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8Standard3

Rock and Fossil Formation
by

Chandra Nelson

on 1 February 2013

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Transcript of 8Standard3

Weathering is the physical breakdown and chemical alteration of rocks at or near Earth's surface. Standard 3: Understand the process of rock and fossil formation Objective 1: Compare rocks and minerals and describe how they are related. Objective 2: Describe the nature of the changes that rocks undergo over long periods of time. Objective 3: Describe how rock and fossil evidence is used to infer Earth's history. Objective 4: Compare rapid and gradual changes to Earth's surface. The Big Ideas •Earth is constantly changing.

•Processes that changed Earth’s surface in the past work the same today.

•Evidence of past surface and climatic changes is found in the rock and fossil records.

•Rocks are made of minerals.

•Rocks and minerals cycle through processes that change their form.

•Many different processes contribute to changing Earth’s surface.

•Earth’s surface is changed by heat flowing from the hot interior toward the cooler surface and by atmospheric processes.

•Earth’s surface can change abruptly through volcanoes and earthquakes.

•Earth’s surface can change gradually through mountain building, weathering, erosion, and deposition.

•Small changes that occur repeatedly over very long time periods can add up to major changes in Earth’s surface.
The study of minerals, rocks, processes that shape the earth, and history of the earth is called Geology.

Within the field of Geology there are many specializations such as paleontology, geologic engineering, structural geology, marine geology, geophysics, geochemistry, and volcanology. Geology Claudia Alexander
Geophysicist
USGS
NASA Giant's Causeway, Northern Ireland
A mineral is a naturally occurring, inorganic, crystalline solid.
•An inorganic substance is not made of living things or the
remains of living things.
•Every mineral has a characteristic chemical composition that
can be either an element or a compound.
•Minerals are the basic material of Earth’s crust.
•Most rocks are mixtures of various minerals. Minerals Characteristics of Minerals Every mineral has characteristics unique to that mineral. These properties can help to identify types of minerals. Color is the most easily observable and the most unreliable property for mineral identification. Many minerals are similar in color and any impurities in the mineral can cause a variation in color, so color alone is not a good indicator of mineral type. Luster is the light reflected from the surface of a mineral. Luster can be metallic or non-metallic (glassy, pearly, waxy, brilliant, or earthy). Streak is the color of a mineral in powder form. Streak is most easily tested by rubbing the mineral against an unglazed ceramic tile. Some minerals have the tendency to split easily along certain flat surfaces. This property is called cleavage and is related to the type of bonds in the structure of the mineral. Minerals that don’t break along cleavage planes fracture, or break, unevenly. Hardness is the measure of the ability of a mineral to resist scratching. Hardness of an unknown mineral can be measured against the hardness of known minerals using Mohs Hardness Scale. Talc is the softest mineral and can be easily scratched by a fingernail. Diamond, the hardest mineral, will scratch all other minerals and can not be scratched. There are six basic shapes in which mineral crystals will form. These specific shapes can be used to help identify a mineral. A certain mineral always forms crystals in the same shape because the atoms always combine in the same pattern. Density is the measure of the amount of matter in a substance in a given space. It is calculated by dividing the mass of the mineral by the volume of the mineral. Each mineral has a specific density that will not change, no matter the size of the mineral sample. All minerals exhibit these properties, but a few minerals have special properties as well. Some of these special properties are magnetism, fluorescence and phosphorescence, radioactivity, and reaction to acid. http://www.galleries.com/Fluorescent_Minerals Some Common Minerals Quartz http://ngm.nationalgeographic.com/2008/11/crystal-giants/shea-text Feldspar Pure quartz form clear, hexagonal crystals, but any contamination from other elements can cause a variety of colors. Quartz has a hardness of 7, which means it can easily scratch glass and steel. Quarts is used to make things such as watches and vases. Feldspar is the most common mineral group, partly because it can form under a wide range of pressures and temperatures. Feldspar has cleavage on two planes at close to 90 degrees, so it cleaves in a rectangular shape. The luster of feldspar ranges from glassy to pearly. Mica There are tow major types of mica, muscovite (light) and biotite (dark). Mica has excellent cleavage in one direction. In thin sheets, micas are clear, which accounts for its use as window "glass" in the Middle Ages. Mica is common in many rocks, often giving them a sparkle. Calcite Calcite is made of calcium-carbonate (CaCO3). It is found in a variety of water related environments. It can be difficult to distinguish calcite from minerals such as halite, but a diagnostic property of calcite is that it fizzes when exposed to hydrochloric acid because HCl breaks the mineral bonds and releases CO2. Hornblende Hornblende is a type of mineral called an amphibole. It is usually dark green to black and has cleavage on two planes at 60 degrees and 120 degrees. Hornblende is found in igneous rocks and often makes up the dark parts of otherwise light colored rocks (like granite). Halite Halite has a molecular composition of NaCl. It has cubic cleavage and a salty taste. It forms from the evaporation of salty water, such as a drying lake or part of a sea that had been cut off. Halite is also called rock salt and is ground up to make table salt. Gypsum Gypsum is a very soft mineral with a hardness of less than 2. It can easily be scratched with your fingernail. It forms in similar conditions to halite. Olivine Olivine is the most common mineral in the upper mantle of the earth. It has a distinctive olive-green color and no cleavage. Graphite Graphite is made of only carbon atoms, like diamond, but because of the molecular structure it is much softer. The most familiar use of graphite is in your pencil. Galena Galena is made mostly of lead, which makes the mineral very dense and heavy for its size. It forms distinctive metallic cubes and has a cubic cleavage. Native Element Minerals Native element minerals are elements that occur in nature in uncombined form with a distinct mineral structure. Many of these are metals such as copper, gold, and silver. Gemstones Many gemstones are minerals. A few examples are garnet, beryl (emerald), corundum (ruby, sapphire), and quartz (amethyst, citrine, crystal). Crystal Growth Factors affecting crystal size 1. The rate at which magma cools
2. The amount of silica present
3. The amount of dissolved gasses in the magma Lava that cools rapidly at the surface of the earth will form very small (microscopic) crystals. Magma that cools slowly below Earth's surface will have much larger crystals.
There are three basic types of rocks, igneous, sedimentary, and metamorphic. These rock types are interchangeable through the rock cycle. Rock Types Igneous Rocks that are formed by the cooling of molten rock in the form of magma or lava. •Most of the Earth’s crust is made up of igneous rocks.

