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Earth Science
Table of Contents
Unit 2 Overview: Earth's Systems and Cycles
2.1 Earth's Structure and Composition
2.2 Plate Tectonics and Earth's Interior
2.3 Rocks and the Rock Cycle
2.4 Weathering, Erosion, and Deposition
2.5 The Water Cycle and Earth's Water Resources
2.6 The Carbon Cycle and Earth's Atmosphere

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2.1 Earth's Structure and Composition

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Earth's structure is like a giant onion, with distinct layers from crust to core. Each layer has unique properties that shape our planet's behavior. Understanding these layers helps us grasp Earth's dynamic nature and its impact on surface processes.

The crust and upper mantle form the lithosphere, Earth's rigid outer shell. Below lies the asthenosphere, a partially molten layer that allows plate movement. This layered structure drives plate tectonics, shaping Earth's surface and influencing geological processes we observe.

Earth's Interior Layers

Layers and Their Characteristics

  • Earth's interior is divided into three main layers: crust, mantle, and core, each with distinct physical and chemical properties
  • The crust is the thin, outermost layer of the Earth, composed of solid rocks and minerals, and is divided into oceanic and continental crust
  • The mantle is the thick, middle layer of the Earth, composed of hot, dense rocks that are primarily solid but can flow slowly over long periods
  • The core is the innermost layer of the Earth, composed of mostly iron and nickel, and is divided into a liquid outer core and a solid inner core

Lithosphere and Asthenosphere

  • The lithosphere is the rigid outer layer of the Earth, which includes the crust and the uppermost part of the mantle
    • It is broken into several large tectonic plates that move and interact with each other (North American Plate, Pacific Plate)
    • The lithosphere is generally cooler and more brittle than the underlying asthenosphere
  • The asthenosphere is the partially molten layer beneath the lithosphere
    • It is composed of hot, semi-solid mantle rock that can flow slowly over geologic time
    • The asthenosphere's plasticity allows the overlying lithospheric plates to move and slide over it

Composition and Properties of Earth's Layers

Crust

  • The crust is composed primarily of silicate rocks rich in elements like oxygen, silicon, aluminum, iron, magnesium, and potassium, with an average density of about 2.7-3.0 g/cm³
    • Felsic rocks (granite) are common in the continental crust, while mafic rocks (basalt) are prevalent in the oceanic crust
    • The crust is the thinnest layer of the Earth, ranging from 5-70 km in thickness
    • It is the most accessible and well-studied layer of the Earth

Mantle

  • The mantle is composed of ultramafic rocks, rich in iron and magnesium silicates like olivine and pyroxene, with an average density of about 3.4-5.6 g/cm³ and temperatures ranging from 500°C to 4000°C
    • The upper mantle is cooler and more rigid, while the lower mantle is hotter and more plastic
    • Convection currents in the mantle are responsible for the movement of lithospheric plates and the generation of heat that drives volcanic and tectonic activity
  • The crust and uppermost mantle form the lithosphere, which is rigid and brittle, while the asthenosphere beneath is more ductile and can flow slowly

Core

  • The outer core is composed of liquid iron and nickel, with an average density of about 9.9-12.2 g/cm³ and temperatures around 4000-6000°C
    • The liquid nature of the outer core allows for the generation of Earth's magnetic field through the geodynamo process
    • Convection currents in the outer core are thought to be driven by heat released from the inner core and the cooling and solidification of the outer core
  • The inner core is composed of solid iron and nickel, with an average density of about 12.8-13.1 g/cm³ and temperatures around 5000-7000°C
    • The solid inner core is believed to have formed through the gradual cooling and solidification of the Earth's interior over billions of years
    • Seismic waves passing through the inner core exhibit unique properties, such as anisotropy, which provides insights into its structure and dynamics

