Grade 12 Geography Notes - Unit 1 PDF

**Unit 1 1. Major Geological Processes Associated With Plate Tectonics 1.1 Continental Drift Theory** - The concept of the continental Drift Theory was proposed by German meteorologists Alfred Wegener in 1912. This theory suggested that the present continents were formed from a single sup...

**Unit 1 1. Major Geological Processes Associated With Plate Tectonics 1.1 Continental Drift Theory** - The concept of the continental Drift Theory was proposed by German meteorologists Alfred Wegener in 1912. This theory suggested that the present continents were formed from a single super continent called Pangaea. - During the Carboniferous period, some 350 million years ago, Pangaea was located in the South Pole. - In the early Triassic period of Mesozoic era, Pangaea started to break in to several parts. The northern part of Pangaea is known as *[Laurasia]*, while the southern part called *[Gondwanaland]*. - Laurasia includes the continent of North America, Greenland, Europe (without Balkan Peninsula) and Asia. (without India). While Gondwanaland comprises Antarctica, Australia, South America, India, and Africa. - As the drifting continuous, these two continents also sub-divided in to different the present continents. **[Evidences]**:- Alfred Wegener introduced the super continent Pangaea to explain the ancient climate similarity, fossil evidence, and similarity of rock structure between Africa and South America, as well as the outlines of the continent, especially the continental shelves which seem to fit together. Thus, Wegener used the following evidences, among others, to support his theory. 1. **[Coastline Similarities]** :- The coastlines of Africa and South America have remarkable similarities in the opposite sides of Atlantic Ocean. 2. **[Rock type and structural similarities]** :- When we fit the continents of Africa and South America "back together again", find that: - Similar rock types extend from one continent to the other; - The rocks are also the same age; and - The mountain belts extend from one continent to the next. 3. **[Fossil Evidences]** /;- Cases of several fossil organisms had been found on either side of the Atlantic ocean implying that the continents were once joined together. Example:- Fossil of Mesosaurus (an extinct small aquatic reptile of the early Permian period, with elongated body, flattened tail and long, narrow snout with numerous pointed teeth). 4. **[Paleo climatic evidence]**: -Similarity of climatic data reconstructed from rock structure are similar in the present continents. - He was not a geologist, by profession, which of course was most welcome by his opponents. - Most influential geoscientist at that time were based in the Northern hemisphere, whereas most of the conclusion data comes from the Southern hemisphere. - Wegener thought that Pangaea did not break up until Cenozoic era, and scientists found it hard to believe that so much continental drift could have occurred in so short a time. - The greatest problem remained the lack of direct evidence for the movement of continents and the needed explanation for the mechanism. Regardless of the controversies, Wegener's most important legacy is to have introduced the idea of lateral mobility of continents, that is, offering a paradigm change from fixism to mobility to the scientific community and the public. **1,2 Plate Tectonic Theory** The structure of the Earth comprises: the crust, mantle, and core. Each of these layers further subdivided. The Crust consists of continental and oceanic crust. The uppermost mantle is solid rock like the crust (Lithosphere), while the rest of the mantle is not weak and plastic (asthenosphere). The core consists of an outer liquid region surrounding a solid center. Tectonic -- is a Greek word which means 'construction'. The plate tectonic theory states that earth's surface made up of slabs of rocks that are slowly shifting right under our feet. Because of this constant movement, today's earth looks a lot different from what it did millions of years ago. Earth's lithosphere is broken in to a dozens of large and small pieces Plate tectonic is the modern theory of the movement of earth's outer shell, or lithosphere. Plates may be entirely made up of continental rocks, both continental and oceanic rocks, or entirely of oceanic rocks. Plate tectonics is a theory developed in the late 1960s to explain how the outer layers of the earth move and deform. The theory states that the earth's lithosphere is divided into plates that move around on the top of the asthenosphere. Continental crust is embodied within the lithospheric plate. The plates move in different directions, and meet each other at plate boundaries. Plate tectonics has been proven to be so useful that it can predict geologic events and explain almost all aspects of what we see on the earth. Plate tectonic process influences the composition of the atmosphere and oceans. It serves as a prime cause of long term climate change, and makes a significant contribution to the chemical and physical environment in which life