Glossary

2.3 Earth's internal structure and dynamics

2 min read•july 24, 2024

Earth's interior is like a cosmic onion, layered with fascinating structures. From the thin we walk on to the blazing hot core, each layer plays a crucial role in shaping our planet's behavior and appearance.

Seismic waves act as Earth's internal X-ray, revealing its hidden depths. These waves travel differently through various materials, helping scientists map out the planet's structure and composition. Understanding these layers is key to grasping Earth's dynamic processes.

Earth's Internal Structure

Layers of Earth's interior

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  • Crust forms outermost layer varying 5-70 km thick composed primarily of silicate rocks divided into oceanic (denser, thinner) and continental (less dense, thicker) types

  • extends to 2900 km depth as largest layer by volume composed mainly of iron and magnesium-rich silicate rocks split into upper and lower sections with partially molten in upper region

  • spans 2900-5150 km depth as liquid layer of iron and nickel with temperatures 4400-6100℃ generating Earth's magnetic field

  • sits at Earth's center as solid sphere 1220 km radius made of iron and nickel under extreme pressure and heat up to 5400℃

Seismic waves for Earth's structure

  • Seismic waves from earthquakes or explosions travel at varying speeds through different materials revealing Earth's internal structure

  • move fastest as compressional waves traversing solids, liquids, and gases arriving first at seismic stations

  • travel slower as shear waves moving only through solids unable to propagate in liquids or gases

  • Wave behavior at boundaries involves refraction (direction change in new medium), reflection (bouncing off layer interfaces), creating shadow zones where waves don't arrive

  • Seismic velocity changes mark transitions between Earth's layers indicating composition and physical state of interior regions

Mantle convection and plate motion

  • drives large-scale circulation of solid material transferring heat from Earth's interior to surface through rising hot lower mantle material and cooling, sinking material at cell boundaries

  • currents power with upwelling at divergent boundaries and downwelling at zones

  • Heat sources include residual heat from Earth's formation and radioactive decay in mantle and crust

  • Surface effects encompass oceanic crust creation/destruction, mountain building, continental drift, and earthquake/volcano distribution

Earth's outer core and magnetic field

  • Outer core contains liquid iron and nickel with lighter elements (sulfur, oxygen) exhibiting low viscosity, high electrical conductivity, and temperatures 4400-6100℃

  • Dynamo theory explains magnetic field generation through convection-induced electrical currents influenced by Earth's rotation via Coriolis effect

  • Magnetic field displays dipolar nature with varying strength protecting Earth from solar wind and cosmic radiation

  • Geomagnetic reversals involve periodic pole flipping recorded in magnetic minerals used for studying past plate motions and rock dating

  • Core-mantle boundary marks transition between outer core and lower mantle hosting complex interactions affecting mantle dynamics and core processes

Key Terms to Review (22)

