2.2 Composition of the Earth's layers
4 min read•august 7, 2024
Earth's layers have distinct compositions that shape our planet's behavior. The , our rocky home, is made of . It's thinner and denser under oceans, thicker and lighter on continents. These differences drive and Earth's ever-changing surface.
Beneath the crust, the and have their own unique makeups. The mantle, mostly , flows slowly to move plates. The -nickel core generates Earth's magnetic field. Together, these layers create a dynamic, living planet.
Crust Composition
Silicate Minerals and Rocks
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- Crust primarily composed of silicate minerals and rocks formed from and atoms bonded with various metal ions (, iron, , calcium, sodium, potassium)
- Silica () content varies between oceanic and continental crust
- Silicate minerals include quartz, feldspar, mica, amphibole, pyroxene, and olivine
- Common silicate rocks include granite, basalt, gneiss, and schist
Oceanic Crust Characteristics
- Covers ~60% of Earth's surface beneath the oceans
- Relatively thin layer averaging 6-7 km in thickness
- Primarily composed of dense, dark-colored mafic igneous rocks like basalt and gabbro
- Higher density due to greater abundance of heavier elements (iron, magnesium)
- Younger geologic age (< 200 million years old) due to continuous recycling at zones and mid-ocean ridges
Continental Crust Properties
- Covers ~40% of Earth's surface forming the continents and shallow seabed close to shores (continental shelves)
- Thicker layer averaging 30-50 km in thickness with roots extending up to 100 km beneath mountain ranges
- Primarily composed of lighter-colored felsic igneous rocks like granite and diorite, along with metamorphic and sedimentary rocks
- Lower density due to greater abundance of lighter elements (silicon, aluminum, oxygen)
- Older geologic age (up to 4 billion years old in cratonic cores) as it is not subducted and recycled like oceanic crust
Mantle Layers
Upper Mantle Characteristics
- Extends from the base of the crust to a depth of ~660 km
- Primarily composed of peridotite, an ultramafic rock rich in olivine and pyroxene minerals
- Uppermost part (lithospheric mantle) is rigid and combined with the crust forms the tectonic plates
- layer beneath the is partially molten and facilitates plate motion due to reduced viscosity
- Transition zone at the base of the upper mantle marked by mineral phase changes (olivine to wadsleyite and ringwoodite) due to increasing pressure
Lower Mantle Properties
- Extends from ~660 km depth to the core-mantle boundary at ~2900 km
- Primarily composed of high-pressure mineral phases of perovskite (bridgmanite) and ferropericlase
- Increased density and viscosity compared to the upper mantle
- Slow currents over millions of years contribute to heat transfer and plate tectonics
- Contains regions of anomalous seismic wave speeds (large low-shear-velocity provinces, ultra-low velocity zones) that may represent hot spots or partial melting
Density Stratification in the Mantle
- Mantle density increases with depth due to compression under the weight of overlying material
- Density stratification creates distinct layers with different physical properties and dynamics
- Transition zone separates upper and lower mantle with sharp increases in seismic wave speeds at 410 km and 660 km depths
- Differences in chemical composition between upper and lower mantle remain uncertain, but may involve changes in iron, aluminum, and silicon content
- Layered convection with limited material exchange between upper and lower mantle is supported by geochemical differences in magmas sourced from different depths
Core Materials
Iron-Nickel Alloy Composition
- Core primarily composed of an with smaller amounts of lighter elements (, oxygen, silicon, , )
- High density (~10-13 g/cm³) due to the abundance of iron, the heaviest element that can be produced in significant quantities by nuclear fusion in stars
- is with convection currents that generate Earth's magnetic field through the geodynamo process
- is due to the extreme pressure despite high temperatures exceeding 5000°C
- Solidification of the inner core releases latent heat and light elements that drive convection in the outer core
Chemical Differentiation and Core Formation
- Earth's layered structure is a result of chemical during its early history
- Differentiation involved the separation of immiscible components (metals, silicates) based on density differences
- Dense metallic iron along with siderophile elements (nickel, gold, platinum) sank to the center to form the core
- Lighter silicates and lithophile elements (silicon, oxygen, aluminum) remained in the outer layers to form the mantle and crust
- Core formation was largely complete within the first 50-100 million years of Earth's history based on geochemical evidence and modeling
- The concentration of heat-producing radioactive elements (potassium, uranium, thorium) in the silicate mantle and crust contributes to Earth's long-term thermal evolution and plate tectonics
Key Terms to Review (29)
Aluminum: Aluminum is a lightweight, silvery-white metallic element with the symbol 'Al' and atomic number 13. It is the third most abundant element in the Earth's crust and plays a significant role in the composition and structure of the Earth's layers, especially in the crust, where it is a major constituent of many minerals such as feldspar and bauxite.
