Physical Geology
CHAPTER ONE
Physical Geology is the science which studies the materials composing the earth and the processes taking place above and beneath the earth’s surface.
- The earth is one of the major nine planets that revolve around the sun.
- It was formed as the other planets of the solar system (the sun and the other planets) from a vast cloud of dust and gases called a nebula.
Origin of the earth (the nebular
hypothesis):
This
hypothesis suggests that the bodies of the solar system formed from an enormous
nebular cloud composed mostly of hydrogen and helium with only small percentage
of the heavier elements. About
5 billion years ago, this cloud began to contract under its own gravitational
influence; this contraction caused rotation of the cloud, which became faster
and faster as it contracted. The
rotation caused the nebular cloud to flatten into a desk. Smaller
accumulations formed nuclei from which the planets formed. The sun was
formed at the center of the desk.
There are
four spheres around the earth:
1- Hydrosphere
2- Atmosphere
3- Biosphere
4- Lithosphere
Hydrosphere
It includes all water bodies on the earth such as the oceans, which constitute about 71% of the earth’s surface, the fresh water streams, lakes as well as the underground water. Fresh water is very important for life and also responsible for sculpting and creating many landforms on the earth.
Biosphere
It includes
all life on the earth either plant life or animal life, in the sea or on land.
Atmosphere
It includes the air envelope surrounding the earth. It is important for breathing and for protection against sun’s heat and ultraviolet radiation. It is divided into several layers arranged upwards from the earth’s surface into Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.
Earth’s Internal Structure
Earth’s interior consists of three major regions that have markedly chemical composition; these three regions are called crust, mantle and core.
Crust
Crust is the rigid outermost layer of the earth. It is divided into oceanic crust and continental crust. The oceanic crust ranges in thickness from 3-15 km and is composed of the rock basalt (alkaline). It is chemically composed of iron and magnesium silicates and hence takes the name Sima (Silicon and magnesium). The continental crust ranges in thickness from 20-70 km and is composed of granite (acidic). It is chemically composed of aluminum and potassium silicates and hence takes the name Sial (silicon and aluminum).
Mantle
The mantle underlies the crust and is separated from it by Moho discontinuity (Mohorovicic). It has a thickness of about 2900 km and is composed of denser material, mainly silicates of iron, magnesium, sodium and aluminum. Although the mantle behaves like a solid in transmitting earthquake waves, mantle rocks are able to flow at a slow rate.
Lithosphere:
The mantle is divided into the lower mantle or mesosphere and the upper mantle or Asthenosphere. The upper rigid part of the Asthenosphere together with the crust is called Lithosphere.
Core
Age of the Earth
Before the 19th century, it wasn’t possible to determine the absolute age of the earth. Only a relative age could be given to the different rock layers.
1- Relative age: Events are placed in their proper sequence or order without knowing their absolute age in years. This is done by applying the following principles:
1- The law of superposition
2- The principle of faunal succession.
Law of
superposition
In any undeformed sequence of sedimentary rocks or lava flows, each layer is older than the one above it and younger than the one below it.
Principle of faunal succession
Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content.
Geologists collected fossils from countless rock layers around the world and used these fossils to identify rocks of the same age in widely separated places and to build the geologic time scale.
Fossils are Remains or traces of ancient animals and plants.
2- Absolute age: This is the age of rocks in years
In the twentieth century, the discovery of radioactive materials helped the scientists to give an absolute age for the events in the earth’s history.
The age of the earth is estimated to be about 4.6 billion years (4600 million years).
Life started on the earth about 600 million years ago, this time is called Phanerozoic. The other 4000 million years are called Precambrian. There was little life in the Precambrian represented by first one-celled and first multi-celled organisms.
The Phanerozoic Eon is divided into three Eras called Paleozoic, Mesozoic and Cenozoic. Each of these eras is further subdivided into periods and epochs.
(Hutton’s
theory)
The physical, chemical and biological processes that operate today have also operated in the geologic past.
So we can use what we observe today to interpret what happened in the past. For example we can use the mode of life of a living organism to interpret the mode of life of a similar fossil organism.
|
Cenozoic |
HolocenePleistocene Pliocene Miocene Oligocene Eocene Paleocene |
|
Mesozoic |
Cretaceous Jurassic Triassic |
|
Paleozoic |
Permian Carboniferous Devonian Silurian Ordovician Cambrian |
|
Precambrian |
|
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Minerals are useful materials in our life since every manufactured product contains materials obtained from minerals. For example: most car parts are made of iron, which is produced from the minerals hematite, magnetite, limonite and goethite.
A mineral: is a solid, naturally occurring, inorganic substance that has an orderly internal structure and a definite (but not fixed) chemical composition. So, the mineral must:
1- be solid
2- occur naturally
3- be inorganic
4- have an orderly internal structure (its atoms must be arranged in a definite pattern).
5- Have a definite chemical composition.
Examples:
* Diamond is a mineral, but synthetic diamond is not a mineral.
* Graphite is a mineral, but coal is not a mineral.
* Quartz is a mineral, but opal is not a mineral because it lacks an orderly internal structure.
* Oil is not a mineral because it is organic in origin.
MINERALS AND ROCKS
(The minerals are the building blocks of rocks)
Definition of a rock
It is any solid mass of minerals or mineral like matter that occurs naturally as part of our planet.