Seismic Waves and Earth's Structure

Types of Seismic Waves

  • Seismic waves, generated by earthquakes or artificial explosions, travel through the Earth's interior and provide information about its structure and composition
  • P-waves (primary waves) are compressional waves that can travel through solids, liquids, and gases
    • They are the fastest seismic waves and are the first to arrive at seismic stations
    • P-waves cause the ground to compress and expand in the direction of wave propagation
  • S-waves (secondary waves) are shear waves that can only travel through solids
    • They are slower than P-waves and arrive at seismic stations after P-waves
    • S-waves cause the ground to oscillate perpendicular to the direction of wave propagation

Seismic Wave Behavior and Earth's Interior

  • The velocity of seismic waves depends on the density and elastic properties of the material they pass through, with waves traveling faster in denser and more rigid materials
    • P-wave velocities range from about 5 km/s in the crust to about 13 km/s in the inner core
    • S-wave velocities range from about 3 km/s in the crust to about 7 km/s in the inner core
  • Seismic waves refract (bend) and reflect at boundaries between layers with different properties, such as the crust-mantle boundary (Mohorovičić discontinuity) and the mantle-core boundary (Gutenberg discontinuity)
    • These boundaries are marked by abrupt changes in seismic wave velocities and are used to delineate the major layers of the Earth
    • The Mohorovičić discontinuity represents a sharp increase in P-wave and S-wave velocities, indicating the transition from crust to denser mantle rocks
    • The Gutenberg discontinuity marks a decrease in P-wave velocity and the disappearance of S-waves, suggesting the presence of a liquid outer core
  • 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
  • The analysis of seismic wave travel times and paths has led to the development of models of Earth's interior, such as the Preliminary Reference Earth Model (PREM)
    • PREM is a one-dimensional model that describes the average properties of the Earth's interior, including seismic wave velocities, density, and pressure, as a function of depth
    • More complex models, such as three-dimensional tomographic models, provide a more detailed picture of lateral variations in Earth's interior structure

Continental vs Oceanic Crust

Thickness and Density

  • Continental crust is thicker (30-50 km) and less dense (2.7 g/cm³) compared to oceanic crust, which is thinner (5-10 km) and denser (3.0 g/cm³)
    • The greater thickness of continental crust is due to its longer and more complex history of formation and deformation
    • The lower density of continental crust allows it to "float" higher on the denser mantle, forming the continents and their elevated topography

Composition

  • Continental crust is composed mainly of felsic rocks, such as granite, which are rich in silica and aluminum
    • Felsic rocks are generally lighter in color and less dense than mafic rocks
    • The presence of felsic rocks in the continental crust is attributed to the differentiation and remelting of mantle-derived magmas over time
  • Oceanic crust is composed mainly of mafic rocks, such as basalt, which are rich in iron and magnesium
    • Mafic rocks are generally darker in color and denser than felsic rocks
    • The mafic composition of oceanic crust is a result of the partial melting of mantle rocks at mid-ocean ridges and the rapid cooling of the resulting magma

Age and Formation

  • Continental crust is older (up to 4 billion years) and more heterogeneous due to its complex history of formation and deformation
    • The oldest continental rocks, such as the Acasta Gneiss in Canada, provide insights into the early history of the Earth and the formation of the first continents
    • Continental crust has undergone multiple episodes of mountain building, metamorphism, and sedimentary deposition, resulting in a diverse array of rock types and ages
  • Oceanic crust is younger (less than 200 million years) and more homogeneous
    • The oldest oceanic crust is found farthest from mid-ocean ridges and is progressively younger towards the ridges
    • Oceanic crust is continuously created at mid-ocean ridges and destroyed at subduction zones, resulting in a relatively short lifespan compared to continental crust

