3(iv). Interior of the Earth lecture.docx

3(iv). Interior of the Earth lecture.docx

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  1 3(iv). Interior ofthe Earth Most of what we know about the interior of the Earth comes from the study of seismic waves from earthquakes. Seismic waves from large earthquakes pass throughout the Earth. These waves contain vital information about the internal structure of the Earth. As seismic waves pass through the Earth, they are refracted, or bent, like rays of light bend when they pass through a glass prism. Because the speed of the seismic waves depends on density, we can use the travel-time of seismic waves to map change in density with depth, and show that the Earth is composed of several layers. The Earth's internal structure can be divided into several layers based on their composition, physical properties, and behavior. These layers include the crust, mantle, outer core, and inner core. 1. Crust The crust ranges from 5–70 km (~3–44 miles) in depth and is the outermost layer. The thin parts are the oceanic crust, which underlie the ocean basins (5–10 km) and are composed of dense (mafic) iron magnesium silicate igneous rocks, like basalt. The thicker crust is continental crust, which is less dense and composed of (felsic) sodium potassium aluminum silicate rocks, like granite. The rocks of the crust fall into two major categories – sial and sima. It is estimated that sima starts about 11 km below the Conrad discontinuity (a second order discontinuity). The uppermost mantle together with the crust constitutes the lithosphere. The crust-mantle boundary occurs as two physically different events. First, there is a discontinuity in the seismic velocity, which is most commonly known as the Mohorovičić discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks. Properties of the earth crust: The Earth's crust possesses several key properties that are fundamental to understanding its composition, behavior, and significance in geological processes. Here are some of the main properties of the Earth's crust:
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  2 1. Composition: TheEarth's crust is primarily composed of various types of rocks and minerals. The exact composition can vary depending on whether it's continental or oceanic crust, but common elements include silicon, oxygen, aluminum, iron, calcium, sodium, and potassium. 2. Thickness: The thickness of the Earth's crust varies significantly. Continental crust is generally thicker, ranging from about 20 to 70 kilometers (12 to 43 miles), while oceanic crust is much thinner, typically around 5 to 10 kilometers (3 to 6 miles) thick. 3. Density: Continental crust has a lower density compared to oceanic crust. This difference in density contributes to the phenomena of isostasy, where less dense continental crust "floats" higher on the denser material of the mantle beneath. 4. Temperature: The temperature of the Earth's crust increases with depth. At the surface, temperatures can range widely depending on location and environmental factors, but on average, the temperature increases by about 25 to 30 degrees Celsius per kilometer of depth. 5. Rigidity: The Earth's crust is relatively rigid, especially compared to the underlying mantle. This rigidity allows for the formation of landforms such as mountains, valleys, and plateaus through processes like tectonic activity and erosion. 6. Heterogeneity: The Earth's crust is not uniform in composition or structure. It exhibits heterogeneity at various scales, including differences in rock types, geological features, and tectonic boundaries. 7. Tectonic Activity: The Earth's crust is involved in tectonic processes such as plate movements, earthquakes, and volcanic activity. These processes reshape the crust over geological timescales and contribute to the dynamic nature of the Earth's surface. Understanding these properties is essential for geologists and scientists studying the Earth's crust to gain insights into its formation, evolution, and role in shaping the planet's surface and processes.
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  3 2. Mantle The mantleis a significant layer within the Earth's structure, lying between the outer core and the Earth's crust. It makes up the largest portion of the Earth's interior by volume, accounting for about 84% of the Earth's total volume. It is divided into two parts; Upper Mantle & Lower Mantle. Here are some key properties and characteristics of the Earth's mantle:
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  4 1. Composition: Themantle is primarily composed of silicate minerals rich in magnesium and iron. Olivine, pyroxene, and garnet are among the most abundant minerals found in the mantle. These minerals undergo high-pressure and high-temperature conditions within the mantle. 2. Physical State: The mantle is in a semi-solid state, behaving as a viscous fluid over geological timescales due to the intense heat and pressure within Earth's interior. However, it is solid rock, capable of transmitting seismic waves. The mantle's ability to flow over long periods of time is known as mantle convection, a crucial process in plate tectonics. 3. Thickness: The mantle is several hundred kilometers thick, extending from about 30 kilometers below the Earth's surface (in oceanic regions) to approximately 2,900 kilometers deep at the boundary with the outer core. 4. Temperature and Pressure: Temperatures in the mantle increase with depth, ranging from around 500°C (932°F) near the upper boundary with the crust to over 4,000°C (7,232°F) at the core-mantle boundary. Pressure also increases significantly with depth due to the weight of overlying rock layers. 5. Convection: Heat from the Earth's core and radioactive decay of elements within the mantle drive convection currents, causing the mantle to slowly flow in a process known as mantle convection. This movement plays a critical role in the motion of tectonic plates and the redistribution of heat within the Earth. 6. Chemical and Physical Processes: Various chemical and physical processes occur within the mantle, including partial melting, crystallization, and chemical reactions between different mineral phases. These processes influence the composition and behavior of magmas that eventually reach the Earth's surface in volcanic eruptions. Understanding the mantle's properties and processes is essential for scientists studying Earth's interior dynamics, plate tectonics, volcanic activity, and the planet's overall evolution.
