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Mohorovicic Discontinuity

• Separates crust and mantle

• Occurs at the depth from 5 to 50km from the surface

Litosphere

• The uppermost mantle is relatively cool and consequently is hard, strong rock. In fact, its mechanical behavior is similar to that of the crust. The outer part of the Earth, including both the uppermost mantle and the crust, make up the lithosphere (Greek for “rock layer”).

• Thickness

• 10km where tectonic plates separate

• 75km beneath ocean basins

• 125km under continents

Asthenosphere

• Low-velocity zone (low speed of seismic layers)

• Under the lithosphere layer

• Plastic ductile layer

• Rocks reach melting point in this layer, therefore rocks are thought to be partially melted

• Thickness – 200km

• Crustal (tectonic) plates move above it

• Low-velocity of seismic waves are caused by melting of rocks and water associated with minerals

• 1-2% liquid

• Depth to 350km

Mesosphere

• About 2500km thick

• Solid but capable of flowing

3. What does “Plate Tectonics” mean? Describe the plate boundaries. What are the consequences of plate tectonics?

Tectonics (Greek - construction)- movement and deformation of the crust, incorporates older theory of continental drift.

Plates- lithospheric plates - about 100 km thick, which move around on top of the asthenosphere.

7 Large major tectonic plates and several others

Plates move slowly – rates range from less than 1 to about 16 cm per year

Plate Boundaries:

Divergent

• At a divergent plate boundary, also called a spreading centerand a rift zone, two lithospheric plates spread apart.

• The underlying asthenosphere then oozes upward to fill the gap between the separating plates. As the asthenosphere rises between separating plates, some of it melts to form molten rock called magma.

• Most of the magma rises to the Earth’s surface, where it cools to form new crust, the top layer of the lithosphere. Most of this activity occurs beneath the seas because most divergent plate boundaries lie in the ocean basins.

• The hot asthenosphere is weak and plastic, but the cooler lithosphere is strong and hard. As the asthenosphere rises, it cools, gains mechanical strength, and, therefore, transforms into new lithosphere. In this way, new lithosphere continuously forms at a divergent boundary.

• Divergent Boundaries occur at Oceanic Ridges, where new Oceanic lithosphere is formed and moves away from the ridge in opposite directions.

• A spreading center lies directly above the hot, rising asthenosphere. The newly formed lithosphere at an oceanic spreading center is hot and therefore of low density. Consequen-tly, the sea floor at a spreading center floats to a high elevation, forming an undersea mountain chain called the mid-oceanic ridge

• Higher than ocean floor by 2 – 3km

• The Earth`s the longest mountain chain

• A divergent plate boundary can rip a continent in half in a process called continental rifting. A rift valleydevelops in a continental rift zone because continental crust stretches, fractures, and sinks as it is pulled apart. Continental rifting is now taking place along a zone called the East African rift.



Convergent

• At a convergent plate boundary, two lithospheric plates move toward each other.

• Convergergence can occur between:

– Oceanic crust and continental crust carrying plates

– 2 plates carrying oceanic crust

– 2 plates carrying continental crust

A subduction zoneis a long, narrow belt where a lithospheric plate is sinking into the mantle. On a worldwide scale, the rate at which old lithosphere sinks into the mantle at subduction zones is equal to the rate at which new lithosphere forms at spreading centers.

 

• Differences in density determine what happens where two plates converge.

• When two plates converge, the denser plate dives beneath the lighter one and sinks into the mantle. This process is called subduction.

• Generally, only oceanic lithosphere can sink into the mantle.

Convergence of Oceanic Crust with Continental Crust

• When an oceanic plate converges with a continental plate, the denser oceanic plate sinks into the mantle beneath the edge of the continent. As a result, many subduction zones are located at continental margins.

Convergence of Two Plates Carrying Oceanic Crust

• When two oceanic plates converge, the denser one sinks into the mantle. Oceanic subduction zones are common in the southwestern Pacific Ocean.

