Geoscience Essentials: Plate Tectonics, Rocks, and Soils

Earth formed about $4.6\times 10^{9}$ years ago; differentiation by density created distinct layers.
Core: innermost zone; inner core solid, outer core liquid; mostly nickel and iron.
Mantle: above the core; includes magma, the asthenosphere, and the solid upper mantle.
Lithosphere: outermost layer (solid upper mantle + crust); thickness ~ $100\ \text{km}$ .
Surface layer above crust is soil; essential for life and nutrient cycling.

Lithosphere is divided into tectonic plates in slow, constant motion driven by mantle convection.
Wegener (1912) proposed Pangaea; evidence included similar rock formations and fossils across continents.
Marie Tharp (1950s) mapped the ocean floor, revealing underwater mountain ranges and canyons, leading to plate tectonics.
Plate tectonics: plates in motion create continents, oceans, mountain ranges, and deep-sea features.

Divergent boundaries: plates move apart; seafloor spreading forms new ocean crust; can create volcanoes, earthquakes, and rift valleys (e.g., Great Rift Valley).
Divergent boundary example: seafloor spreading at mid-ocean ridges.
Convergent boundaries: plates move toward each other; outcomes depend on plate types.
- Oceanic vs. continental: oceanic plate subducts beneath continental plate; island arcs form; earthquakes/volcanoes common.
- Oceanic vs. oceanic: subduction occurs; island arcs form.
- Continental-continental: no subduction; collision uplifts to form mountain ranges (e.g., Himalayas).
Transform boundaries: plates slide sideways past each other; major faults form; frequent earthquakes (e.g., San Andreas Fault).
Oceanic vs. continental crust density difference drives subduction; continental crust is lighter and often overrides oceanic crust.
Ring of Fire: circle of tectonic activity around the Pacific Ocean due to boundaries; hot spots like Hawaii also occur away from boundaries.

Typical plate movement rate: $36\ \text{mm/year}$ .
Time to move a distance: $t = \frac{D}{r}$ .
Example: distance $D = 630\ \text{km}$ , rate $r = 36\ \text{mm/year}$ .
- Convert distance: $630\ \text{km} = 6.30\times 10^{8} \text{ mm}$ .
- Time: $t = \frac{6.30\times 10^{8}}{36} \approx 1.75\times 10^{7}$ years (≈ 18 million years).
Question: How long for a plate moving at $20\ \text{mm/year}$ to traverse one football field ( $91.44\ \text{m}$ )?
- Answer: $t = \frac{91.44\times 10^{3}\ \text{mm}}{20\ \text{mm/yr}} = 4.572\times 10^{3}\ \text{yr} \approx 4.6\times 10^{3}$ years.

Earthquakes: sudden crust movements due to fault release; epicenter is surface point above rock rupture; magnitude on a logarithmic scale (Richter/Moment Magnitude).
Frequency: many small quakes daily; large quakes (< few per year globally) cause major damage depending on population and building codes.
Notable recent events: Alaska 8.7 (2021) at a convergent boundary; Mexico 2017; Haiti 2010/2021 impacts.
Volcanoes: eruptions release ash, lava, and gases; 85% occur along plate boundaries; hot spots (e.g., Hawaii) are independent of boundaries; volcano activity linked to plate motion.
Human infrastructure: nuclear plants are designed to withstand certain ground movement; some operate in seismic zones; notable accidents linked to earthquakes/tsunamis (e.g., 2011 Japan).

Igneous rocks form from magma; classified as basaltic (oceanic crust) or granitic (continental crust).
Basaltic: iron, magnesium, calcium; dense; dark color.
Granitic: feldspar, mica, quartz; less dense; lighter color; major in continental crust.
Fractures form as rocks cool; mineral-rich fluids in fractures yield ore deposits (veins).
Sedimentary rocks form from compressed sediments (muds, sands, gravels); can be uniform (sandstones, mudstones) or heterogeneous (conglomerates); hold fossils.
Metamorphic rocks form from existing rocks under high temperature/pressure (without melting); examples: slate, marble, anthracite; often used in construction.
Rock cycle: interlinked pathways among igneous, sedimentary, metamorphic rocks driven by tectonics, weathering, erosion, and subduction.

Weathering: breakdown of rocks at/near the surface; physical (mechanical) and chemical (reactions).
Physical weathering: freeze–thaw cycles, abrasion, root wedging, and biological activity.
Chemical weathering: dissolution by acids (e.g., carbonic acid from CO₂, sulfuric/nitric acids from pollution).
Acid precipitation: sulfuric and nitric acids from pollutants dissolve rocks, alter soils, and degrade limestone statues.
Erosion: physical removal of weathered material by water, wind, ice, and organisms; leads to deposition elsewhere.
Weathering/erosion link to soil formation and nutrient cycling.

Soil forms from weathering of rocks plus accumulation of organic detritus; takes hundreds to thousands of years.
Five soil-forming factors: parent material, climate, topography, organisms, and time.
Horizons: O (organic detritus/humus), A (topsoil), E (eluviation, leaching), B (subsoil), C (parent material).
Humus: fully decomposed organic matter in the O horizon.
Loam (ideal agricultural soil): roughly 40% sand, 40% silt, 20% clay.
Physical properties: particle size (sand > silt > clay); porosity; permeability.
Texture triangle: method to classify soil texture from sand/silt/clay percentages.
Water holding capacity vs permeability:
- Sand: high drainage, low water retention; easy root penetration.
- Clay: high water retention, low permeability; dense root zone.
Chemical properties: cation exchange capacity (CEC) and base saturation.
CEC: ability of soils to adsorb/release cations; higher in clay-rich soils; supports nutrient availability.
Base saturation: proportion of bases (Ca, Mg, K, Na) to acids (Al, H); affects nutrient availability.
Trade-off: high CEC but very high clay can reduce aeration; ideal productivity requires balanced CEC and permeability.
Biological properties: fungi, bacteria, protozoa; earthworms; detritivores; some nitrogen-fixing bacteria.

Agriculture, forestry, and development can degrade soils; erosion is a major issue.
Dust Bowl (1930s): reduced vegetation and drought caused massive wind erosion and dust storms.
Soil erosion reduces soil depth quickly; recovery can take centuries.
Soil supports plant growth, filters water, provides biodiversity habitat, and buffers pollutants; protecting soil protects water quality.

Lithosphere: rigid outer shell including crust and upper mantle; ~ $100\ \text{km}$ thick.
Asthenosphere: semi-molten layer beneath the lithosphere.
Mantle: includes mantle magma, asthenosphere, and upper mantle.
Core: inner solid core and outer liquid core; mainly Fe/Ni.
Divergent boundary: plates move apart; seafloor spreading.
Convergent boundary: plates move toward each other; subduction or collision.
Transform boundary: plates slide horizontally past one another; faulting and earthquakes.
Subduction: oceanic plate sinks beneath another plate.
Island arc: chain of volcanic islands formed by subduction.
Hot spot: magma plumes beneath plates creating volcanoes away from boundaries (e.g., Hawaii).
Ring of Fire: circum-Pacific zone of high tectonic activity.
Rock cycle: interlinked processes producing igneous, sedimentary, and metamorphic rocks.
Weathering vs. erosion: breakdown vs. removal of rock material.
CEC: cation exchange capacity; base saturation: proportion of nutrient bases in soil.
Loam: balanced soil texture favorable for agriculture.