•The mantle, which accounts for 82% of the Earth’s volume is also igneous.

•Igneous rock forms as molten rock cools and solidifies. Igneous rocks are classified as extrusive or intrusive.

•Extrusive: through volcanoes, lava cools at the earth’s surface. These rocks cool very quickly, so there is little time for crystals to form, resulting in very small and sometimes microscopic crystals.

•Intrusive: Magma in the mantle of the earth rises and eventually begins to cool and crystallize. Intrusive igneous rocks only outcrop at the surface is the crust is uplifted and the overlying rocks are stripped away by erosion.

•Igneous rocks are most often classified by their texture and mineral composition. Obsidian Diorite Scoria Sedimentary Rocks that are formed by the weathering, deposition, and cementation of sediments. •Surface of earth is covered with sedimentary rock or sediment.

•Weathering

•Erosion

•Deposition

•Lithification or cementation

•Rock layers contain evidence of past conditions and events. Metamorphic Rocks that have been changed with heat and pressure. •Metamorphism means to “change form”. Every metamorphic rock is changed from a parent rock into a new rock.

•Metamorphic rocks are changed through heat, pressure, or chemically active fluids.

•Textures include foliated (mineral grains are arranged into a nearly flat order) or nonfoliated (minerals are not flattened or elongated).

•Metamorphism occurs in several different ways.

Thermal metamorphism: rock immediately surrounding molten rock are “baked”.

Hydrothermal metamorphism: hot, ion rich fluid fills cracks and holes in a rock.

Burial metamorphism: Occurs with layers of sedimentary rock in a subduction zone, where one tectonic plate is going under another.

Regional metamorphism: Most common type of metamorphism, caused by mountain forming, when two tectonic plates are pushing against each other.

Impact metamorphism: Occurs when a meteorite strikes the surface of the earth, causing rocks to shatter and melt.

