🌋Geochemistry Unit 1 – Earth's structure and composition

Core Concepts

• Earth is a dynamic planet with a layered structure consisting of the crust, mantle, outer core, and inner core
• Plate tectonics theory explains the movement and interaction of Earth's lithospheric plates driven by convection currents in the mantle
• Minerals are naturally occurring, inorganic solids with a definite chemical composition and crystalline structure
• Rocks are composed of minerals and are classified into three main types: igneous, sedimentary, and metamorphic based on their formation processes
• Geochemical cycles describe the movement and exchange of elements between Earth's reservoirs (atmosphere, hydrosphere, lithosphere, and biosphere)
• Analytical methods such as X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), and scanning electron microscopy (SEM) are used to study Earth materials and processes
• Understanding Earth's structure and composition has practical applications in resource exploration, hazard assessment, and environmental management

Earth's Layers

• Earth's outermost layer is the crust, which is thin (5-70 km) and composed of lighter silicate rocks (granitic continental crust and basaltic oceanic crust)
• The mantle lies beneath the crust and extends to a depth of ~2,900 km consisting of ultramafic rocks rich in olivine and pyroxene
  • The upper mantle is solid and includes the lithosphere (rigid plates) and asthenosphere (ductile layer)
  • The lower mantle is solid but can deform plastically over long timescales
• The outer core is a liquid layer (~2,900-5,100 km depth) composed primarily of iron and nickel alloy
• The inner core is a solid layer (~5,100-6,371 km depth) also composed of iron and nickel but with a higher pressure causing it to be solid
• Seismic waves (P-waves and S-waves) are used to study Earth's interior structure by analyzing their velocities and behavior at layer boundaries
• Earth's layered structure results from differentiation during its formation where denser materials sank to the center and lighter materials rose to the surface

Plate Tectonics

• Plate tectonics is the unifying theory that explains the movement and interaction of Earth's lithospheric plates
• Lithospheric plates are composed of the crust and uppermost mantle and move relative to each other at rates of a few centimeters per year
• Plate boundaries are classified into three main types: divergent (plates move apart), convergent (plates collide or subduct), and transform (plates slide past each other)
  • Divergent boundaries occur at mid-ocean ridges and rift valleys where new oceanic crust is formed (East Pacific Rise)
  • Convergent boundaries include subduction zones where oceanic crust sinks beneath another plate (Mariana Trench) and continental collisions that form mountain ranges (Himalayas)
  • Transform boundaries are characterized by horizontal motion along faults (San Andreas Fault)
• Plate motions are driven by convection currents in the mantle caused by heat transfer from the core and radioactive decay in the mantle
• Evidence for plate tectonics includes the fit of continents (Pangaea), distribution of earthquakes and volcanoes, and magnetic anomalies on the seafloor
• Plate tectonics has implications for the distribution of resources (mineral deposits), natural hazards (earthquakes, volcanoes, tsunamis), and the evolution of Earth's surface features

Mineral Composition

• Minerals are the building blocks of rocks and are defined by their chemical composition and crystalline structure
• The most abundant elements in Earth's crust are oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium which form common rock-forming minerals
• Silicate minerals are the most common mineral group and include quartz, feldspar, mica, amphibole, pyroxene, and olivine
  • Silicate minerals are characterized by the presence of silicon-oxygen tetrahedra (SiO4) in their structure
  • The arrangement of these tetrahedra determines the mineral's properties and classification (framework, sheet, chain, or isolated)
• Non-silicate minerals include carbonates (calcite, dolomite), oxides (hematite, magnetite), sulfides (pyrite, galena), and native elements (gold, copper)
• Mineral properties such as color, streak, luster, hardness, cleavage, and specific gravity are used for identification
• Minerals form through various processes including crystallization from magma, precipitation from aqueous solutions, and metamorphic reactions

Rock Types and Formation

• Rocks are classified into three main types based on their formation processes: igneous, sedimentary, and metamorphic
• Igneous rocks form from the cooling and solidification of magma (intrusive) or lava (extrusive)
  • Intrusive igneous rocks (granite, diorite, gabbro) have large crystals due to slow cooling rates
  • Extrusive igneous rocks (basalt, rhyolite, obsidian) have small crystals or are glassy due to rapid cooling rates
• Sedimentary rocks form from the deposition and lithification of sediments (weathered rock fragments, organic matter, or chemical precipitates)
  • Clastic sedimentary rocks (sandstone, shale, conglomerate) are composed of physically weathered rock fragments
  • Chemical sedimentary rocks (limestone, chert, evaporites) form from the precipitation of minerals from aqueous solutions
  • Organic sedimentary rocks (coal, oil shale) form from the accumulation and alteration of organic matter
• Metamorphic rocks form from the transformation of pre-existing rocks under high temperature and/or pressure conditions
  • Foliated metamorphic rocks (gneiss, schist, slate) have a layered or banded appearance due to the alignment of minerals
  • Non-foliated metamorphic rocks (marble, quartzite, hornfels) have a more uniform texture
• The rock cycle describes the dynamic transitions between rock types through various processes (weathering, erosion, deposition, metamorphism, and melting)