Classification of rocks
1- Monominerallic rock: a rock composed of one mineral. E.g. limestone is composed of the mineral calcite.
2- Polyminerallic rock: a rock composed of several minerals. E.g. granite is composed mainly of three minerals called quartz, feldspar and hornblend.
3- Nonminerallic rock: a rock composed on nonminerallic matter. E.g. Obsidian and pumice (noncrystalline glassy substances).
The 4000 minerals discovered up till now are characterized each by certain physical properties which allow us to distinguish each mineral from the other. These physical properties are:
1. Crystal form
A crystal is a solid substance that has regular faces resulting from an orderly arrangement of atoms.
Minerals form crystals with well-developed faces when they find space for crystal growth. When there is no space for crystal growth, they form intergrown masses of crystals without a definite crystal form.
2. Luster
It is the appearance or quality of light reflected from the surface of a mineral.
- Metallic Luster: minerals that show the appearance of metals.
- Nonmetallic luster: minerals that show other nonmetallic appearance. This may be vitreous (glassy), pearly, silky, resinous and earthy (dull).
- Submetallic luster: minerals that appear partially metallic in luster.
3. Color
Color is a diagnostic property for identifying minerals. Sulfur for example has a yellow color, and malachite has a bright green color. In many cases, a mineral can have several colors due to the presence of impurities in its crystalline structure. Ex. quartz is usually white but it has also other colors including pink, purple and even black.
4. Streak
It is the color of the powder produced by rubbing the mineral across a streak plate made of unglazed porcelain. The streak is not necessary to be same as the color of the mineral. For example some black minerals have a brown streak. The streak of a mineral is the same whatever the colors of the mineral. Minerals with metallic luster have darker and denser streak than minerals with nonmetallic luster.
5. Hardness
It is the degree of mineral resistance to scratching. It is determined by rubbing a mineral with unknown hardness against one of known hardness or vice versa. Minerals With known hardness belong to Mohs scale of Hardness.
It consists of ten minerals arranged in order from 1 (softest) to 10 (hardest) as follows: Talc (1) - gypsum-calcite- fluorite - apatite - orthoclase - quartz - topaz - corundum - Diamond (10).
So, a mineral with hardness 4.5 will scratch fluorite but is scratched by apatite.
There are other objects of known hardness such as a fingernail (2.5), a copper penny (3), and a piece of glass 5.5.
So the mineral gypsum, which has a hardness (2) is scratched by the fingernail. Quartz (7) is the hardest of the common minerals and will scratch glass (5.5).
6. Cleavage
It is the tendency of a mineral to break along planes of weak bonding. Some minerals have one cleavage plane such as micas, some have four such as fluorite or three such as calcite. Quartz on the other hand has no cleavage.
7. Fracture
It is the shape of minerals that don’t have cleavage when broken. Some minerals break into smooth curved surface like broken glass, this is called conchoidal fracture others break into splinters or fibers but most minerals fracture irregularly.
8. Specific gravity
It is a number representing the ratio of the weight of a mineral to the weight of an equal volume of water.
For example if a mineral weighs three times as much as an equal volume of water, its specific gravity is 3.
Other properties of minerals:
1- Taste: halite has a salty taste.
2- Smell: sulfur streak smells like rotten eggs.
3- Elasticity: mica sheets bend easily and elastically snap back.
4-Malleability: copper & gold are malleable and can be easily shaped.
5-Feel: Talc feels soapy and graphite feels greasy.
6- Magnetism: a magnet picks up Magnetite because it has high iron content.
7-Double refraction: when a transparent calcite is placed over a written word, the letters appear twice.
8-Chemical reactions to HCl: carbonates such as calcite effervesce (fizz) with HCl
9- Transparency: some minerals are transparent; others are translucent whereas others are opaque.
Mineral groups
Although nearly 4000 minerals are known, few minerals are abundant and make up the rocks of the earth’s crust. These minerals are called rock– forming minerals.
Eight elements (silicon - oxygen - aluminum - iron - calcium - sodium - potassium and magnesium) make up the bulk of these minerals. Silicon and oxygen are the two most abundant elements and combine together to form the most common mineral group, which is called silicates. Following silicates in abundance is a mineral group called carbonates, of which calcite is the most abundant mineral. Other rock-forming minerals include gypsum and halite.
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CHAPTER THREE
ROCKS
There are three kinds of rocks:
1- Igneous rocks: are those rocks, which were formed from the cooling of molten material (magma or lava).
2- Sedimentary rocks: are those formed from the deposition of sediments.
3- Metamorphic rocks: are either igneous or sedimentary rocks subjected to temperature or pressure or both to form a new rock.
The name igneous is derived from the Latin word (ignis) which means fire. This is because these rocks were formed from cooling and solidification of the molten material either beneath the earth’s surface (magma) or when it rises up above the earth’s surface in the form of volcanoes (lava).
Magma and lava:
Magma is the molten material beneath the earth’s surface. It is composed of liquid material with solid rocks and gases.
Lava is similar to magma but loses its gases when it rises to the surface in the form of volcanoes.
How is magma formed?
Magma is formed by a process called partial Melting within the earth’s crust and the upper mantle. This occurs at depths that may exceed 200km.