Fault metamorphism: When heat and pressure along fault lines between tectonic plates change rock. The Rock Cycle http://www.tangostangos.com.ar/video/video/53lMdHzvGCQ&feature=youtube_gdata_player http://studyjams.scholastic.com/studyjams/jams/science/rocks-minerals-landforms/rock-cycle.htm rock cycle song Study Jams: animation, song, quiz. Weathering Weathering is a constant process that is powered by the sun. Mechanical Weathering Chemical Weathering Physical forces break rock into smaller and smaller pieces without changing the mineral composition of a rock. A chemical transformation of rock into one or more new compounds. a. Frost wedging: Water expands when it freezes. When water seeps into cracks in rocks and freezes, it expands the cracks and causes rock to break.



b. Salt crystal growth: This process occurs along rocky shorelines or arid regions with salty groundwater. Salt dissolved in water seeps into cracks or holes in rock. When the water evaporates, the salt minerals are left behind and form crystals that push rocks apart as they grow.



c. Unloading: When igneous rocks cool underground, there is a lot of pressure exerted all around them. As the overlying rock is eroded away, this pressure decreases and the rock expands, breaking off in sheets.



d. Biological activity: Living things can influence weathering (both mechanical and chemical) in several different ways. Tree roots can dig into rock in search of water and wedge rocks apart as they go. Burrowing animals bring fresh material to the surface where it can be weathered more easily, humans dig mines big enough to be seen from space, etc, etc. a. Water: Water is the most important agent of chemical weathering. Pure water is non-reactive, but it dissolves and carries many different substances, such as salt, which can then chemically alter rocks.


b. Carbonic Acid: When carbon dioxide dissolves in water, it forms carbonic acid. CO2 is dissolved in water as rain falls through the air, or when water percolates through soil with decaying organic matter. Many rocks are dissolved by this acid and sediments are turned back into sedimentary rock in the form of limestone, often found in subsurface caverns.


c. Oxidation: Oxidation occurs when water dissolves oxygen. Oxygen is highly reactive and forms compounds with iron. This is what gives some rocks a reddish or yellowish color and weakens rock over time. 1880 1993 Many factors influence the type and rate of weathering.
Mechanical weathering increases the rate of chemical weathering by exposing more surface area to be weathered. Rock Characteristics Climate Differential Weathering Rock characteristics include mineral composition of a rock, solubility, and physical features like cracks. If a rock is made of more resistant minerals, such as quartz, it will withstand weathering better than rocks made of easily weathered minerals, like calcite. If a rock has cracks, it can be exposed to weathering much sooner than a rock with no cracks. The only common mineral that is very resistant to both mechanical and chemical weathering is quartz. Climatic factors, especially temperature and moisture, heavily influence the rate of weathering. Mechanical weathering in the form of frost wedging is greatly affected by the frequency of freeze-thaw cycles. Chemical weathering occurs at the greatest rate where there is warm temperatures and high moisture because there will be a thick layer of soil with decaying organic matter to produce acids. Masses of rock do not weather at the same rate. This is called differential weathering. The main cause of this is mineral composition, but spacing of cracks plays a significant role as well. Differential weathering is responsible for many unusual and spectacular landforms. http://studyjams.scholastic.com/studyjams/jams/science/rocks-minerals-landforms/weathering-and-erosion.htm Erosion Erosion is the movement of sediment and rock fragments to a different location. The weathered material is carried by wind, water, or ice. Weathering and erosion work together to change the environment. weathering and erosion (9m) Turning dunes into architecture (14m) Rock Layers The earth is constantly changing through processes such as weathering and erosion. Processes that changed Earth’s surface in the past work the same today. Evidence of past surface and climatic changes is found in the rock and fossil records. Finding the Age of Rocks The relative age of a rock is its age compared to the ages of other rocks.

The absolute age of a rock is the number of years since the rock formed. Radioactive Dating Geologists can find the absolute age of a rock by measuring the radioactive decay of a rock.

Radioactive elements occur naturally in igneous rocks. These elements (like uranium, carbon, or potassium to name a few) decay over time. This means the elements break down, lose energy and particles to become another element.

Geologists can measure how much of these certain elements a rock contains, then use the known half-life of the element to determine how old the rock is. Relative Dating To find the relative age of rocks, geologists compare the rock to other layers using the law of superposition.

The law of superposition says that in horizontal sedimentary rock layers the oldest layer is at the bottom and each higher layer is younger than the layers below it.

By examining the order of rock layers, geologists can determine which rocks are younger and which rocks are older. Rock layers give us a clue as to how old rocks are. But rock layers don't always stack up perfectly. There are different processes that can change the order of the layers, offset them, or interrupt them. The six layers of rock show no evidence that they have been disturbed or rearranged by earth movements. Which layer do geologists assume is the oldest layer? Which layer on the diagram is the oldest? What event happened at the area labeled by the heavy black line? What event most likely created the offset of layers J and I? Clues from igneous rock Remember that igneous rocks form from molten rock.

