuL_blog. html IGNEOUS ROCKS Fire Rocks Yosemite Igneous formations PikesPeak in the Rockies--Colorado Mount Rushmore, South Dakota White Mountains of New Hampshire Stone Mountain in Georgia Yosemite-- California Igneous Rock Facts Form from the slow cooling of magma or the quick cooling of lava on the surface Most abundant rock on earth Magma can rise from depths of 200 km or more This cross section of the earth shows how magma can crystallize under the surface or how lava can erupt and then cool on the surface Aboveground or Underground? Igneous rocks can be subdivided into two main categories: intrusive or extrusive Intrusive – igneous rocks formed inside the earth at depth from cooling of magma Extrusive – igneous rocks formed on the earth’s surface from cooling of lava A note about magma: it’s mostly hot molten rock, but it does contain some solid material and some dissolved gasses Classifying all the Igneous Rocks Two criteria are used when classifying igneous rocks: TEXTURE – overall appearance of the rock based upon the size and arrangement of the crystals MINERAL COMPONENTS - Chemistry Textures Four textures used for all igneous rocks Coarse-grained texture (large) Fine-grained texture (small) Porphyritic texture (mixed) Glassy texture (none) Igneous Textures Coarse-grained – rocks that cool slowly at depth and crystals are large enough to be seen with the naked eye Granite is a very common This texture is example of a coarse- inherently intrusive grained igneous rock Rhyolite is the fine grained version of granite – same mineral different cooliing history Igneous Textures Fine-grained – rock cools more rapidly on or near surface and crystals are small and can only be seen with a microscope. This texture is inherently extrusive Basaltic lava flows are Hawaiian Islands and very common and are good examples of fine- Iceland composed grained igneous rocks primarily of basalt. Igneous Textures Porphyritic – rock that has large visible crystals embedded in a matrix of fine- grained crystals Porphyritic rocks usually This texture is both have the appearance of intrusive & extrusive chocolate chip cookies Igneous Textures Glassy – rocks that cool so rapidly that crystals do not form This Volcanic Glass is the result of a violent eruption Obsidian is volcanic glass and is the result of rapidly This texture is cooling lava (no crystals) inherently extrusive The Chemistry of Magma Magma has different chemicals in it The chemicals determine what minerals will form The same magma can actually produce a wide variety of minerals and therefore a wide variety of different igneous rocks N.L. Bowen came up with a way to predict which minerals will form in a cooling magma body He charted mineral formation in the BOWEN REACTION SERIES The Sliding Scale of Igneous Rocks Granitic Light-colored Rich in SiO2 Least Dense Sparse Fe & Mg (Felsic) Andesitic Medium-colored (Intermediate) Basaltic Dark-colored (Mafic) Rich in Fe & Mg Ultramafic Very-dark Sparse in SiO2 Highest Density Chemistry of Magma & Minerals The Bowen Reaction Series (pg 39) How do we know the names? Igneous rocks are classified by their texture and their chemistry right? And Bowen figured out which minerals will form when right? So we make a chart that correlates all of these characteristics Then we simply give each rock a name Igneous Rock Names (pg 41) To Review: Igneous Rocks form from There are 4 textures that the cooling of either you need to know magma or lava The four chemistry types Igneous rocks can form are: granitic, andesitic, on the surface or deep basaltic, & ultramafic within the earth – Know how to read and extrusive and intrusive understand the Bowen’s We classify igneous reaction series rocks by two criteria – Know how to read and texture & chemistry understand an igneous naming chart SEDIMENTARY ROCKS Rocks of Wind & Water Sedimentary Rock Facts Remember, sediments are broken down rock fragments from other rocks due to weathering & transport Sedimentary rocks are only 5-10% of all rocks in the crust But, sed rocks are over 75% of all visible land rocks Sed Rocks are great story tellers Sedimentary rocks tell the story of the past They show what kind of climate and area the rock formed in They show what kind of depositional environment the rock was formed in Ex. dunes, streams, ocean bottoms, salt flats, etc… And most importantly, they are the only rocks that contain fossils Two types of Sed Rocks DETRITAL CHEMICAL Sed rocks that Sed rocks that form from the solid form from material weathered particles that precipitates of other existing out of an aqueous rocks solution DETRITAL ROCKS All minerals can be weathered down into sediments and transported Therefore you will