Asthenosphere: The asthenosphere is a semi-fluid layer of the Earth's mantle located beneath the lithosphere, extending from about 100 to 700 kilometers below the Earth's surface. This layer plays a critical role in tectonic plate movement, providing the necessary flow and flexibility that allows the rigid lithospheric plates to move atop it. Its properties and behaviors are vital for understanding geological processes like earthquakes and volcanic activity.
Carbon: Carbon is a fundamental chemical element represented by the symbol C and atomic number 6, essential for all known forms of life. It is a key component of organic molecules, forming the backbone of biological compounds such as carbohydrates, proteins, lipids, and nucleic acids, playing a crucial role in the Earth's systems and the composition of its layers.
Convection: Convection is the transfer of heat through the movement of fluids, which occurs when warmer, less dense areas of a fluid rise while cooler, denser areas sink. This process plays a critical role in the interactions among different layers of the Earth and its atmosphere, influencing weather patterns, geological activity, and the distribution of energy within various Earth systems.
Core: The core is the innermost layer of the Earth, located beneath the mantle, primarily composed of iron and nickel. It plays a critical role in Earth's magnetic field generation and contributes to its overall geodynamics. Understanding the core helps reveal insights into the internal structure and composition of the Earth as well as the processes that drive tectonic activity and magnetic phenomena.
Crust: The crust is the outermost solid layer of the Earth, forming a thin shell that covers the underlying mantle. It is composed of a variety of rocks and minerals and is where all terrestrial life exists. This layer is crucial because it not only provides the foundation for ecosystems but also contains vital natural resources, impacting both the environment and human activity.
Differentiation: Differentiation refers to the process by which the Earth’s layers formed and evolved over time due to varying densities and compositions of materials. This process led to the stratification of the Earth's interior, resulting in distinct layers such as the crust, mantle, outer core, and inner core. The differences in material properties, like temperature and pressure, contribute to the separation and formation of these layers.
Geophysical surveys: Geophysical surveys are systematic methods used to investigate the physical properties of the Earth's subsurface, typically employing techniques like seismic, magnetic, electrical, and gravitational measurements. These surveys provide valuable insights into the composition and structure of the Earth's layers, helping geoscientists understand geological formations, locate resources, and assess hazards. By analyzing variations in physical properties, such as density or magnetism, geophysical surveys can reveal hidden features beneath the surface.
Hydrogen: Hydrogen is the simplest and most abundant element in the universe, consisting of just one proton and one electron. In the context of Earth's layers, hydrogen plays a crucial role as a component of water and various minerals, influencing geochemical processes and the formation of the Earth's crust. Its presence is vital for understanding the chemical composition and behavior of both the atmosphere and hydrosphere.
Inner core: The inner core is the solid, innermost layer of the Earth, composed primarily of iron and nickel. It is extremely hot, with temperatures reaching up to 5,700 degrees Celsius (10,300 degrees Fahrenheit), and is under immense pressure due to the weight of the overlying layers. Understanding the inner core helps to reveal the dynamic processes that occur within the Earth, influencing geological activity and the planet's magnetic field.
Iron: Iron is a chemical element with the symbol Fe and atomic number 26. It is one of the most abundant elements on Earth and is a key component in the composition of the Earth's core, mantle, and crust. Its properties make it essential for understanding the geological and geophysical processes that shape the planet.
Iron-nickel alloy: An iron-nickel alloy is a metallic material primarily composed of iron and nickel, often found in the Earth's core and meteorites. This alloy is significant in understanding the internal structure of the Earth as it contributes to the density and magnetic properties of the planet's innermost layers. The presence of iron and nickel also helps explain the formation and differentiation of the Earth’s layers during its early history.
Liquid: A liquid is one of the fundamental states of matter, characterized by its ability to flow and take the shape of its container while maintaining a constant volume. In the context of Earth's layers, liquids play a crucial role in processes such as magma formation, plate tectonics, and the behavior of the outer core, influencing geological and geophysical phenomena.
Lithosphere: The lithosphere is the rigid outer layer of the Earth, composed of the crust and the uppermost part of the mantle. It plays a crucial role in geological processes, forming tectonic plates that float on the semi-fluid asthenosphere beneath, contributing to phenomena such as earthquakes and volcanic activity.