Lava that cools on the surface of the earth is called an extrusion. Rock layers below an extrusion are always older than the extrusion.

Magma below the earth's surface pushed up into rock layers and forms an intrusion. Intrusions are always younger than the rocks around and beneath them. Clues from faults A fault is a break in the earth's crust. Forces inside the earth cause movement of the rock on opposite sides of the fault. Faults are most commonly caused by earthquakes.

A fault is always younger than the rock it cuts through.

To find the relative age of a fault, scientists find the relative age of the youngest rock layer it cuts through. Other clues Pressure and heat from metamorphism causes rock layers to bend and fold, which means the layers also bend, fold, and tilt. Over time the surface of the earth is weathered and eroded. Sometimes entire layers or parts of them can be missing. As time continues, new layers of sediment will be deposited, creating nice even layers again until a force acts on them to upset the order. Which rock layer is youngest?

What is #3 called?

What happened at B?

Why is there a chunk missing from layers 1 and 6? Fossils (ba-ba-baaaaaa!) Fossils are the preserved remains or traces of living things.

Most fossils form when living things die and are buried by sediments. The sediment eventually becomes sedimentary rock and the cast of the organism is preserved. Kinds of Fossils Petrified Fossils Fossils in which minerals replace all or part of an organism. These fossils form when sediment covers the organism, then water rich in dissolved minerals seeps into the spaces in the organisms cells. Over time, water evaporates, leaving behind the hardened minerals. Carbon Films Trace Fossils Preserved Remains Molds and Casts Molds and casts are the most common type of fossil. Both copy the shape of an ancient organism. A mold is a hollow area in a rock that forms when something hard, like a shell is buried. Over time the organism decomposes and water seeps into the space, depositing minerals into the empty mold and forming a cast in the shape of the organism that was once there. A carbon film is an extremely thin coating of carbon on a rock. All living things contain carbon in their cells. When an organism dies and is buried by sediment, the materials that make up the organism become gasses. Eventually only carbon remains. This process can preserve delicate parts like insects and plant leaves. Trace fossils do not preserve an organism, but they provide evidence of the activities of ancient organisms. One example of a trace fossil is a dinosaur footprint. Sediment fills in the space left by the footprint and eventually cements into rock. A footprint can tell us a lot about the dinosaur like how big it was, how fast it moved, how many legs it walked on, and if it lived alone or with others of its kind. Preserved remains are fossils that are protected from decay by a substance. The body of an organism is preserved in a condition unchanged from when they were living. Sometimes even tiny hairs on insect legs or the internal organs of a mammoth are perfectly preserved. Preserved remains can be found in tar pits, amber (hardened pine sap) or ice. Using Fossils to Date Rocks Sometimes rock layers don't match up, especially rocks in different locations. This makes it difficult to determine the relative age of a rock. Index fossils can help geologists match rock layers.

An index fossil is a widely distributed type of organism that existed only briefly. Index fossils are useful because they tell the relative age of the rock layer in which they are found. The Fossil Record The fossil record provides evidence about the history of life on Earth. The fossil record also shows that different groups of organisms have changed over time.

Older rocks contain fossils of simpler organisms and younger rocks contain fossils of more complex organisms. In other words, the fossil record shows that life on Earth has evolved, or changed over time.

Evolution is the gradual change in living things over time.

The fossil record shows that millions of organisms have evolved, but many others have become extinct. An organism is extinct if it no longer exists and will never again live on Earth. Fossils and Past Environments Fossils can help Geologist infer the history of Earth. Not only what lived here, but also what the environment was like. Fossils of coral are common in the midwestern US, but coral definitely don't live in corn fields. The presence of these fossils tell us that the area was once covered by a shallow sea. Coal in Antarctica tells us that the continent was once warm and swampy. Fossils provide evidence that tells us about the earth's past climates and ancient life on Earth. Earthquakes http://video.nationalgeographic.com/video/environment/environment-natural-disasters/earthquakes/earthquake-101/ Volcanoes http://video.nationalgeographic.com/video/kids/forces-of-nature-kids/volcanoes-101-kids/ Faults Richter Scale Pyroclastic Flow Lava Flow Explosive Lava
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