Convergence of Two Plates Carrying Continents

• If two converging plates carry continents, neither can sink into the mantle because of their low densities. In this case, the two continents collide and crumple against each other, forming a huge mountain chain. The Himalayas, the Alps, and the Appalachians all formed as results of continental collision.

Transform

• A transform plate boundary forms where two plates slide horizontally past one another as they move in opposite directions. California’s San Andreas fault is the transform boundary between the North American plate and the Pacific plate. This type of boundary can occur in both oceans and continents

Plate movement consequences:

• Mountain ranges building

• Volcanic eruptions

• Earthquakes

• Oceanic trencehs

• Migration of continents and oceans

• Mid-oceanic ridges

4. What is mineral? Properties of minerals. Formation of minerals.

A mineral is a naturally occurring inorganic solid with a characteristic chemical composition and a crystalline structure or Chemical compositionand crystalline structure are the two most important properties of a mineral: They distinguish any mineral from all others. Naturally formed– it forms in nature on its own. Solid(can not be liquid or gas). With definite chemical composition(the same mineral has the same chemical composition and chemical formula). A characteristic crystalline structure(atoms are arranged within the mineral in a specific ordered manner).

Properties of minerals:

1. Chrystal Habit - the characteristic shape of a mineral and the manner in which aggregates of crystals grow. If a crystal grows freely, it develops a characteristic shape controlled by the arrangement of its atoms. If the growth is obstructed then mineral cannot develop its characteristic habit.

2. Color - Color is the most obvious property of a mineral, but it is commonly unreliable for identification. Color would be a reliable identification tool if all minerals were pure and had perfect crystal structures.

3. Streak - the color of a fine powder of a mineral. It is observed by rubbing the mineral across a piece of unglazed porcelain known as a streak plate. Many minerals leave a streak of powder with a diagnostic color on the plate. Streak is commonly more reliable than the color of the mineral itself for identification.

4. Luster - the manner in which a mineral reflects light. A mineral with a metallic look, irrespective of color, has ametallic luster. The luster of nonmetallic minerals is usuallydescribed by self-explanatory words such as glassy, pearly, earthy, and resinous.

5. Cleavage - the tendency of some minerals to break along flat surfaces. The surfaces are planes of weak bonds in the crystal.

6. Density(mass/volume)

7. Specific gravity- the weight of a substance relative to that of an equal volume of water. If a mineral weighs 2.5times as much as an equal volume of water, its specific gravity is 2.5.

8. Hardness - the resistance of a mineral to scratching. It is easily measured and is a fundamental property of each mineral because it is controlled by bond strength between the atoms in the mineral. To measure hardness more accurately, geologists use a scale based on ten minerals, numbered 1 through 10. Each mineral is harder than those with lower numbers on the scale, so 10 (diamond) is the hardest and 1 (talc) is the softest.

a. Talc

b. Gypsum (fingernail)

c. Calcite (penny)

d. Fluorite

e. Apatite (knife blade)

f. Orthoclase (glass)

g. Quartz

h. Topaz

i. Corundum

j. Diamond

9. Fracture – a pattern in which a mineral breaks other than along planes of cleavage. Many minerals fracture into characteristic shapes.

Formation of minerals:

Minerals are formed in nature by a variety of processes.

– Crystallization from melt (igneous rocks)

– Precipitation from water (chemical sedimentary rocks, hydrothermal ore deposits)

– Change to more stable state (weathering, metamorphism, diagenesis)

– Precipitation from vapor (not common, sometimes occur around volcanic vents)

• Since each process leads to different minerals and different mineral polymorphs, we can identify the process by which minerals form in nature. Each process has specific temperature and pressure conditions that can be determined from laboratory experiments. Example: graphite and diamond, as shown previously.

5. What is mineral? Describe each classification group of minerals.

A mineral is a naturally occurring inorganic solid with a characteristic chemical composition and a crystalline structure or Chemical compositionand crystalline structure are the two most important properties of a mineral: They distinguish any mineral from all others. Naturally formed– it forms in nature on its own. Solid(can not be liquid or gas). With definite chemical composition(the same mineral has the same chemical composition and chemical formula). A characteristic crystalline structure(atoms are arranged within the mineral in a specific ordered manner).