find all types of minerals in sedimentary rocks However, quartz & clay minerals are predominant Quartz because it is very durable and, Clay because they are weathered silicates DETRITAL ROCKS Names are based upon sediment particle size Sediments can range from microscopic dust specks to large house size boulders Sand grains Boulder size sediments DETRITAL ROCKS Conglomerate – a sed rock with large rounded particles of varying sizes Breccia – a sed rock that has large jagged-edge particles of varying sizes DETRITAL ROCKS Sandstone – sed rock that has uniform sand grain size particles only Shale – sed rock that has very fine grained particles, the particles are usually flat and platy Energy of Transport The size of the sediment will tell how much energy was present when the rock was formed in the past The larger the sediment sizes = the higher the energy required to transport the particles Examples: conglomerate and breccia rocks could only have formed in high energy environments; whereas, shale and sandstone will usually form in much lower energy environments Energetic Environments Low energy depositional environments might include lakes, ponds, bays, and small streams High energy environments might include raging rivers, flood prone areas, and glaciers Zones of Deposition High energy environments might include the steep mountain slopes, rushing rivers, and the rugged continental shelf Low energy zones would include lakes, lagoons, swamps, and the deep ocean floor CHEMICAL ROCKS These are different from detrital rocks in that the sediments are already dissolved in water The sediments then precipitate out of that water to form a rock Three types: EVAPORITES BIOCHEMICAL COAL CHEMICAL ROCKS Evaporites – rocks form when a sea or lake dries up and leaves behind minerals – Sometimes these are called “salt flats” Usually halite and Rocksalt is a common name for the evaporite rock halite gypsum are the minerals CHEMICAL ROCKS Biochemical – plants and animals in water excrete CaCO3 as waste or they make their shells out of it Over time the CaCO3 will settle and be deposited to make LIMESTONE Most abundant chemical sed rock! CHEMICAL ROCKS Coal – is nothing more than buried swamps or buried peat bogs Plant material is “cooked” over long time spans and transformed into a fossil fuel Examples of modern day swamps CHEMICAL ROCKS Coal can differ depending upon how much it has been cooked. The more it is cooked the harder it becomes, the more dense it becomes, the less water it will have in it, and the cleaner it burns. Swamp Peat Lignite Bituminous Anthracite These two charts show how the different stages of coal develop CHEMICAL ROCKS We have a lot of Lignite in Texas... The four stages of coal are shown in progressive order in this diagram How do seds become Sed Rocks? There are two main ways that sediments can be lithified into sedimentary rocks. Remember, “lithified” literally means turned into a rock Compaction Cementation Shale is the frock used in fracking Lithification Part I Compaction – seds accumulate over time, the weight of overlying seds compress the deeper ones, pore space in between seds reduces and the rock is squeezed together Lithification Part II Cementation – water percolates through pores, minerals dissolved in the water are left behind in the pore spaces, these minerals now act as a glue holding the sediments together It Looks Like a Sed Rock! The most characteristic feature of sedimentary rocks are the strata or beds Strata – layers of sed rocks that represent a specific depositional period --most striking feature of sed rock Bedding Planes – these separate strata from each other and represent the end of one depositional period and the beginning of a new one 1000s of feet of strata To Review: Sed rocks are made up of The particle size gives weathered particles from you clues to the energy of other pre-existing rocks the past environment Sed rocks can tell a vivid Chemical rocks are named by the process story of the past that forms them The two types of sed Lithification can occur rocks are Detrital & either by compaction or Chemical by cementation Detrital rocks are named Strata is the most by the particle size distinguishing feature of sedimentary rocks METAMORPHIC ROCKS Rocks of Change The Keys to Metamorphic Rocks Remember, “metamorphism” literally means to change form How does the original parent rock change?? The size, the shape, the texture, and even the chemistry of the minerals can change! Which kind of rocks can be changed into a metamorphic rock? All Types: – Igneous rocks can be changed – Sedimentary rocks can be changed – Even metamorphic rocks can be changed into other kinds of metamorphic rocks Meta-why? What causes a