evolves. Tectonic plates are divided in to major and minor tectonic plates. The major tectonic plates are: African, Antarctic, 1.3 **Plate movements and Plate boundaries** The way the plates interact at their margins depends on whether the crust forming the top of the plate (at the point of contact) is oceanic or continental. Continental crust, made largely of granite, is less dense than oceanic crust made largely of basalt. As the plates move, they may: 1\. Converge or come together; forming convergent (Destructive) boundary; 2\. Diverge, or spread apart; forming divergent (Constructive) boundary or 3\. Slide past each other along fractures called transform (Conservative) boundary. **1.4 Major Geological Processes** The term \"geological processes\" describes the natural forces that shape the physical makeup of our planet earth. Plate tectonics, erosion, chemical weathering and sedimentation are all examples of forces that significantly affect the Earth's surface and account for its major features. The geological processes are closely studied by geologists and earth scientists to: - Improve the understanding of the planet's history; - Help to locate useful resources, such as metal ores; and - Aid the prediction of potentially disastrous events, such as earthquakes, tsunamis and volcanic eruptions. Some of the geological processes bring about changes on the surface of the earth. The forces that bring about changes on the earth's surface are divided into two. They are: 1\. Endogenic forces (also called [internal forces], are used to describe pressure that originates inside the earth) 2\. Exogamic forces (The forces which derive their strength from the earth's exterior or originate within the earth's atmosphere are called exogamic forces or [external forces]) Folding occurs when rock layers are pushed by earth movement's sideways. The layers of rock bend up from an [upfold or anticline]. Those which bend down form a downward [arch or syncline]. If compression continues then simple folds are changed into asymmetrical folds, over folds and over thrust folds. - **Symmetrical fold**: both limbs gas the same steepness. - **Asymmetrical fold**: one limb is steeper than the other. - **Over fold**: One limb is pushed over the other limb. - **Over thrust fold**: When pressure is very great, a fracture occurs in the fold and one limb is pushed forward over the other limb. **Types of Fold Mountains** The Fold Mountains of the world are grouped into two: They are: 1. Young Fold Mountains 2. Old Fold Mountains During the last 400 million years, there have been three main mountain-building periods. These periods experienced mountain-building processes known as orogenesis. Fold Mountains formed during the first and second mountain-building periods are known as old fold mountains. The last and the recent is known as Alpine orogeny. Mountains belonging to this period include the Andes, Rockies, Alps, Himalayas, Atlas and Australian Alps. They are called young fold mountains. The old fold mountains are the oldest, dating back 250 to 300 million years, and they are characterized by lower heights are more weathered. Scandinavian (Calidonides) Mountains, Appalachian Mountains, and the Urals are some examples of old fold mountains. **FAULTING** A fault is a crack on the earth's crust. It is formed by the forces of tension and compression. A fault may occur in the rocks along a single line. When this happens, rocks are displaced either upward or downward. Usually a series of faults could develop on the surface of the earth. These faults may be roughly parallel to each other. Where parallel faults have occurred, the land in between may sink down or may be forced to move upwards. These movements of the earth will result in the formation of the following landforms: 1. **Rift valleys (Draben)** - They are formed when the land between two faults sinks down. The blocks on both sides of the valley form plateaus For example the East African Rift Valley. 2. **Block Mountains** - They are formed when the land between two parallel faults is pushed upward. A block mountain is called horst. For example: the Afar Horst. **EARTHQUAKES** The sudden shaking of the ground that occurs when masses of rock change position below Earth's surface is called an earthquake. The shifting of the rock releases a great amount of energy, sending out shock waves that travel through the rock, and cause the ground to shake. These shock waves, called seismic waves by Earth scientists, may be powerful enough to alter the surface, thrusting up cliffs and opening great cracks in the ground. Earthquakes occur most often along geologic faults, which are fractures in the rocks of Earth's crust. Along faults, the rock masses on opposite sides of the fracture strain against each other and sometimes "slip," causing an earthquake. The major fault lines of the world are located at the fringes of the huge tectonic plates that make up the crust. Earthquakes, called temblors by scientists, occur almost continuously. Fortunately, most of them can be detected only by sensitive instruments called