🪨Intro to Geophysics Unit 1 – Earth's Structure in Geophysics

Key Concepts and Terminology

• Geophysics studies the physical properties and processes of the Earth using quantitative methods from physics, chemistry, and mathematics
• Lithosphere consists of the crust and uppermost mantle, forming rigid plates that move over the Earth's surface
• Asthenosphere is a layer of the upper mantle that exhibits plastic deformation and allows for plate motion
• Seismic waves, including P-waves (primary) and S-waves (secondary), propagate through the Earth's interior and provide information about its structure
  • P-waves are compressional waves that can travel through solids, liquids, and gases
  • S-waves are shear waves that can only travel through solids
• Isostasy describes the gravitational equilibrium between the lithosphere and the underlying mantle, with the lithosphere "floating" on the denser mantle
• Curie temperature is the temperature above which a material loses its permanent magnetic properties (~580°C for magnetite)
• Geoid is the equipotential surface of the Earth's gravity field that coincides with mean sea level

Earth's Layers and Composition

• Earth is divided into three main layers: crust, mantle, and core, each with distinct physical and chemical properties
• Crust is the outermost layer, ranging from 5-70 km thick, and is composed of silicate rocks
  • Continental crust is thicker (~30-50 km), older, and less dense than oceanic crust
  • Oceanic crust is thinner (~5-10 km), younger, and denser than continental crust
• Mantle extends from the base of the crust to ~2,900 km depth and is composed primarily of silicate minerals (olivine, pyroxene, garnet)
  • Upper mantle is cooler and more rigid than the lower mantle
  • Lower mantle is hotter and undergoes slow convection
• Core is the innermost layer, composed primarily of iron and nickel, and is divided into the outer and inner core
  • Outer core is liquid and undergoes convection, generating Earth's magnetic field
  • Inner core is solid due to high pressure despite high temperatures
• Mohorovičić discontinuity (Moho) is the boundary between the crust and mantle, marked by a sharp increase in seismic wave velocities
• Gutenberg discontinuity is the boundary between the mantle and core, marked by a decrease in seismic wave velocities

Plate Tectonics and Continental Drift

• Plate tectonics is the theory that Earth's lithosphere is divided into several large plates that move and interact with each other over geological time
• Plates are driven by convection currents in the mantle, which are caused by heat transfer from the Earth's interior
• Three main types of plate boundaries: divergent, convergent, and transform
  • Divergent boundaries occur where plates move away from each other, forming new oceanic crust (mid-ocean ridges)
  • Convergent boundaries occur where plates collide, resulting in subduction, mountain building, and volcanic activity
  • Transform boundaries occur where plates slide past each other, causing earthquakes (San Andreas Fault)
• Seafloor spreading is the process by which new oceanic crust is formed at mid-ocean ridges and older crust is destroyed at subduction zones
• Continental drift, proposed by Alfred Wegener, is the idea that continents have moved over Earth's surface throughout geological history
  • Evidence for continental drift includes matching coastlines, similar rock formations, and fossil distributions across continents
• Wilson cycle describes the opening and closing of ocean basins over geological time due to plate tectonic processes

Seismic Waves and Earth's Interior

• Seismic waves are vibrations that propagate through the Earth's interior, generated by earthquakes, explosions, or other sources
• Two main types of seismic waves: body waves (P and S) and surface waves (Rayleigh and Love)
  • Body waves travel through the Earth's interior and are used to study its structure
  • Surface waves travel along the Earth's surface and cause the most damage during earthquakes
• P-waves (primary waves) are compressional waves that travel fastest and can pass through solids, liquids, and gases
• S-waves (secondary waves) are shear waves that travel slower than P-waves and can only pass through solids
• Seismic wave velocities depend on the density and elastic properties of the material they pass through
  • Velocity increases with depth in the Earth due to increasing pressure and temperature
  • Velocity decreases at certain boundaries (Moho, Gutenberg) due to changes in composition or phase
• Shadow zones are areas on the Earth's surface where certain seismic waves are not detected due to refraction or reflection at internal boundaries
  • P-wave shadow zone occurs between 103° and 143° from the earthquake epicenter
  • S-wave shadow zone occurs beyond 103° from the epicenter due to the liquid outer core
• Seismic tomography uses seismic wave travel times to create 3D images of the Earth's interior structure

Gravity and Earth's Shape

• Earth's gravity is the force that attracts objects towards its center due to its mass
• Gravity varies across the Earth's surface due to variations in density, elevation, and rotation
  • Gravity is stronger at the poles and weaker at the equator due to the Earth's rotation and equatorial bulge
  • Gravity anomalies are local variations in gravity caused by differences in crustal thickness or density
• Earth's shape is an oblate spheroid, slightly flattened at the poles and bulging at the equator due to its rotation
• Geoid is an equipotential surface that coincides with mean sea level and represents the shape of the Earth's gravity field
  • Geoid undulations are deviations of the geoid from a perfect ellipsoid, caused by variations in gravity
• Isostasy is the state of gravitational equilibrium between the lithosphere and the underlying mantle
  • Lithosphere "floats" on the denser mantle, with thicker crust (mountains) having deeper roots and thinner crust (ocean basins) having shallower roots
• Glacial isostatic adjustment is the ongoing rise of land masses that were depressed by the weight of ice sheets during the last glacial period
• Gravity measurements are used in geophysics to study the Earth's internal structure, density variations, and geodynamic processes