Geochemical Cycles

• Geochemical cycles describe the movement and exchange of elements between Earth's reservoirs (atmosphere, hydrosphere, lithosphere, and biosphere)
• The carbon cycle involves the exchange of carbon between the atmosphere (CO2), oceans (dissolved CO2, carbonate rocks), biosphere (photosynthesis, respiration), and lithosphere (fossil fuels, carbonate rocks)
  • Human activities such as burning fossil fuels and deforestation have altered the carbon cycle by increasing atmospheric CO2 concentrations
• The water cycle (hydrologic cycle) describes the continuous movement of water through evaporation, transpiration, precipitation, infiltration, and runoff
  • The water cycle is driven by solar energy and is critical for maintaining Earth's climate and supporting life
• The rock cycle is driven by plate tectonic processes and involves the transformation of rocks through weathering, erosion, deposition, metamorphism, and melting
• Other important geochemical cycles include the nitrogen cycle (fixation, nitrification, denitrification), phosphorus cycle (weathering, biological uptake, sedimentation), and sulfur cycle (weathering, reduction, oxidation)
• Understanding geochemical cycles is crucial for addressing environmental issues such as climate change, ocean acidification, and nutrient pollution

Key Analytical Methods

• X-ray diffraction (XRD) is used to identify mineral phases and determine their crystal structure by measuring the diffraction of X-rays by the mineral's atomic lattice
• X-ray fluorescence (XRF) is used to determine the elemental composition of rocks and minerals by measuring the secondary X-rays emitted when a sample is excited by high-energy X-rays
• Inductively coupled plasma mass spectrometry (ICP-MS) is used to measure the concentrations of trace elements in rock and mineral samples by ionizing the sample in a plasma and measuring the mass-to-charge ratios of the resulting ions
• Scanning electron microscopy (SEM) is used to image the surface topography and composition of rock and mineral samples by scanning the sample with a focused electron beam and detecting the secondary electrons and backscattered electrons
  • Energy-dispersive X-ray spectroscopy (EDS) is often coupled with SEM to determine the elemental composition of specific points or areas on the sample
• Transmission electron microscopy (TEM) is used to image the internal structure of minerals at the atomic scale by transmitting a beam of electrons through a thin sample and detecting the scattered electrons
• Stable isotope analysis (δ13C, δ18O, δ34S) is used to trace the sources and pathways of elements in geochemical cycles by measuring the relative abundances of isotopes in samples
• Radiometric dating methods (U-Pb, K-Ar, Rb-Sr) are used to determine the absolute ages of rocks and minerals by measuring the abundances of radioactive parent isotopes and their stable daughter isotopes

Real-World Applications

• Understanding Earth's structure and composition is essential for exploring and managing natural resources such as mineral deposits, fossil fuels, and groundwater
  • Geochemical exploration methods (soil and stream sediment sampling, geophysical surveys) are used to locate mineral deposits
  • Petroleum geologists use seismic surveys and well logging to identify and characterize oil and gas reservoirs
• Geohazard assessment and mitigation rely on knowledge of Earth's structure and processes
  • Seismic hazard maps are developed based on the distribution of faults, historical seismicity, and ground motion models
  • Volcanic hazard assessment involves monitoring seismic activity, ground deformation, and gas emissions to predict eruptions
  • Landslide hazard assessment considers factors such as slope stability, rock type, and precipitation patterns
• Environmental management and remediation require an understanding of geochemical cycles and the behavior of contaminants in the environment
  • Geochemical modeling is used to predict the fate and transport of contaminants in groundwater and soils
  • Remediation strategies (pump-and-treat, bioremediation, permeable reactive barriers) are designed based on the properties of the contaminants and the site geology
• Climate change research relies on paleoclimatology, the study of past climate conditions using geochemical proxies such as stable isotopes, trace elements, and biomarkers preserved in sediments, ice cores, and fossils
  • Understanding past climate variations helps to contextualize current climate change and improve predictions of future climate scenarios
• Geochemistry plays a crucial role in the search for extraterrestrial life and the study of planetary evolution
  • The composition of meteorites provides insights into the early solar system and the building blocks of planets
  • The geochemistry of Mars, as determined by rovers and orbiters, helps to assess the planet's potential for past or present habitability