seismographs. A scale is used for reading it. It is called a Richter scale. It gives reading from 0 (no movement) to 9 (extremely severe). **Causes of Earthquakes** Most of the worst earthquakes are associated with changes in the shape of the crust. These earthquakes are generated by the rapid release of strain energy that is stored within the rocks of the crust. The strongest and the most destructive quakes are associated with ruptures of the crust, which are known as faults. Although faults are present in most regions of the world, earthquakes are not associated with all of them. **Shock Waves** The shifting rock in an earthquake causes shock waves called *[seismic waves]* to spread through the rock in all directions. In a great earthquake, shocks may be felt by people thousands of miles or kilometers away from the center. *Seismographs* can pick up the waves on the other side of the world. There are two broad classes of seismic waves: *[Body waves]* and *[surface waves]*. Body waves travel within the body of the Earth. They include *P, or primary waves and S, or secondary waves*. P waves spread in the crust from the point of rupture, which is called the *[focus of the earthquake]*. The point of which the wave originates is what we call the ***[Earthquake focus]***. The point on the Earth's surface immediately above the focus is termed the ***[epicenter of an earthquake]***. P waves alternately compress and expand the rock through which they pass, and vibrate in the same direction in which the waves travel. **S waves** vibrate at right angles to the direction of wave travel. After both P and S waves have moved through the body of the Earth, they are followed by *[two types of surface waves]*, which travel along the Earth's surface. These are named ***[Love and Rayleigh waves]***, after the scientists who identified them. Because of their larger ***amplitude***, surface waves are responsible for much of the *destructive shaking* that occurs far from the *epicenter*. Surface waves, which travel more ***slowly than body waves***, are the [most powerful shake waves]. **Effects of Earthquakes** Earthquakes often cause *dramatic changes* at the Earth's surface. In addition to the ground movements, other surface effects include *changes in the flow of groundwater, landslides, and mudflows*. Earthquakes can do significant *damage to buildings, bridges, pipelines, railways, embankments, dams, and other structures*. Earthquakes can lead to *devastating fires.* Fire produced the greatest property loss following the 1906 San Francisco earthquake, when 521 blocks in the city center burned uncontrollably for three days. Fire also followed the 1923 Tokyo earthquake, causing much damage and hardship for the citizens. Underwater earthquakes can cause giant waves called tsunamis. Violent shaking of the seafloor produces waves that spread over the ocean surface in ever-widening circles. In deep water a tsunami can travel as fast as 800 kilometers per hour. By the time a tsunami reaches shore, it has gained tremendous size and power, reaching heights as great as 30 meters. Tsunamis can be catastrophic, with the potential to wipe out coastal settlements. **Occurrence** Most earthquakes take place on one of the two great earthquake belts that girdle the world. The belts coincide with the more recently formed mountain ranges and with belts of volcanic activity. 1. Earthquake belt circles the Pacific Ocean along the mountainous west coasts of North and South America and runs through the island areas of Asia. It is estimated that 80 percent of the energy released in earthquakes comes from this belt, which is called the ***[Circum-Pacific Belt or the Ring of Fire]*** 2. **Less active belt** passes between Europe and North Africa through the Mediterranean region. It then runs eastward through Asia and joins the Ring of Fire in the East Indies. The energy released in earthquakes in this belt is about 15 percent of the world total. 3. There are also remarkably **connected belts of seismic activity**, mainly along mid oceanic ridges including those in the Arctic Ocean, the Atlantic Ocean, and the western Indian Ocean and along the Great Rift Valley of East Africa. The belts of Most earthquakes take place The **focus** of an earthquake may occur from **quite close to the surface down to** a maximum **depth** of about **700 kilometers**. More than 75 percent of the seismic energy produced each year, however, is released by shallow focus earthquakes, that is, quakes with foci less than about 60 kilometers deep. Most parts of the world experience at least occasional shallowfocus earthquakes. About 12 percent of the total energy released in earthquakes comes from intermediate earthquakes, those with foci ranging from about 60 to 300 kilometers deep. About 3 percent of the total energy *[comes from deeper earthquakes]*. The deeper focus earthquakes commonly occur in **Benioff zones**, which dip down into the mantle at places where [two tectonic plates converge]. A Benioff zone extends down along the plate that is being subducted. **Volcanism** **[Volcanism]** :- is