Modes of Igneous rock formation:
1- Intrusive or plutonic Igneous rocks:
These are formed when magma loses its mobility before reaching the surface and crystallizes at depth.
Pluto: the god of the lower world
Plutonic Igneous rocks are exposed on the surface now as a result of uplift and erosion of portions of the earth’s crust.
2- Extrusive or volcanic Igneous rocks:
These are formed when molten-rock material (lava) rises up and solidifies at the earth’s surface during volcanoes.
Texture of Igneous rocks:
The word texture means the size and arrangement of minerals within the rock. Texture is important because it reflects the environment in which the rock formed. For example rocks formed beneath the earth’s surface consist of large crystals because of the slow cooling of magma (tens or hundreds of Thousands of years). On the other hand rocks formed at the earth’s surface consist of small crystals because of the rapid cooling of magma.
Types of igneous rock texture:
Igneous rock composition:
Igneous rocks are composed mainly of silicate minerals. These silicate minerals consist of the eight elements Silicon, Oxygen, Sodium, Calcium, Potassium, Aluminum, Magnesium and Iron.
Silicon and oxygen are the most abundant elements of these eight elements and together constitute about 98% by weight of magma. In addition, magma contains small amounts of titanium and manganese and trace amounts of rare elements such as gold, silver and uranium.
Silicate minerals, which are rich in iron and magnesium but low in silica, are called dark (or ferromagnesian) silicates. These minerals include olivine, pyroxene, amphibole and biotite. On the other hand, silicate minerals, which are rich in sodium, potassium and calcium and also rich in silica, are called light silicates. These minerals include quartz, muscovite and feldspar group (orthoclase and plagioclase).
Bowen’s Reaction Series:
This is a
diagram, which illustrates the sequence of minerals crystallization from a
basaltic magma under laboratory conditions.
As magma cools, the ferromagnesian minerals first crystallize (olivine followed by pyroxene, amphibole and biotite). At the same time calcium- rich feldspars also crystallize. With cooling the remaining magma becomes poor in iron and magnesium but rich in sodium, potassium and aluminum. Also it is rich in silica, which is little incorporated in the composition of the ferromagnesian minerals. As a result, light minerals such as quartz and muscovite crystallize. At the same time sodium-rich feldspars also crystallize.
Types of Igneous rocks
1. Mafic (basic) rocks:
These are igneous rocks composed of ferromagnesian minerals like olivine and pyroxene as well as calcium–rich plagioclase. These minerals are rich in iron, magnesium or calcium and low in silicon.
The mafic rocks are darker and denser than the other igneous rocks.
Mafic: ma (magnesium), fe (ferrum)
Examples of these rocks include Gabbro (plutonic) and basalt (volcanic).
2. Felsic (acidic) rocks:
These are igneous rocks composed of potassium feldspar and quartz.
Felsic: feldspar, silica (quartz)
These rocks are lighter in color. Examples include granite (plutonic) and Rhyolite (volcanic).
3. Intermediate Rocks:
These are igneous rocks containing minerals found near the middle of Bowen’s reaction series like amphibole and plagioclase feldspar.
Examples of intermediate igneous rocks include diorite (plutonic) and andesite (volcanic).
4. Ultramafic (ultrabasic) rocks:
These are igneous rocks composed mostly of olivine and pyroxene.
Examples include peridotite (the main constituent of the upper mantle).
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CHAPTER FOUR
Sedimentary rocks are rocks, which formed from deposition of sediments by processes of compaction and cementation.
The word sedimentary is derived from the Latin word sedimentum, which means “settling” referring out to solid material from fluids (water or air).
Sediments and sedimentary rocks:
Rock particles such as gravel, sand and mud, which result from weathering processes, are called sediments. Examples include sands forming the sand dunes, gravels in a streambed or mud on the floor of a swamp.
Over long periods, these sediments accumulate; the materials near the bottom are compacted and finally cemented together by mineral matter deposited in the spaces between particles forming a solid sedimentary rock.
Types of sedimentary rock
1- Detrital sedimentary rocks:
These are sedimentary rocks formed from deposition of particles resulted from weathering processes.
2- Chemical sedimentary rocks:
These are sedimentary rocks formed from precipitation of soluble material by inorganic or organic processes.
A detrital sedimentary rock is a rock that is formed from grains; these grains are the products of mechanical and chemical weathering.
The most common constituents of detrital rocks are clay minerals and quartz. Clay minerals are the most abundant product of chemical weathering of feldspars. Quartz is also abundant because it is very resistant to chemical weathering.
CLASSIFICATION OF DETRITAL SEDIMENTARY ROCKS
Detrital sedimentary rocks are classified on the basis of particle size into the following:
1- Conglomerate or breccia: >2mm
Conglomerate and breccia are similar in having grains more than 2mm in size but conglomerate is composed of rounded grains whereas breccia is composed of angular grains.
2- Sandstone: 2 – 1/16 mm
3- Shale or mudstone: < 1/16
Shale is usually fissile (able to split into thin layers) whereas mudstone is compact and breaks into blocks. Shale is the most common detrital sedimentary rock followed by sandstone then conglomerate or breccia.
Shale:
- Shale is a fine-grained, fissile sedimentary rock consisting of silt and clay-sized particles.
- It is deposited in quiet-water environment such as lakes, lagoons, swamps, river floodplains and portions of the deep-ocean basins.