Arthur Holmes: Arthur Holmes was a prominent British geologist known for his influential work in understanding the Earth's internal structure and the concept of radioactivity in geology. His research laid the groundwork for modern theories of geochronology and plate tectonics, connecting geological processes with the thermal history of the Earth and its dynamic internal mechanisms.
Asthenosphere: The asthenosphere is a semi-fluid layer of the Earth's upper mantle located beneath the lithosphere, characterized by its ability to flow and deform slowly over time. This layer plays a crucial role in tectonic plate movement and interactions, allowing the rigid plates of the lithosphere to slide over it. The asthenosphere's properties are vital for understanding geological processes such as volcanism, earthquakes, and the overall dynamics of Earth's interior.
Basalt: Basalt is a dark, fine-grained volcanic rock that forms from the rapid cooling of lava at the Earth's surface. This rock is primarily composed of plagioclase and pyroxene minerals and is significant in understanding Earth's internal structure because it typically forms the oceanic crust, which plays a key role in plate tectonics and mantle dynamics.
Conduction: Conduction is the process of heat transfer through a material without the movement of the material itself. This occurs when higher-energy particles collide with lower-energy particles, allowing thermal energy to spread from warmer regions to cooler regions. In the context of Earth's internal structure and dynamics, conduction plays a crucial role in how heat moves from the core to the mantle and then to the crust, influencing geological processes like plate tectonics and volcanic activity.
Convection: Convection is the process of heat transfer in fluids (liquids and gases) where warmer, less dense areas rise while cooler, denser areas sink, creating a circular motion. This mechanism plays a crucial role in the dynamics of Earth's internal structure, as it drives the movement of materials in the mantle and influences plate tectonics. Understanding convection helps explain various geological phenomena such as volcanic activity and earthquakes, connecting thermal energy and material behavior within the Earth.
Crust: The crust is the outermost layer of the Earth, composed of solid rock, and varies in thickness from about 5 kilometers under the oceans to up to 70 kilometers beneath some mountain ranges. It is divided into two main types: oceanic crust, which is thinner and denser, and continental crust, which is thicker and less dense. The crust plays a crucial role in Earth's internal structure and dynamics, serving as the foundation for landforms and geological processes such as plate tectonics and earthquakes.
Granite: Granite is a coarse-grained igneous rock composed mainly of quartz, feldspar, and mica, characterized by its light color and durability. This rock forms from the slow crystallization of magma beneath the Earth's surface, making it a significant indicator of the processes occurring in the Earth's crust. Its formation is closely tied to tectonic activity and the cooling of magma, linking it to the structure and dynamics of the Earth’s interior.
Gravity surveying: Gravity surveying is a geophysical method used to measure variations in the Earth's gravitational field caused by differences in subsurface density. This technique helps geologists understand the internal structure of the Earth, revealing information about rock types, geological formations, and even the presence of minerals or oil deposits beneath the surface. By analyzing gravity anomalies, scientists can infer the dynamics and processes occurring within the Earth.
Inge Lehmann: Inge Lehmann was a Danish seismologist known for her groundbreaking work in the early 20th century, particularly for discovering the existence of the Earth's inner core. Her research helped to enhance the understanding of Earth's internal structure and dynamics by providing evidence of the solid inner core surrounded by a liquid outer core, which has profound implications for the study of seismic waves and geophysical processes.
Inner core: The inner core is the Earth's innermost layer, composed primarily of solid iron and nickel, and it is located beneath the outer core. This extreme environment has temperatures that can reach up to 5,700 degrees Celsius (about 10,300 degrees Fahrenheit), creating a dense and solid state despite the immense pressure. Understanding the inner core helps explain Earth's internal dynamics and contributes to our knowledge of its overall structure and composition.
Iron-nickel alloy: An iron-nickel alloy is a mixture of iron and nickel that is primarily found in the Earth's core. This alloy plays a crucial role in understanding the composition and behavior of the inner and outer core, contributing to Earth's magnetic field generation and overall internal dynamics.
Isostasy: Isostasy is the equilibrium between the Earth's crust and the denser mantle beneath it, which allows landforms to float at a certain elevation based on their density and thickness. This concept explains how changes in topography, such as mountain formation or erosion, can affect the vertical movement of the crust. Understanding isostasy is crucial for studying the dynamics of Earth's internal structure and how geological processes shape our planet's surface.
Lithosphere: The lithosphere is the outermost layer of the Earth, composed of the crust and the upper part of the mantle. It is rigid and relatively cool compared to the underlying asthenosphere, playing a crucial role in the dynamics of tectonic plates and the geological processes that shape our planet. This layer's interactions with both the atmosphere and hydrosphere contribute to various geological phenomena, including earthquakes and volcanic activity.
Mantle: The mantle is a thick layer of semi-solid rock located between the Earth's crust and the outer core, extending to about 2,900 kilometers below the surface. This layer plays a crucial role in Earth's internal dynamics, influencing tectonic activity and convection processes that drive plate movements. Understanding the mantle's composition and behavior is essential for comprehending the structure of the Earth and how seismic waves propagate during earthquakes.
Mantle Convection: Mantle convection is the slow, churning motion of the Earth's mantle caused by heat from the core, which drives the movement of tectonic plates on the Earth's surface. This process is crucial for understanding how heat is transferred within the Earth and how it influences geological activity, including earthquakes and volcanic eruptions. Mantle convection is a key driver in the dynamic behavior of Earth's internal structure and plays a vital role in the formation and movement of plate boundaries.
Outer core: The outer core is a fluid layer composed mainly of molten iron and nickel, located between the solid mantle and the inner core of the Earth. This layer is crucial for generating the Earth's magnetic field through the movement of its molten metals, which create electric currents and, consequently, magnetic fields. Understanding the outer core is essential in studying Earth's internal structure and dynamics, as it plays a significant role in geophysical processes and the overall composition of the planet.
P-waves: P-waves, or primary waves, are the fastest type of seismic waves generated by earthquakes and travel through solids, liquids, and gases. These waves are compressional in nature, meaning they move by alternating compressing and expanding the material they pass through, which provides crucial information about Earth's internal structure and helps in understanding earthquake mechanisms.
Plate Tectonics: Plate tectonics is the scientific theory that describes the large-scale movements of Earth's lithosphere, which is broken into tectonic plates that float on the semi-fluid asthenosphere beneath. This theory explains how these plates interact at their boundaries, leading to geological phenomena such as earthquakes, volcanic activity, and the formation of mountains and oceanic trenches.
S-waves: S-waves, or secondary waves, are a type of seismic wave that moves through the Earth during an earthquake. These waves are characterized by their shear motion, meaning they move the ground perpendicular to their direction of travel, which can cause significant shaking and damage. S-waves cannot travel through liquids, which helps scientists determine the composition of Earth's internal layers.
Seismic Tomography: Seismic tomography is a geophysical technique that uses seismic waves generated by earthquakes or artificial sources to create detailed images of the Earth's internal structure. By analyzing how these waves travel through different materials and how they are affected by variations in density and composition, scientists can gain insights into the dynamics of the Earth's crust, mantle, and core. This method is crucial for understanding geological processes such as plate tectonics and volcanic activity.
Subduction: Subduction is the geological process where one tectonic plate moves under another and sinks into the mantle as the plates converge. This movement is crucial for understanding plate tectonics, as it drives many geological processes, including volcanic activity and earthquake generation, and plays a significant role in shaping Earth's internal structure and dynamics.
Volcanism: Volcanism refers to the processes and phenomena associated with the movement of magma from beneath the Earth's crust to the surface, resulting in volcanic eruptions and the formation of various volcanic features. This term is closely linked to the behavior of magma, the structure of the Earth, and the dynamics occurring at plate boundaries, playing a crucial role in shaping the planet's surface and contributing to geological cycles.
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