the process by which molten rock or magma, together with gaseous and solid materials is forced out on the surface of the earth. This movement could be *[very slow or sudden]*. With slow movement, the materials could spread over the surrounding area gently. If the movement is violent, the materials will be thrown high into the sky and then fall back in the surface of the earth. Magma may reach the surface of the earth through two types of openings: 1. Vents and 2. Fissures [Vents] are holes or openings like a pipe throw which magma flows out into the surface of the earth. If lava emerges via vent, it builds up a **[volcano (a cone shaped mound)]**. Fissures are large and narrow cracks or fractures in a rock. Molten materials may move upward along the cracks and spread out over the surroundings. If the lava emerges via a fissure, it builds up a **[plateau]**. During the formation of lava plateau, there is little or no explosive activity through the fissures; lava gently spreads over large area. The word **[volcano]** refers to the form or structure, usually conical, produced by accumulations of erupted material. In some volcanic eruptions, the molten rock called **[magma]** when it is underground and **[lava]** when it reaches the surface, flows slowly out of the vent. *Occurrence Volcanoes occur mainly near the boundaries of tectonic plates*. They form along belts of tension, where plates diverge, and along belts of compression, where plates converge. Nearly 1,900 volcanoes are active today or are known to have been active in historical times. Of these, almost 90 percent are situated in the **Pacific Ring of Fire**. The **[Mediterranean-Asian belt]**, which accounts for most of the world's earthquakes outside the Ring of Fire, has few volcanoes except the Indonesia and Mediterranean which have more numerous ones. [Oceanic volcanoes] are strung along the world's oceanic ridges, while the remaining *[active volcanoes]* are associated with the **Great Rift Valley of East Africa**. Volcanic activity typically alternates between short active periods and much longer dormant periods. 1. An extinct volcano is one that is not erupting, and is not likely to erupt in the future. Example, Mount Zuqualla, Ethiopia. 2. A dormant volcano is currently inactive but has erupted within historic times, and is likely to do so in the future. Example: Mount Kilimanjaro, Tanzania 3. An active volcano is one that has erupted or thought to have erupted during the last 500 years. Example: Erta Ale, Ethiopia. Volcanoes are usually *classified by* **shape** and **size**. These are determined by such factors as - the volume and type of volcanic material ejected, - the sequence and variety of eruptions, and - the environment. Among the most common types are 1.Shield volcanoes 2.Strato volcanoes 3.Cinder cones 4. Craters, and 5. Calderas Shield ***[Shield volcanoes]*** are volcanoes that have a low, but broad profile created by highly fluid lava flows that spread over wide areas. The lava, usually composed of basalt, cannot build up a cone with sides much steeper than 7 degrees. The Hawaiian Islands are composed of shield volcanoes that have built up from the seafloor to the surface some 5 kilometers above. ***[Strato volcanoes]*** are the most common volcanic form. They are steep cones composed of alternating layers of lava and pyroclastics, or rock fragments. These volcanoes are characterized by a steep profile and periodic, explosive eruptions. The lava that flows from them is highly viscous, and cools and hardens before spreading very far. ***[Cinder cone volcano]*** is a conical hill of mostly cinder-sized pyroclastics. The profile of the cone is determined by the angle of repose, that is, the steepest angle at which debris remains stable and does not slide downhill. Other landforms created by volcanoes include ***craters and calderas***. [Craters] are formed either by the massive collapse of material during volcanic activity, by unusually violent explosions, or later by erosion during dormancy. [Calderas] are large, basin-shaped depressions. Most of them are formed after a magma chamber drains and no longer supports the overlying cone, which then collapses inward to create the basin. When magma cools and solidifies within the [crust intrusive or plutonic igneous rocks] are formed deep beneath the Earth's surface. Thus, intrusive land forms are the results of part of the magma that solidifies within the crust: Some of the intrusive igneous rocks include ***[batholith], [dyke], and [sill]***. ***Batholith*** is a very large dome shaped intrusion of igneous rock. It is located several kilometers deep in the crust, and extends over hundreds of square kilometers. Sometimes, it forms the core of a mountain. ***Sill*** is a near horizontal intrusion of igneous rock between two rock layers. The cooled rock forms a sheet of more or less parallel to the surrounding layers of rocks. ***Dike*** is formed as the magma rises up through a near vertical crack. As the magma cools, it forms a vertical sheet of rock or a wall like structure. **EXTERNAL (EXOGENIC) FORCES**