- Shale is impermeable and doesn’t allow fluids like water and petroleum to penetrate its microscopic tiny pores. Rocks that contain underground water are usually underlain by shale, which prevents downward movement. On the contrary; rocks that contain oil are capped by shale beds, which prevent oil and gas from escaping to the surface.
Uses of shale:
1- Raw material for pottery, brick and china.
2- Mixed with limestone to make Portland cement.
3- Oil shale will be a valuable energy resource in the future.
Sandstone:
Sandstone is a detrital sedimentary rock consists of sand-sized grains.
The degree of similarity in particle size is called sorting. If all the grains are about the same size the rock is well sorted, on the other hand if the rock contains mixed large and small grains, it is poorly sorted. Sandstone is a good reservoir rock for the underground water.
Chemical sedimentary rocks are formed from material that is carried in solution to lakes and seas and then precipitates. The precipitation occurs in two ways:
1- Inorganic processes: such as evaporation and chemical activity. Example: limestone, chert and rock salt.
2- Organic processes: by organisms living in water which extract dissolved mineral matters to form their skeletons. Example: Mollusk and corals
Limestone:
Limestone is a rock that is composed chiefly of the mineral calcite (CaCO3) and is formed by inorganic as well as biochemical process.
An example of limestone formed by biochemical processes includes coral reefs which secrets huge calcareous skeletons such as the great barrier reef of Australia (2000 km long). Another example is coquina, which is composed of poorly cemented shells and shell fragments.
Another example of limestone formed by inorganic process is the travertine which is deposited in caves when groundwater rich in CaCO3 losses its dissolved CO2 after exposure to air causing CaCO3 to precipitate.
Turning sediment into sedimentary rock (LITHIFICATION):
Lithification is a process by which unconsolidated sediments are transformed
into solid sedimentary rocks. This process requires two stages: compaction and
cementation.
Compaction results when sediments accumulate through time and the weight of the overlying sediments compresses the deeper sediments causing reduction of the total volume as well as the pore spaces of the sediments.
Cementation follows compaction and results when cementing materials like calcite, silica or iron oxides, which are dissolved in water, precipitate and fill the open spaces and join the particles together.
Classification of sedimentary rocks:
Sedimentary rocks are classified according to their mode of formation into clastic and nonclastic sedimentary rocks.
1- Clastic (terrestrial) sedimentary rocks:
These are formed from grains and particles cemented and compacted together e.g. conglomerate, breccia, sandstone and shale.
2- Nonclastic sedimentary rocks:
These are sedimentary rocks formed by chemical and biochemical process. e. g. coral reefs, chalk, coquina, travertine, chert, and chalk.
SEDIMENTARY STRUCTURES
1- Bedding
Bedding means the occurrence of sedimentary rocks in the form of layers called strata or beds. Beds differ from each other in grain size, mineral composition, texture. …etc.
The planes separating these beds are called bedding planes. These are flat surfaces along which rocks tend to separate or break. Each bedding plane marks the end of one episode of sedimentation and the beginning of another.
2- Cross bedding
Normally, most beds are originally deposited horizontal. In some case, the layers within a bed are inclined to the horizontal. This is called cross bedding and is characteristic of sand dunes; rivers deltas are stream channel deposits.
3- Graded Bedding
This is also another kind of bedding where the particles within a single layer gradually change from coarse at the bottom to fine at the top. It is characteristic of rapid deposition from water containing sediments of varying sizes. As the energy of water decreases, coarse grains deposit first, and in time smaller particles settle to produce graded bedding.
4- Ripple Marks
Ripple marks are undulations of sand that develop on the surface of a sediment layer by the water or wind. Ripple marks formed by water or wind moving in one direction are called current ripple marks. They are asymmetric and have steep slops in the down-current side and gentle slope in the up-current side. Current Ripples include those formed on the surface of sand dunes or those formed by a stream flowing in a sandy channel. Ripple marks formed by the back- and forth movement of waves in a shallow near shore environments are called oscillation ripple marks and are usually symmetric.
5- Mud cracks
These are polygonal shapes result from drying and shrinkage of mud. They are found in shallow lakes and desert basins where mud is alternately wet and dry.
Fossils
Fossils are remains or traces of ancient organisms, which are usually found in sedimentary rocks.
Conditions of fossil preservation:
Two conditions are necessary for fossil preservation:
1- Possession of hard parts.
2- Rapid burial.
Modes of fossil preservation:
Fossils were preserved by several ways, in some cases the entire organism is preserved as in the mammoth of Siberia and the insects in amber. But generally the soft parts of the organism decay and only the hard part is preserved. Fossil hard skeletons are common in rocks especially those of marine origin. The original hard skeleton of fossils doesn’t stand unchanged for long time, but usually undergoes alteration due to diagenetic processes. For this reason it is rare to find the original skeletons without alteration in their mineralogical composition, especially in older rocks. The original mineralogy of fossil skeletons may recrystallize to another mineral or in many cases dissolve and replaced by other minerals or sediments that fill the voids left after the skeleton is removed. This change in skeletons is represented in fossil record by various forms such as recrystallization, replacement, permineralization, cast, mold and imprint. Also the organisms sometimes leave traces of their activity such as burrowing, boring, tracks, trails and coprolites.
Importance of fossils in sedimentary rocks:
1- Understanding the environment, in which the rock formed.
