1.2 Earth's structure and composition

Earth's structure and composition are key to understanding geophysics. The Earth is divided into layers: crust, mantle, outer core, and inner core. Each layer has unique properties that affect geological processes.

Seismic waves reveal Earth's internal structure. P-waves and S-waves behave differently in solid and liquid materials, helping scientists map the planet's layers. This knowledge is crucial for studying plate tectonics and other Earth processes.

Earth's Interior Layers

Major Layers and Their Depths

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• The Earth's interior is divided into four main layers: the crust, mantle, outer core, and inner core, each with distinct physical and chemical properties
• The crust is the outermost layer of the Earth, ranging from 5-70 km in thickness, and is composed of solid rocks and minerals
• The mantle is the layer beneath the crust, extending to a depth of about 2,900 km, and is composed of hot, dense rocks that are partially molten in the upper mantle
• The outer core is a liquid layer, approximately 2,900-5,100 km deep, composed primarily of iron and nickel alloys
• The inner core is the innermost layer, extending from about 5,100-6,371 km deep, and is a solid layer composed of iron and nickel alloys under immense pressure and temperature

Temperature and Pressure Changes with Depth

• Temperature and pressure increase with depth in the Earth's interior
• The increase in temperature and pressure leads to changes in the physical state and behavior of the materials in each layer
• The outer core remains liquid despite high temperatures due to the extreme pressure at that depth
• The inner core is solid because the pressure is high enough to overcome the effects of the high temperature

Composition and Properties of Earth's Layers

Density and Composition Variations

• The crust is the least dense layer, composed primarily of silicate rocks rich in elements such as oxygen, silicon, aluminum, and potassium, with a density ranging from 2.7-3.0 g/cm³
• The mantle is denser than the crust, with a density ranging from 3.3-5.7 g/cm³, and is composed of silicate rocks rich in elements such as magnesium, iron, calcium, and aluminum
• The outer core is much denser than the mantle, with a density of about 9.9-12.2 g/cm³, and is composed primarily of liquid iron and nickel alloys
• The inner core is the densest layer, with a density of about 12.8-13.1 g/cm³, and is composed of solid iron and nickel alloys under extreme pressure and temperature conditions

Physical State and Behavior of Materials

• The crust and upper mantle are composed of solid rocks, while the lower mantle is solid but can deform plastically over long time scales due to high temperatures and pressures
• The outer core is liquid, allowing for the generation of Earth's magnetic field through convection currents
• The inner core is solid due to the extreme pressures at that depth, despite the high temperatures
• The physical state and behavior of materials in each layer influence processes such as plate tectonics, volcanism, and seismic wave propagation

Seismic Waves and Earth's Structure

Types of Seismic Waves and Their Propagation

• Seismic waves, generated by earthquakes or artificial sources, travel through the Earth's interior and provide valuable information about its structure and composition
• P-waves (primary waves) are compressional waves that travel through both solid and liquid materials
• S-waves (secondary waves) are shear waves that only travel through solid materials
• The velocity of seismic waves changes as they pass through different layers of the Earth, depending on the density and elastic properties of the materials

Evidence for Earth's Internal Structure

• Seismic wave refraction and reflection at the boundaries between layers, such as the Mohorovičić discontinuity (Moho) between the crust and mantle, provide evidence for the existence and depth of these boundaries
• The absence of S-waves in the outer core indicates that it is liquid, while the presence of P-waves and S-waves in the inner core suggests that it is solid
• Seismic tomography, which uses seismic wave data to create 3D images of the Earth's interior, has greatly improved our understanding of the Earth's internal structure and heterogeneity
• Seismic wave shadow zones, where certain types of waves are not detected due to refraction or reflection, help constrain the depths and properties of the Earth's layers

Rocks and Minerals of the Crust and Mantle

Rock Types and Formation Processes

• The Earth's crust and upper mantle are composed of various types of rocks and minerals, classified based on their formation processes and chemical composition
• Igneous rocks, such as basalt and granite, form from the cooling and solidification of magma or lava and are abundant in the Earth's crust
• Sedimentary rocks, such as sandstone and limestone, form from the accumulation and lithification of sediments derived from weathering and erosion of pre-existing rocks
• Metamorphic rocks, such as gneiss and marble, form from the transformation of pre-existing rocks under high temperature and pressure conditions within the Earth's crust and upper mantle

Common Rock-Forming Minerals

• Common rock-forming minerals in the Earth's crust and upper mantle include silicates (quartz, feldspar, mica, and olivine), carbonates (calcite and dolomite), and oxides (magnetite and hematite)
• Silicate minerals are the most abundant in the Earth's crust and upper mantle, forming the majority of igneous and metamorphic rocks
• Carbonate minerals are common in sedimentary rocks, particularly limestone, and are often formed through biological processes or chemical precipitation
• Oxide minerals are important components of some igneous and metamorphic rocks and can have significant economic value as ore deposits

Factors Influencing Rock and Mineral Distribution

• The distribution and abundance of rocks and minerals in the Earth's crust and upper mantle vary depending on factors such as tectonic setting, magmatic processes, and the history of the region
• Tectonic settings, such as divergent boundaries (mid-ocean ridges), convergent boundaries (subduction zones), and transform boundaries, influence the types of rocks and minerals formed and their spatial distribution
• Magmatic processes, including partial melting, crystallization, and differentiation, control the composition and distribution of igneous rocks and their associated minerals
• The history of a region, including past tectonic events, sedimentary processes, and metamorphic episodes, can significantly impact the distribution and characteristics of rocks and minerals in the crust and upper mantle