Geographical Distribution

  • Continental crust is found above sea level and forms the continents and continental shelves
    • Continents cover about 29% of the Earth's surface but account for most of the land area
    • Continental shelves are submerged extensions of the continents that gradually slope towards the ocean basins
  • Oceanic crust is found beneath the oceans and forms the ocean floor and oceanic plateaus
    • Ocean basins cover about 71% of the Earth's surface and are characterized by abyssal plains, mid-ocean ridges, and deep-sea trenches
    • Oceanic plateaus are large, elevated areas of the ocean floor that are often capped by thick accumulations of basaltic lava flows
  • The boundary between continental and oceanic crust is marked by the continental margin, which includes the continental shelf, continental slope, and continental rise
    • The continental margin represents the transition from the thick, low-density continental crust to the thin, high-density oceanic crust
    • Passive continental margins (Atlantic-type) are characterized by a gradual transition and are often associated with rifting and the opening of ocean basins
    • Active continental margins (Pacific-type) are characterized by a more abrupt transition and are often associated with subduction zones and volcanic arcs

Key Terms to Review (34)

Volcanism: Volcanism refers to the processes and phenomena associated with the movement of molten rock (magma) from beneath the Earth's crust to the surface, resulting in the formation of volcanoes and volcanic eruptions. This term encompasses not just the act of an eruption but also the various geological activities that lead to the creation of different types of volcanic landforms, including lava flows, ash deposits, and pyroclastic flows. Volcanism plays a crucial role in shaping the Earth's landscape and has significant impacts on climate and ecosystems.
Lithosphere: The lithosphere is the rigid outer layer of the Earth, comprising the crust and the uppermost part of the mantle. This layer is crucial as it plays a significant role in geological processes such as plate tectonics, influencing both the movement of tectonic plates and the formation of various geological features. The lithosphere interacts with other Earth systems, impacting not just geology but also ecosystems and climate.
Erosion: Erosion is the process by which soil, rock, and other surface materials are worn away and transported by natural forces such as wind, water, and ice. This process shapes landscapes, influences ecosystems, and plays a critical role in the rock cycle by breaking down materials and redistributing them across different environments.
Plate tectonics: Plate tectonics is the scientific theory that describes the large-scale movement and interaction of Earth's lithosphere, which is divided into tectonic plates. This theory explains many geological phenomena, including the formation of mountains, earthquakes, and volcanic activity, and it connects to the structure and composition of Earth, as well as its geological history.
Preliminary Reference Earth Model: The Preliminary Reference Earth Model (PREM) is a comprehensive, spherically symmetric model that describes the physical properties of the Earth’s interior. It serves as a standard reference for geophysicists and researchers, incorporating data on the composition, density, and elastic properties of various layers within the Earth. This model is crucial for understanding seismic wave propagation, which helps in interpreting the Earth's structure and composition.
Gutenberg Discontinuity: The Gutenberg Discontinuity is the boundary that separates the Earth's crust and the overlying mantle from the underlying outer core, typically located at a depth of about 2,900 kilometers beneath the Earth's surface. This significant transition point is marked by a notable change in material composition, density, and physical state, where the solid silicate rocks of the mantle shift to the liquid iron-nickel alloy of the outer core, illustrating the complex layering of Earth's interior.
Elasticity: Elasticity is the property of materials to deform when a force is applied and then return to their original shape once the force is removed. This characteristic is crucial in understanding how various layers of the Earth respond to stress, which affects geological processes such as the formation of mountains, earthquakes, and the behavior of magma.
Mica: Mica is a group of silicate minerals characterized by their sheet-like crystal structure, which allows them to be easily split into thin, flexible sheets. These minerals are commonly found in igneous and metamorphic rocks and play a significant role in the Earth's structure and composition by contributing to the overall texture and properties of rocks.
Melting point: The melting point is the temperature at which a solid becomes a liquid. This crucial transition occurs when the thermal energy of the solid's particles overcomes the forces holding them in a fixed position, allowing them to move freely and form a liquid. In the context of Earth's structure and composition, the melting point is significant because it influences the behavior of rocks and minerals, particularly during processes like magma formation and plate tectonics.