2- Determining the age of the rock.
3- Correlating rock of similar ages in different places.
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Weathering is the physical breakdown and chemical alteration of rocks at or near earth’s surface. Weathering is one of three processes that continually remove earth’s material from higher elevations to lower elevations. These processes include mass wasting (the transfer of rock and soil downslope under the influence of gravity) and erosion (the physical removal of material by mobile agents such as water, wind or ice.
Kinds of weathering:
1- Mechanical weathering: This is the physical breakdown (disintegration) of rocks into smaller pieces without changing their mineralogical composition
2- Chemical weathering: This is the chemical transformation of a rock into one or more new compounds.
CHEMICAL WEATHERING
Chemical weathering is the alteration of the original components of the rock into new substances. The new minerals produced, are stable in the new environment and remain unchanged.
Water is the most important agent of chemical weathering especially when it contains dissolved materials.
The major processes of chemical weathering are dissolution, oxidation and hydrolysis.
Dissolution
Dissolution is the process by which water dissolves the mineral constituents of the rocks. Water is effective in dissolving minerals especially when it contains small amounts of acids (an acidic solution contains the reactive hydrogen ion, H+). These acids are:
- Carbonic acid: produced when CO2 in the atmosphere dissolves in raindrops forming acidic rainwater.
- Organic acids: produced from the decay of organisms.
- Sulphuric acid: produced from the weathering of pyrite and other sulfide minerals.
These acidic substances decompose rocks and produce new minerals. For example calcite, which composes the common building stones; marble and limestone is easily dissolved by acidic solutions.
CaCO3 +2[H+ (H2)O] Ca2+ + CO2 + 3(H2O)
According, most limestones are dissolved and carried by underground water producing caves and holes.
Also monuments and buildings made of limestone or marble such as the pyramids and sphinx in Egypt are also subjected to the corrosive work of acids, particularly in industrial areas that have smoggy, polluted air.
On the other hand, the soluble ions like Ca2+ are retained in our underground water supply making it hard water. The hard water is undesirable in cleaning with soap.
Oxidation
Oxidation is the reaction of minerals with oxygen. For example the reaction of iron-rich minerals with oxygen to form iron oxides (rusting).
4Fe +3O2 2Fe2O3 ( hematite)
Oxidation
is important in decomposing the ferromagnesian minerals as olivine, pyroxene and
Hornblend. Oxygen reacts with iron in these minerals to form the reddish brown
iron oxide called hematite (Fe2O3) or a yellowish colored
rust called limonite {(FeO (OH))}. These products are responsible for the rusty
color on the surfaces of dark igneous rocks such as basalt.
Oxidation has also serious environmental hazards, particularly in humid areas where abundant rainfall infiltrates spoil banks (waste material left after coal or other minerals are removed). This mine acid eventually makes its way to streams killing aquatic organisms and degrading aquatic animals
Hydrolysis
Hydrolysis is the reaction of any substance with water. Water molecules dissociate to form H+ (very reactive) and (OH)- ions. H+ attacks and replaces other positive ions found in the crystal lattice, destroying the internal regular arrangement of atoms and so mineral decomposes.
Water usually contains other substances like H2CO3 (carbonic acid) that contribute additional H+ and (HCO3)- (bicarbonate).
Hydrolysis of granite (in the presence of carbonic acid)
Granite, a common continental igneous rock consists mainly of quartz and orthoclase (potassium feldspar).
2KAlSi3 O8 + 2(H+ + HCO-3) + H2O Al2 Si2 O5 (OH)4 (Kaolinite) + 2K+ + 2 HCO-3 +4 SiO2
H+ attacks and replaces K in the feldspars thereby disturbing the crystalline network.
Products of granite hydrolysis:
1- K+ which is used as a nutrient for plants or becomes the soluble salt KHCO3 which is carried out to the ocean.
2- Kaolinite, a clay mineral used in ceramic industry. Generally clay minerals make up a high percentage of the inorganic material in soils.
3- Silica which is carried out by underground water. This dissolved silica will eventually precipitate producing nodules of chert or flint or fill in the pore spaces between grains of sediment or carried out to the ocean where it is used by microscopic animals to build their shells.
Soil:
Such as loose leaves and other organic debris. The lower portion is made up of partly decomposed organic matter (humus) in which plant structure is teeming with microscopic life including bacteria, fungi, algae and insects. All of these organisms contribute oxygen, CO2 and acids to the developing soil.
A-horizon:
Underlies the O-horizon and is largely composed of mineral matter. Biological activity is high and humus is generally present up to 30%.
E-horizon:
Underlies the A-horizon. It is a light colored layer that contains little organic material. As water percolates downward through this zone, finer particles are carried away. This washing out of fine soil components is termed eluviation. Water percolating downward also dissolves soluble inorganic soil components and carries them to deeper Zones. This depletion of soluble materials from the upper soil is called leaching.
B-horizon:
Lies below the E-horizon and in which much of the material removed from the E-horizon by eluviation is deposited. For this reason it is often referred to this zone as the zone of accumulation. The accumulation of fine clay particles enhances water retention in the subsoil (B-horizon).
C-horizon:
Lies below the subsoil (B-horizon) and is composed of partially altered parent material.
Below this horizon lies the parent rock, which is not yet weathered.