Magnesium: Magnesium is a chemical element with the symbol Mg and atomic number 12, known for its essential role in various geological and biological processes. In the context of Earth's composition, magnesium is a crucial component of many minerals, such as olivine and pyroxene, and plays a significant role in the formation of the Earth's mantle and crust. Its presence influences the physical and chemical properties of these layers, impacting processes like plate tectonics and volcanic activity.
Mantle: The mantle is the thick layer of rock located between the Earth's crust and the outer core, making up about 84% of Earth's total volume. It plays a crucial role in tectonic processes and the movement of materials within the Earth, significantly influencing geological activity like earthquakes and volcanic eruptions.
Mantle plumes: Mantle plumes are columns of hot, solid material that rise from deep within the Earth's mantle to the lithosphere, where they can cause volcanic activity and form geological features. These plumes play a crucial role in the dynamics of plate tectonics and can lead to the formation of hotspots, which are areas of intense volcanic activity not necessarily associated with tectonic plate boundaries.
Outer core: The outer core is the layer of the Earth located beneath the mantle and above the inner core, composed primarily of liquid iron and nickel. This layer plays a crucial role in generating Earth's magnetic field through its movement and convection processes, connecting it to the overall dynamics of the planet's interior structure.
Oxygen: Oxygen is a colorless, odorless gas that is essential for life on Earth, making up approximately 21% of the atmosphere. It plays a crucial role in various biological and geological processes, including respiration in living organisms and the formation of minerals within the Earth's crust. Its presence and abundance have significant implications for both the atmosphere and the development of life as we know it.
Peridotite: Peridotite is a dense, coarse-grained igneous rock primarily composed of olivine, with lesser amounts of pyroxene and other minerals. It is significant as the main rock type found in the Earth's mantle, providing insights into the composition and processes occurring deep beneath the Earth's crust.
Plasticity: Plasticity refers to the ability of a material to undergo permanent deformation without breaking when subjected to stress. This property is especially significant in understanding how the Earth's layers respond to various forces, as they can behave both elastically and plastically depending on temperature and pressure conditions, affecting geological processes such as tectonics and volcanism.
Plate Tectonics: Plate tectonics is the scientific theory that describes the large-scale movement of the Earth's lithosphere, which is divided into tectonic plates that float on the semi-fluid asthenosphere beneath. This movement explains many geological processes, including the occurrence of earthquakes, volcanic activity, and the formation of mountain ranges, as well as how ocean basins evolve over time.
Seismic waves: Seismic waves are energy waves that travel through the Earth's layers, generated primarily by earthquakes or artificial explosions. They are crucial for understanding the internal structure and composition of the Earth, as they provide valuable information about how different layers interact and their physical properties.
Seismology: Seismology is the scientific study of earthquakes and the propagation of seismic waves through the Earth. It involves understanding the mechanisms that cause earthquakes, how seismic waves travel, and what these waves reveal about the Earth's internal structure and composition. This field also plays a crucial role in assessing volcanic activity and potential eruptions, as seismic data can indicate movement beneath the surface.
Silicate minerals: Silicate minerals are the most abundant class of minerals on Earth, characterized by the presence of silicon and oxygen in their structure, typically forming the fundamental building blocks of rocks. These minerals are crucial in understanding the internal structure and composition of the Earth, as they dominate the Earth's crust and are key to the processes that shape geological formations.
Silicon: Silicon is a chemical element with the symbol Si and atomic number 14, known for its pivotal role in the composition of the Earth's crust. As the second most abundant element in the Earth's crust, silicon primarily exists in the form of silicate minerals and is a fundamental building block of many rocks and minerals, influencing geological processes and the formation of various Earth layers.
Solid: In the context of Earth's layers, a solid is a state of matter characterized by structural rigidity and resistance to changes in shape or volume. This rigidity arises from tightly packed particles that vibrate in place, contributing to the unique physical properties of solid materials. Understanding the solid state is crucial as it plays a vital role in defining the composition and behavior of various layers within the Earth, such as the crust and mantle.
Subduction: Subduction is the geological process where one tectonic plate moves under another and sinks into the mantle as the plates converge. This process is crucial for understanding plate tectonics, as it leads to the recycling of the Earth’s crust and significantly influences geological activity such as earthquakes, volcanic eruptions, and mountain building.
Sulfur: Sulfur is a non-metallic element, symbolized by 'S', known for its distinct yellow color and pungent smell. It plays a vital role in the Earth's layers, particularly within the mantle and crust, influencing geological and biological processes. Sulfur is an essential component of various minerals and compounds, contributing to the formation of sulfide and sulfate minerals, which are significant for understanding the Earth's chemistry and geology.