Mafic rocks: Mafic rocks are igneous rocks that are rich in magnesium and iron, resulting in a darker color and higher density compared to felsic rocks. They are primarily composed of minerals such as pyroxene and olivine and are typically formed from the partial melting of the Earth's mantle. Mafic rocks are crucial to understanding the composition of the Earth's crust and mantle, as well as the processes that lead to volcanic activity.
Pyroxene: Pyroxene is a group of important rock-forming silicate minerals that are characterized by their high temperature and pressure stability, commonly found in igneous and metamorphic rocks. These minerals play a significant role in understanding Earth's structure and composition, particularly in relation to the mantle and crust where they are abundant. Pyroxenes are typically distinguished by their crystal structure and composition, which can help identify the processes that formed the rocks they are part of.
Mohorovičić Discontinuity: The Mohorovičić Discontinuity, commonly referred to as the 'Moho,' is the boundary that separates the Earth's crust from the underlying mantle. This discontinuity is significant because it marks a change in the composition and properties of Earth's materials, transitioning from the lighter, less dense rocks of the crust to the denser, more metallic rocks of the mantle below.
Ultramafic rocks: Ultramafic rocks are a type of igneous rock that contain very low amounts of silica and are composed mainly of iron and magnesium-rich minerals. These rocks are typically dark in color and dense, often forming from the partial melting of the Earth's mantle. They play a crucial role in understanding the composition of the Earth's interior and contribute to the geochemical cycles that affect the planet's crust.
Olivine: Olivine is a magnesium iron silicate mineral that is typically green in color and is one of the most abundant minerals in Earth's upper mantle. It plays a crucial role in understanding Earth's structure and composition, as it contributes to the formation of igneous rocks and helps to reveal information about mantle dynamics and the conditions under which rocks form.
Felsic rocks: Felsic rocks are a category of igneous rocks that are rich in silica and contain high amounts of lighter minerals such as quartz and feldspar. These rocks are typically light in color and low in density, forming from the slow crystallization of magma beneath the Earth's surface. Felsic rocks are important because they often indicate the geological processes at play in continental crust formation and volcanic activity.
Seismic waves: Seismic waves are energy waves generated by the sudden release of energy in the Earth's crust, often due to tectonic activity like earthquakes. These waves travel through the Earth and are crucial for understanding its internal structure, as they provide insights into the layers of the Earth, including the crust, mantle, and core.
Seismology: Seismology is the scientific study of earthquakes and the propagation of seismic waves through the Earth. This field involves understanding how seismic waves are generated by tectonic activities and how they travel through various layers of the Earth's structure. Seismology is crucial for assessing earthquake risk, understanding Earth's internal composition, and developing early warning systems to mitigate damage from seismic events.
P-waves: P-waves, or primary waves, are the fastest type of seismic waves produced during an earthquake. They travel through solids, liquids, and gases by compressing and expanding the material in the same direction as the wave travels. Their ability to move through different states of matter makes them crucial for understanding the Earth's internal structure and the nature of seismic activity.
S-waves: S-waves, or secondary waves, are a type of seismic wave that move through the Earth during an earthquake. They are shear waves that only travel through solids, making them critical in understanding the Earth's internal structure and the composition of its layers, as they provide insights into the behavior of materials beneath the surface.
Weathering: Weathering is the process that breaks down rocks into smaller particles through physical, chemical, and biological means. This process is crucial as it contributes to soil formation, influences landform development, and plays a significant role in the rock cycle, affecting how rocks change over time and interact with the environment.
Subduction: Subduction is the geological process in which one tectonic plate moves under another and sinks into the mantle as the plates converge. This process plays a critical role in the recycling of Earth's crust, leading to the formation of features such as deep ocean trenches, volcanic arcs, and mountain ranges. Subduction also impacts the rock cycle and Earth's interior dynamics, influencing major geological events over time.
Continental drift: Continental drift is the theory that continents have moved over geological time and were once part of a single supercontinent called Pangaea. This movement is driven by the process of plate tectonics, which explains how tectonic plates shift and interact beneath the Earth's surface. The idea of continental drift helps us understand the distribution of fossils, geological formations, and earthquakes, revealing a dynamic Earth shaped by its interior processes.