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CHAPTER SIX
Metamorphic rocks are rocks that are transformed from igneous, sedimentary or even metamorphic rocks, by the effect of temperature, pressure or both.
Metamorphism is the process which causes this change. This process always occurs deep within the earth beyond our direct observation.
The agents of metamorphism include heat, pressure and chemically active fluids. Those changes that occur are both textural and mineralogical.
According to the degree of transformation metamorphism may be of low-grade or of high-grade. Under low-grade metamorphism, rocks like shale becomes a more compact metamorphic rock called slate. On the other hand high-grade metamorphism causes a transformation so complete that the identity of the original rock cannot be determined. Features as bedding planes, fossils and vesicles are obliterated.
The zone of metamorphism extends from few kilometers below earth’s surface to the crust-mantle boundary.
Types of metamorphism
1-
Contact metamorphism:
Occurs when rock is near or touching a mass of magma. In this case changes are caused primarily by the high temperatures of the molten material.
2-
Cataclastic metamorphism:
This is the least common type of metamorphism and occurs along fault zone. Here the rock is broken and pulverized, as crustal blocks on opposite sides of a fault grind past each other.
3- Regional metamorphism: this occurs during mountain building processes where great quantities of rock are subjected to directed pressures and high temperatures causing large-scale deformation.
Effect of metamorphism
The metamorphic processes cause many changes in rocks; these changes are either textural or mineralogical.
Textural changes
Texture as mentioned before means the size, shape and distribution of particles that constitute a rock. Texture of metamorphic rocks is either foliated or nonfoliated.
I. Foliated texture
Foliated texture is produced when rocks are subjected to pressure. Pressure causes not only realignment of mineral grains but also recrystallization and growth of larger crystals. In such case minerals are said to have preferred orientation. The new orientation will be perpendicular to the direction of the compressional force.
The resultant mineral alignment usually gives the rock a layered or banded texture termed foliation. So, a foliated texture results whenever the minerals and structural features of a metamorphic rock are forced into parallel alignment.
There are various types of foliation depending upon the grade of metamorphism and the mineralogy of the parent rock. The main types are: rock or slaty cleavage, schistosity and gneissic texture.
1- Rock or slaty cleavage
Under low-grade regional metamorphism, a fine-grained sedimentary rock like shale is converted into a metamorphic rock called slate. The slate is more compact and thus more dense. During this transformation the directed pressure and flattening causes the microscopic clay minerals in shale to align into the more compact arrangement found in slate. At the same time the clay minerals recrystallize into minute mica flakes, these platy mica crystals become aligned so that their flat surfaces are nearly parallel. Consequently, slate can be split into flat slabs; this property is called rock or slaty cleavage.
2-
Schistosity
This texture is formed under more extreme temperature-pressure regimes, where the very fine mica grains in slate will grow many times larger. This gives the rock a type of foliation called schistosity. The rock that has this texture is called schist such as talc schist or mica schist.
3-
Gneissic texture
This texture is formed under high-grade metamorphism where ion migration is extreme enough to cause minerals to segregate. For example granite gneiss in which dark and light silicate minerals have separated giving the rock a banded appearance called gneissic texture. The rock that has this texture is called gneiss such as diorite gneiss and gabbro gneiss.
II.
Nonfoliated texture
Metamorphic rocks that don’t have a foliated texture are described as nonfoliated. For example marble which results from the metamorphism of limestone is nonfoliated and quartzite which resulted from the metamorphism of sandstone is also nonfoliated.
Mineralogical changes
Mineralogical changes result from the recombination or reaction between the existing original minerals and the available ions in the water to form new minerals that are stable in the new environment. For example: the recrystallization of clay minerals in the shale to form mica crystals.
Examples of metamorphic rocks
1- Slate: slate is a very fine-grained foliated rock, composed of minute mica flakes. It is produced from the low-grade metamorphism of shale and is characterized by its rock or slaty cleavage.
Slate’s color depends on the mineral constituents; black slate contains organic material (carbon-bearing), red slate from iron oxides and green slate from chlorite (a mica-like mineral formed by the metamorphism of iron-rich silicates).
2- Schist: schist is a strongly foliated rock that can be split into thin flakes or slabs. It contains mica minerals and amphibole. It is produced from a more extensive metamorphism of shale. Examples of schist include mica schist, graphite schist, talc schist and chlorite schist.
3- Gneiss: Gneiss is a banded metamorphic rock results from the high-grade metamorphism of shale or granite. Examples include granite gneiss and gabbros gneiss.
4- Marble: Marble is a coarse, crystalline nonfoliated rock results from the metamorphism of limestone. Pure marble is white in color; other colors of marble include pink, grey, green or black. These colors are due to the impurities present in the limestone.
Marble is used as a building stone in banks and government buildings, to carve monuments and statues.
5- Quartzite: Quartzite is a very hard metamorphic rock formed from a moderate to high grade metamorphism of quartz sandstone.
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CHAPTER SEVEN
CRUSTAL DEFORMATION
The results of tectonic activities are apparent in the earth's major mountain belts, which were once ocean basins. Now these rocks, some of which contain fossils of marine organisms may be found thousands of meters above sea level.