rock to change? Three factors: 1. Pressure from deep burial 2. Heat from deep burial or a nearby magma body 3. Chemically active fluids percolating through the rock (we don’t get into this) Meta-how much? 2 levels of Metamorphism Low-grade metamorphism – hardly changed from the original parent rock High-grade metamorphism – cannot even recognize the original parent rock The hotter and deeper the rocks go the greater the metamorphism will be Different types of Change There are 3 types of metamorphism: – Burial Contact Regional Burial Metamorphism Burial Metamorphism – usually a low grade change due to pressure caused by shallow to medium burial; heat is not a major factor Burial Metamorphism will not drastically change the parent rock Contact Metamorphism Contact Metamorphism – also usually a low grade change; the parent rock comes in contact with a magma body and gets cooked enough to metamorphose Contact Metamorphism Rising magma bodies will metamorphose nearby rocks Regional Metamorphism Regional Metamorphism – both heat and pressure change rock on a grand scale; this usually occurs during major tectonic events such as mountain building Regional Metamorphism The highest grade of change will occur during regional metamorphism Texture You Say? 2 types of texture for metamorphic rocks Foliated Non-Foliated Foliated Texture In Foliated rocks mineral grains align parallel because of the pressure & heat This recrystallized mineral alignment causes the rock to have a banded look Rocks with multiple mineral types will exhibit this texture Notice the foliation of this gneiss sample Non-Foliated Texture In Non-Foliated rocks the minerals do not line up parallel These rocks have a uniform appearance and no banding Rocks that have only one kind of mineral will usually exhibit this metamorphic texture Non-Foliated rocks do not have banding in them Naming & Classifying Metamorphic Rocks We can group these into the two categories based upon the types of texture the rocks exhibit Foliated Rocks Non-Foliated Rocks •Slate •Marble •Schist •Quartzite •Gneiss •Anthracite Naming Foliated Rocks Slate – a low grade metamorphism of the parent rock shale The thin platy mica flakes line up and give the rock foliation Slate has great “rock cleavage” and it used as roof & floor tile, Slate is a low grade metamorphic rock which chalkboards, and was once used as hand held billiard table tops chalk boards in schools Naming Foliated Rocks Schist – higher grade metamorphism of the rock slate Schist is “platy” due to the mica minerals, and it can be split into flakes & slabs Schist is a high grade metamorphic change Schist is a high grade metamorphic rock with obvious foliation Naming Foliated Rocks Gneiss (“nice”) – A foliated rock from the parent rock granite Minerals are banded, but instead of being platy like schist, they are granular and rounded Gneiss is a very high Notice the good grade metamorphic foliation in this sample of gneiss change Naming Non-Foliated Rocks Marble – coarse grained crystalline rock from the parent New reduced rock limestone price! It’s Jesus!!! It has large crystals of interlocking calcite Calcite has a hardness of 3 so it is relatively The calcite crystals that make up soft and great for limestone do not change drastically carving and sculpting when marble is made; if anything, Marble is a low-med they will grow in metamorphic change size as they fuse together Naming Non-Foliated Rocks Quartzite – very hard rock made from the parent rock sandstone With heat/pressure the quartz grains simply fuse together Quartzite is a The rock quartzite is medium-high grade not too different than metamorphic change a large quartz crystal Naming Non-Foliated Rocks Anthracite – the highest grade of coal It forms from the continued cooking of bituminous coal Anthracite is a high grade metamorphism, it is extremely hard, and it burns almost completely clean Both of these anthracite samples are hard and shiny Naming Metamorphic Rocks Metamorphic Parent Grade of Foliated or Rock Name Rock Metamorphism Non-Foliated Slate Shale Low Foliated Schist Slate High Foliated Gneiss Granite Very high Foliated Marble Limestone Low-Med Non-Foliated Quartzite Sandstone Med-High Non-Foliated Anthracite Bit. Coal High Non-Foliated To Review: All aspects of a rock There are two types can change during of textures in metamorphism metamorphic rocks: All rock types can foliated & non-foliated undergo metamorphism Foliated rocks look Heat, pressure, and banded due to mineral fluids do the changing alignment There are three types Non-foliated rocks do of metamorphism: burial, not look banded contact, & regional Learn the six rocks!
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