Minerals classification:

• Minerals are classified according to their anions (negatively charged ions).

• Each mineral group (except the native elements) is named for its anion. For example, the oxides all contain O2-, the silicates contain (SiO4)4-, and the carbonates contain (CO3)2-.

Native elements

• About 20 elements occur naturally in their native states as minerals. Fewer than ten, however, are common enough to be of economic importance. Gold, silver, platinum, and copper are all mined in their pure forms.

• Pure carbon occurs as both graphite and diamond. The minerals have identical compositions but different crystalline structures and are called polymorphs, after the ancient Greek for “several forms.”

Oxides

• The oxides are a large group of minerals in which oxygen is combined with one or more metals. Oxide minerals are the most important ores of iron, manganese, tin, chromium, uranium, titanium, and several other industrial metals.

Sulfides

Sulfide minerals consist of sulfur combined with one or more metals. Many sulfides are extremely important ore minerals. They are the world’s major sources of copper, lead, zinc, molybdenum, silver, cobalt, mercury, nickel, and several other metals. The most common sulfides are pyrite (FeS2), chalcopyrite (CuFeS2), galena (PbS), and sphalerite (ZnS).

Sulfates

• The sulfate minerals contain the sulfate complex anion (SO4)2-. Gypsum (CaSO4 - 2H2O) and anhydrite (CaSO4) are two important industrial sulfates used to manufacture plaster and sheetrock. Both form by evaporation of seawater or salty lake water.

Phosphates

Phosphate minerals contain the complex anion (PO4)3-. Apatite, Ca5(F,Cl,OH)(PO4)3,is the substance that makes up both teeth and bones. Phosphate is an essential fertilizer in modern agriculture.

Carbonates

• The complex carbonate anion (CO3)2- is the basis of two common rock-forming minerals, calcite (CaCO3) and dolomite [CaMg(CO 3) 2].

Silicates

• The silicate minerals contain the (SiO4)4- complex anion. Silicates make up about 95 percent of the Earth’s crust. They are so abundant for two reasons. First, silicon and oxygen are the two most plentiful elements in the crust. Second, silicon and oxygen combine readily.

Ionic substitution

• The replacement of one ion by another in the crystal structure of a mineral. Generally, one ion can substitute for another if the ions are of similar size and if their charges are within 1+ or 1-.

6. What is rock? Describe each group of rock types.

Rocks - a collection of mineral aggregates composing independent geological body and forming the crust. Rocks are loose or dense formations, formed as a result of geological processes in a particular condition within the Earth's crust or on its surface.

3 main types

– Igneous

Magmais formed when rocks of lower crust and upper mantle melt under certain conditions

95%of the crust is igneous and metamorphosed igneous rocks

Granite and basaltare common and familiar igneous rocks

Holocrystalline igneous rocks which are formed by solidification of magma - silicate melt coming from the bowels of the Earth.

Abyssal(deep) intrusive rocks - granite, diorite, gabbro, and others;

Hypoabyssal (intermediate between effusive and deep) - gabbro-porphyrin, granite-porphyry

Formation of rocks from the magma can be proved in several ways:

– direct observation of the formation of igneous rocks due to hardening of lava during volcanic eruptions

– the presence of volcanic glass in igneous rocks, and in some cases being entirely of glass (obsidian)

– experimental – i.e. obtaining many igneous rocks by cooling a specially formulated molten mixtures

– the structure of the igneous rocks and the conditions of their contacts with the host rocks that detect telltale signs of matter flow of igneous rocks during formation process.

– Sedimentary

Rocks which are formed by deposition of a substance in an aqueous medium, at least from the air and as a result of the glaciers.

Precipitation occurs by mechanical, chemical and biogenic way.