Magnetic Field and Paleomagnetism

• Earth's magnetic field is generated by convection currents in the liquid outer core, which acts as a self-sustaining dynamo
• Magnetic field lines originate at the south magnetic pole and terminate at the north magnetic pole
  • Magnetic poles do not coincide with geographic poles and wander over time
  • Magnetic field intensity varies across the Earth's surface, with stronger fields near the poles and weaker fields near the equator
• Magnetic reversals occur when the Earth's magnetic field flips polarity, with the north and south magnetic poles exchanging positions
  • Reversals occur irregularly, with intervals ranging from thousands to millions of years
  • Geomagnetic polarity timescale is a record of magnetic reversals used for dating rocks and sediments
• Paleomagnetism is the study of the Earth's past magnetic field as recorded in rocks, sediments, and archaeological materials
  • Magnetic minerals (magnetite) align with the Earth's magnetic field as they cool below their Curie temperature, preserving a record of the field orientation
  • Apparent polar wander paths trace the movement of continents over time relative to the Earth's magnetic poles
• Magnetic anomalies are local variations in the Earth's magnetic field caused by differences in the magnetic properties of rocks
  • Seafloor spreading creates a pattern of magnetic anomalies parallel to mid-ocean ridges, providing evidence for plate tectonics
• Magnetotellurics is a geophysical method that uses natural variations in the Earth's magnetic and electric fields to study its electrical conductivity structure

Methods for Studying Earth's Structure

• Seismic methods use seismic waves generated by earthquakes or artificial sources to study the Earth's interior structure
  • Seismic refraction measures the travel times of seismic waves refracted at interfaces between layers with different velocities
  • Seismic reflection measures the travel times of seismic waves reflected at interfaces between layers with different densities
  • Seismic tomography creates 3D images of the Earth's interior using seismic wave travel times from multiple sources and receivers
• Gravity methods measure variations in the Earth's gravity field to study its internal structure and density distribution
  • Gravimeters measure the absolute value of gravity at a given location
  • Gravity gradiometers measure the spatial rate of change of gravity in different directions
• Magnetic methods measure variations in the Earth's magnetic field to study its internal structure and composition
  • Magnetometers measure the strength and direction of the Earth's magnetic field
  • Aeromagnetic surveys measure the magnetic field from aircraft to cover large areas quickly
• Electrical and electromagnetic methods measure the electrical conductivity and permittivity of the Earth's subsurface
  • Direct current (DC) resistivity measures the electrical resistance of the subsurface by injecting current and measuring voltage
  • Magnetotellurics (MT) measures natural variations in the Earth's magnetic and electric fields to determine the subsurface conductivity structure
• Borehole methods involve lowering instruments into drilled holes to measure physical properties of the subsurface
  • Well logging measures the electrical, acoustic, and nuclear properties of rock formations along the borehole wall
  • Vertical seismic profiling (VSP) measures the travel times of seismic waves from a surface source to receivers in the borehole

Real-World Applications and Case Studies

• Exploration for oil, gas, and mineral resources relies heavily on geophysical methods to identify potential reservoirs and deposits
  • Seismic reflection is widely used in the oil and gas industry to image subsurface structures and stratigraphic traps
  • Gravity and magnetic surveys are used to identify dense or magnetic mineral deposits (iron ore, chromite)
• Geothermal energy exploration uses geophysical methods to locate and characterize geothermal reservoirs
  • Magnetotellurics is used to map the electrical conductivity structure of geothermal systems, which is related to the presence of hot fluids and clay minerals
  • Seismic methods are used to image faults and fractures that control fluid flow in geothermal reservoirs
• Earthquake hazard assessment and risk mitigation rely on understanding the Earth's structure and seismic wave propagation
  • Seismic microzonation studies use geophysical methods to map the local site response and ground motion amplification during earthquakes
  • Seismic building codes and design standards are based on the expected ground motions and soil conditions at a given location
• Groundwater exploration and management use geophysical methods to map aquifers and monitor groundwater resources
  • Electrical resistivity tomography (ERT) is used to image the subsurface electrical conductivity structure, which is related to the presence of water and clay content
  • Time-lapse gravity measurements are used to monitor changes in groundwater storage over time
• Climate change research uses geophysical methods to study the Earth's past climate and predict future changes
  • Paleomagnetic studies of ocean sediment cores provide a record of past magnetic reversals and climate changes
  • Seismic and radar methods are used to measure the thickness and volume of ice sheets and glaciers, which are sensitive to climate change
• Planetary exploration uses geophysical methods to study the internal structure and evolution of other planets and moons
  • Gravity and magnetic measurements from orbiting spacecraft are used to map the internal density and composition of planets (Mars, Venus)
  • Seismic measurements from landers and rovers are used to study the internal structure and seismic activity of planets and moons (Mars, Moon)