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  5 3. Outer core Theouter core is a layer of the Earth located beneath the mantle and surrounds the inner core. It is one of the Earth's major structural components, and it plays a crucial role in the planet's magnetic field generation. Here are some key characteristics of the outer core: 1. Composition: The outer core is primarily composed of liquid iron and nickel. It is in a molten state due to the high temperatures and pressures found in the Earth's interior. The exact composition is inferred from seismic wave behavior and laboratory experiments simulating high-pressure conditions. 2. Thickness: The outer core extends from the boundary with the mantle, approximately 2,900 kilometers beneath the Earth's surface, to the boundary with the inner core, which lies at a depth of about 5,150 kilometers. 3. Temperature and Pressure: Temperatures in the outer core are extremely high, ranging from about 4,000°C (7,232°F) near the top to over 6,000°C (10,832°F) near the bottom. Pressure also increases with depth, reaching immense levels due to the weight of the overlying materials.
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  6 4. State: Theouter core is in a liquid state due to its high temperature and pressure conditions. This fluid state allows for convective motion within the outer core, which generates Earth's magnetic field through a process known as the geodynamo. 5. Magnetic Field Generation: The convective motion of molten iron and nickel within the outer core generates electrical currents. These currents, combined with the Earth's rotation, produce a magnetic field through a process called the geodynamo effect. This magnetic field extends far into space and plays a crucial role in protecting the Earth from harmful solar radiation and in guiding compass needles. 6. Density: The outer core is denser than the mantle and the Earth's crust. Its high density contributes to the overall mass and gravitational pull of the Earth. Understanding the properties and dynamics of the outer core is essential for comprehending Earth's magnetic field generation, the behavior of seismic waves, and the internal processes driving geological phenomena such as plate tectonics and mantle convection.
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  8 Name of discontinuitiesinside the earth Inside the Earth, there are several important discontinuities, or boundaries between different layers, which mark significant changes in composition, density, and physical properties. Some of the most notable discontinuities include: 1. Mohorovičić Discontinuity (Moho): This is the boundary between the Earth's crust and the underlying mantle. Named after the Croatian seismologist Andrija Mohorovičić, the Moho is characterized by a rapid increase in seismic wave velocity, indicating the transition from less dense rocks of the crust to the denser rocks of the mantle. 2. Gutenberg Discontinuity: Also known as the core-mantle boundary, the Gutenberg Discontinuity marks the boundary between the Earth's mantle and outer core. It is named after the seismologist Beno Gutenberg. Seismic waves experience a sharp decrease in velocity at this boundary, indicating the transition from the solid mantle to the liquid outer core. 3. Lehmann Discontinuity: This discontinuity separates the Earth's outer core from the inner core. It was discovered by Danish seismologist Inge Lehmann. Seismic waves
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  9 exhibit a suddenincrease in velocity upon crossing this boundary, indicating the transition from the liquid outer core to the solid inner core. 4. Conrad Discontinuity: Found within continental crust, the Conrad Discontinuity marks a change in seismic wave velocity, suggesting a transition from less dense upper crustal rocks to denser lower crustal rocks. It is not as prominent or well-defined as the Moho. 5. Wiechert-Gutenberg Discontinuity: This discontinuity lies within the mantle, about 220 kilometers (137 miles) below the Earth's surface. It is characterized by a change in the behavior of seismic waves, possibly indicating a transition zone with different material properties. These discontinuities are significant features that help geoscientists understand the Earth's internal structure, composition, and behavior. They play a crucial role in interpreting seismic data and studying the dynamics of the Earth's interior.