These geologic features such as folds, faults, joints and fractures are called geologic structures. The study of such structures has an economic important especially most occurrence of oil and natural gas are associated with geologic structures that act to trap these fluids in valuable "reservoirs" Rock fractures can also be the site of hydrothermal mineralization, which is a major source of metallic ores. Moreover, the orientation and characteristics of rock structures must be considered when selecting sites for major construction projects such as bridges, hydroelectric dams and nuclear power plants. The basic geologic structures associated with deformation are folds, faults and joints.
Deformation:
Deformation is a general term that refers to all changes in volume and or shape of a rock body. Most crustal deformation occurs along plates margins. As the plates interact along their boundaries, tectonic forces deform the involved rock units.
Rocks respond to deformation in various ways; at first they deform elastically (return to their original shape and size when the stress is removed). Once the elastic limit is surpassed, rocks either deform plastically (don’t return to their original size and shape when the stress is removed) or fracture. Plastic deformation is characteristic to ductile rocks and causes folding and flowing. On the other hand, fracturing occurs when the rock is brittle and in this case faults are formed.
Folds are structures formed during mountain building, where flat-lying sedimentary and volcanic rocks are bent into a series of wave-like undulations.
Folds result from compressional stress and are either broad flexures (bending), in which rock units hundreds of meters thick have been slightly warped. Other folds are very tight microscopic structures such as those found in metamorphic rocks.
Folds are found singly, but most often they occur as a series of undulations.
Parts of the fold:
Limbs: are the two sides of the fold.
Fold axis: is a line drawn along the points of maximum curvature of each layer.
The fold axis is usually horizontal (parallel to the surface). However in more complex folding, it is often inclined at an angle known as the plunge.
Axial plane: is am imaginary surface that divides a fold as symmetrically as possible.
Types of folds:
There
are two main types folds known as
anticlines
and
synclines.
The anticline is a structure formed by the up folding or arching of rock layers. In the anticline the oldest strata are found in the center and the two limbs dip away from each other.
On the other hand, the syncline is a structure formed by the down folding of rock layers. The youngest strata are found in the center and the two limbs dip towards each other.
Anticlines and synclines are usually found in associations where the limb of an
anticline is also a limb of the adjacent syncline.
Both anticlines and synclines are described as symmetrical when the limbs on either side of the axial plane diverge at the same angle and asymmetrical when they diverge at different angles.
Overturned fold:
Overturned fold is an asymmetrical fold in which one limb is tilted beyond the vertical.
Recumbent fold:
Recumbent fold is a fold in which the fold plane is horizontal.
Folds don't continue forever; rather, their ends die out much like the wrinkles in cloth. Some folds plunge because the axis of the fold penetrates into the ground. When erosion removes the upper layers of the structures, the outcrop pattern of the plunging anticline points in the direction of plunging, the plunging syncline on the other hand points in the opposite direction of plunging.
Monoclines:
Monoclines are broad flexures (bendings) that have only one limb. They usually result from vertical displacement and not from compressional stresses as the other folds. These monoclinal folds are produced from vertical faulting in deep-lying basement rocks. The rigid basement complex responds to vertical stress by fracturing, On the other hand the relatively flexible sedimentary strata above these basement rocks were deformed by folding.
Domes and basins:
Domes
are circular or elongated structures found in sedimentary rocks. They are formed
as a result of broad up warping in basement rocks underlying such domes. Domes
are also formed by the intrusion of magma (laccoliths) or upward migration of
salt formation (salt domes).
Basins
on the other hand are down warped structures having a similar shape to domes.
Basins are formed when large accumulations of sediments accumulate, the weight
of these sediments caused the crust to subside. Some structural basins may have
been the result of a large asteroid impact.
Faults are fractures in rocks across which there has been some relative movement has occurred relative to a flat surface described as a Fault Plane.
Parts of the fault:
Hanging Wall and the Footwall:
These are the two blocks on either side of an inclined fault. The hanging wall lies above the fault plane and the footwall lies below.
Throw:
This is the amount of relative displacement across a fault plane.
Strike of the fault:
This is the geologic bearing of the line formed by the intersection of the fault with the horizontal plane.
The dip of the fault:
This is the angle between the fault plane and the horizontal measured perpendicular to the strike.
Types of faults:
1- Normal Faults: when the hanging wall moves down relative to the footwall.
2- Reverse Fault: when the hanging wall moves up to the footwall. Reverse faults having a very low angle are also referred to as Thrust faults; in such case the fault plane is often parallel to the bedding planes.
3- Strike-slip fault: in which the dominant displacement is horizontal and parallel to the strike of the fault surface. The two blocks of rock simply slide past each other with no compression or extension. If the motion is to the right, it is called a Right-lateral fault. If the motion is to the left, it is called a left-lateral fault. The faulting can be combinations of strike-slip and normal or reverse fault, in such case the fault is called Oblique slip Fault. Strike-slip faults are the most common style of faulting at transform plate margins.
In the fault zone, repeated movements may crush the rock to form a fault
breccia, or grind up
the rock to powder called
Fault gouge.
As one block slides the other, stretches or striations are formed on the polished fault surface called Slickensides.
At depth, some faults curve into a near-horizontal orientation called a Listric fault.
Joints
Joints are fractures or discontinuities in a rock with no relative sliding or
movement on either
side of a joint. They result from the release confining
stresses.