How do sediments become sedimentary rocks:

  1. To become a sedimentary rock, sediments must be lithified, or cemented into stone. As sediments pile atop other sediments, the earliest deposits become deeply buried. Weight of overlying sediment places pressure on those below. Sediment under pressure compacts. The tighter the sediment is squeezed, the less room there is for water or air between grains and grains to stick together. Something else has to happen to turn these sediments into rock.

2. Cementation is the word given to the sticking together of sediments by additional material between grains. Water flowing between grains contain silica, carbonate or other dissolved materials that precipitate out of solution and attach to grain surfaces. Eventually, enough mineral.

• Sedimentary rocks are created when two sediments press other small rocks together therefor creating a sedimentary rock.

• Sedimentary rocks are classified by texture, composition, and how they were formed:

  1. Clastic

Clastic Sedimentary Rocksare composed of fragments of weathered rocks, called clasts, that have been transported, deposited, and cemented together.

• Clastic rocks make up more than 85 percent of all sedimentary rocks –sandstone, siltstone and shale.

  1. Chemical

• Water traveling through rock dissolves minerals and transports them away. Chemical sedimentary rocks form from these minerals. Eventually dissolved minerals are redeposited, or reprecipitate, when the water evaporates or when the solution becomes over-saturated with minerals or when the water enters an area of low pressure, like a cavity in the rock. Some chemically precipitated rocks include stalactites and stalagmites you see in caves (also known as travertine). Chert is a precipitated rock from silica-saturated waters. Rock salt, known as table salt, is the result of water evaporating from salt-rich water.

  1. Biogenic (organogenic)

Organic sedimentary rocks consist of the remains of plants or animals. Coal is an organic sedimentary rock made up of decomposed and compacted plant remains.

4. Bioclastic

Bioclastic sedimentary rocks. Most limestone is composed of broken shell fragments. The fragments are clastic, but they form from organic material. As a result, limestone formed in this way is called a bioclasticrock.

– Metamorphic - rocks formed as a result of changes (metamorphism) of sedimentary or igneous rocks with a complete or nearly complete change of their mineral composition, structure and texture (eg, gneisses, shale, quartzite)

Metamorphism is the transformation of rock by temperature and pressure.

Metamorphic rocks are produced by transformation of:

Sedimentary and igneous rocks and by the further alteration of other metamorphic rocks.

Metamorphism(from the Greek words for “changing form”) is the process by which rising temperature—and changes in other environmental conditions—transforms rocks and minerals.

Metamorphism occurs in solid rock—like the transformations in the vase as the potter fires it in her kiln.

Metamorphism can change any type of parent rock: sedimentary, igneous, or even another metamorphic rock.

• The metamorphic rocks have holocrystalline structure.

• Crystal grain sizes, usually increases as the metamorphism temperature.

• Metamorphic rocks generally have typical oriented texture.

Metamorphic rocks are formed deep in the earth. Heat, pressure, and some chemical reactions cause one type of rock to change into another type of rock.

7. What is geochronology? Describe methods of calculation of rock age. Explain the main principles of relative geologic time (principles of original horizontality, superposition, crosscutting relations). Correlation. Key Beds.

Geochronology – calculation absolute age and duration of eras, periods, epochs, and centuries in millions of years. Calculation is based on the radioactive methods to determine the absolute age of rocks and minerals. It studies the chronological order of formation and age of the rocks that form the Earth's crust. It is classified as relative (Relative - Know Order of Events But Not Dates )and absolute (Know Dates) (or nuclear) geochronology.

PRINCIPAL OF ORIGINAL HORIZONTALITY:

• Based on the premise that sedimentary rock is deposited in flat-lying layers.

• This means that if a sedimentary layer is not horizontal, it has been subjected to forces that have moved or changed it.

• If it is horizontal, we can use this to study organisms that lived on the Earth during the same periods of time.

• The principle of original horizontality:

(A) Sediments tend to be deposited in horizontal layers.

(B) Even where the sediments are draped over an irregular surface, they tend toward the horizontal.


Date: 2015-12-11; view: 1126


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