Quartz: Quartz is a hard, crystalline mineral composed of silicon and oxygen (SiO₂) and is one of the most abundant minerals in the Earth's crust. Its unique properties, including its hardness and resistance to weathering, make it a key component in various geological processes and a significant rock-forming mineral. Quartz also plays an important role in understanding mineral formation, classification, and the structure of Earth's lithosphere.
Feldspar: Feldspar is a group of rock-forming minerals that make up about 60% of the Earth's crust. They are primarily composed of aluminum silicate combined with other elements such as potassium, sodium, and calcium. Feldspar plays a crucial role in the formation and classification of various igneous, metamorphic, and sedimentary rocks, making them essential for understanding Earth’s structure and composition.
Period: In geology, a period is a unit of geological time that represents a specific span of time during which certain rock layers were formed and specific events occurred. Periods are subdivisions of eras and help in organizing Earth's history into manageable segments for understanding the evolution of life, climate changes, and geological transformations.
Era: An era is a significant period in geological time characterized by distinct developments in Earth's history, marked by changes in climate, life forms, and geological features. These periods are essential for understanding the evolution of the planet and its inhabitants, as they help to organize the vast timeline of Earth's past into manageable segments for analysis. Eras are used in conjunction with relative and absolute dating methods to determine the age of rocks and fossils, and they are integral to the geologic time scale that divides Earth's history into intervals based on major events.
Eon: An eon is the largest division of geologic time, spanning hundreds of millions to billions of years. Eons are typically divided into smaller units called eras, which help to organize Earth's history and its significant geological and biological events. Understanding eons is crucial for interpreting the geologic time scale and aids in both relative and absolute dating methods by placing events and formations within a broader context.
Density: Density is the measure of mass per unit volume of a substance, usually expressed in grams per cubic centimeter (g/cm³) for solids and liquids, and in kilograms per cubic meter (kg/m³) for gases. It plays a crucial role in understanding how materials interact in different states, influencing phenomena such as buoyancy in fluids and the behavior of Earth's layers.
Big bang theory: The big bang theory is the leading explanation for the origin of the universe, proposing that it began as an extremely hot and dense point roughly 13.8 billion years ago and has been expanding ever since. This model accounts for the observed redshift of galaxies and the cosmic microwave background radiation, which are key pieces of evidence supporting the theory. Understanding the big bang theory is crucial as it provides insight into the formation of stars, galaxies, and ultimately, the structures we observe in the universe today.
Radiometric Dating: Radiometric dating is a method used to determine the age of rocks and fossils based on the decay of radioactive isotopes. This technique relies on the principle that certain isotopes are unstable and will break down into stable forms at a predictable rate, allowing scientists to calculate the time elapsed since the rock or fossil formed. It plays a crucial role in understanding Earth's history, providing precise numerical ages for geological events and helping to establish a timeline of Earth's formation and evolution.
Asthenosphere: The asthenosphere is a semi-fluid layer of the Earth's upper mantle located below the lithosphere, characterized by its ability to flow slowly over geological time. This layer plays a crucial role in the movement of tectonic plates, allowing for the dynamic processes that shape the Earth's surface, such as earthquakes and volcanic activity. The asthenosphere's properties are essential for understanding Earth's structure and how the interior influences surface phenomena.
Crust: The crust is the outermost layer of the Earth, characterized by its solid rock composition and varying thickness. This layer is where all terrestrial life exists, containing both continental crust, which forms the continents, and oceanic crust, which forms the ocean floors. The crust is essential for understanding geological processes and Earth's overall structure.
Core: The core is the innermost layer of the Earth, primarily composed of iron and nickel, and is divided into two parts: the solid inner core and the liquid outer core. This layer plays a crucial role in generating Earth's magnetic field and affects plate tectonics and volcanic activity through convection currents.
Mantle: The mantle is a thick layer of rock located between the Earth's crust and core, making up about 84% of the Earth's total volume. It plays a crucial role in tectonic activity, as it is involved in the movement of tectonic plates and the recycling of materials through processes like convection and subduction.