If the joints are planar, parallel, and very closely spaced, the fracturing process is called Sheeting. If the joints are curved, concentric, parallel, and with uniform spacing, the fracturing process is termed Exfoliation.
Joints can also occur in igneous rocks as a result of cooling and called "Columnar Jointing" in which the rock cracks into columns with hexagonal cross section.
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CHAPTER EIGHT
PLATE TECTONICS
Continental Drift Theory (Wegener’s Theory):
Alfred Wegener (German Climatologist and geophysicist) proposed that continents were once joined together as a super continent that he called “Pangea” (about 200 m. y. ago). This supercontinent subsequently broke up and drifted apart, a process known as “ continental drift” (Fig.18.1)
Supports:
Fit of the continents:
He noticed the remarkable similarity between the coastlines on opposite sides of the south Atlantic.
Fossil Evidence:
He recognized that the observed paleontologic correlations (Fossil plant remains named Glossopetris and small reptile called Mesosaurus) between South America and Africa meant that these two continents must have been joined at some time in the past.
The similarities in the rock type and structures, and mountain belts on both sides of the Atlantic.
Paleoclimatic Evidence:
For instance, glacial deposits were found in the same stratigraphic position (end of Paleozoic) in South Africa and South America, as well as in India and Australia.
Objections:
1- Inability to provide a mechanism for continental drift. He proposed two possible energy sources for drift. One of these, the tidal influence of the moon. Secondly, he suggested that the continents plowed through the ocean basins. This assumption could be considered a predecessor to the idea of plate subduction. However, no evidence existed to suggest that the ocean floor was weak enough to permit passage of the continents without themselves being deformed in the process.
2- The assumption that the contents are not attached and move independently is also incorrect, because the continents are actually an integral part of lithospheric plates.
3- The postulation that the shapes of the continents- as defined by their coastlines- are permanent is in error because continental outlines are not fixed as a result of sedimentation or erosion.
II) Geosynclines and Mountain Building:
Prior to the emergence of “ The New Global Tectonics”, as plate tectonics was known as in the early 1960s, other explanations accounted for some of Earth’s geologic features, such as mountain ranges. These observations were commonly explained by a “geosynclinal theory”. It was thought that elongate belts of deep subsidence and thick sedimentation existed, which were called “geosynclines”. The thick geosynclinal strata were folded and metamorphosed and then uplifted and eroded to form the mountain belts that we see today.
III) Sea-Floor Spreading:
The first clues leading to plate tectonics came from research in the ocean basins.The American geophysicist Harry Hess proposed the concept of sea floor spreading, which was based primarily on the topography of the ocean floor (bathymetry). The ocean floor bathymetry that was determined formed an unexpected and remarkable picture. Huge submarine mountain ranges- the ocean ridges – were discovered. The distribution of these ridges and the symmetry of the ocean floor led Hess to suggest that the sea floor is created at these ridges and spreads outward from them.
Paleomagnetism and Polar Wandering:
The magnetic poles appeared to wander through the geologic time.
Apparent polar wander curves were used to plot the positions of the magnetic north pole at different geologic times and were generated for several continents.
For instance, N. America and Europe curves are similar in shape but away from each other (Fig.6.4.a) as a result of divergent margin activity along the Mid-Atlantic Ridge. If the plate motion is reversed, then the curves are nearly identical (Fig.6.4b).
Therefore, we can use paleomagnetic data to determine the relative plate motions through time.
Paleomagnetism and Sea Floor Spreading:
Two geophysicists from California Univ. collected samples from Hawaiian islands to measure the strength and orientation of Earth’s magnetic field.
Using these samples to document reversals of Earth’s magnetic field and the age of each reversal.
Many magnetic reversals have occurred irregularly in the geologic record.
Over the past 5 million years, Earth’s magnetic field has reversed, on the average, once every 200,000 years (Fig.6.5).
Magma rises new sea floor as previously described. As the magma cools, the magnetism in the minerals in the basalt becomes oriented into alignment with Earth’s magnetic field.
If the magnetic orientation is more or less parallel to the modern north, it is called normal polarity.
If the magnetic orientation is more or less parallel to the present-day south, it is called reversed polarity.
Sea Floor at oceanic ridges is geologically young, becoming progressively older as it moves away from the ridge (Fig.6.7).
Plate Tectonics and Earthquakes:
By 1968, the basic outline of global tectonics was firmly established. In the same year, three seismologists published papers demonstrating how successfully the new plate tectonics model accounted for the global distribution of earthquakes. They noticed the close association between plate boundaries and earthquakes by comparing their maps.
Evidence From Ocean Drilling:
Through Deep Sea Drilling Project 1968-1983.
Hot Spots:
Mapping of seamounts in the Pacific revealed a chain of volcanic structure extending from the Hawaiian Islands continuing northward.
K-Ar dating of volcanoes in this chain revealed an increase in age with an increase in distance from Hawaii.
Researches have proposed that a rising plume of mantle material is located below the island of Hawaii. Melting of this hot rock as it enters near the surface generates a volcanic area or hot spots.
The Driving Mechanism:
- The plate tectonics theory describes plate motion and the effects of this motion.
- Nevertheless, it is clear that the unequal distribution of heat within the earth is underlying driving force for plate movement.
- Three models were proposed to explain the plate movement:
1) large convection currents.
2) Slab-pull/slab-push model.
3) Hot plumes model.
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