Earth Systems, Tectonics, and Volcanic Processes
L1 – Earth Shape, Gravity, and Systems
Earth rotates, creating centrifugal force that leads to an equatorial bulge and polar flattening, resulting in an oblate ellipsoid. The equatorial radius is approximately 7 km greater than the average, while the polar radius is 7 km less, with an average radius of 6371 km. This ellipsoid assumes uniform mass distribution, but Earth’s mass is uneven due to ice sheet fluctuations, mantle plumes, variable crust thickness, and density differences between oceans and continents.
Gravitation is the force caused by mass attraction, while gravity is the combination of gravitation and centrifugal force. Gravity is stronger at the poles (9.83 m/s²) and weaker at the equator (9.78 m/s²) due to the bulge and centrifugal effect; 9.8 m/s² is merely an approximation.
The Geoid is the surface representing mean sea level controlled by Earth’s gravity field. Water deforms easily, responding directly to gravity variations. Satellites measure these global variations, and on continents, researchers extrapolate sea level using geophysical measurements. The geoid is an irregular, slightly pear-shaped surface used as a reference for elevations.
A gravity anomaly is the difference between actual gravity and expected gravity from a perfect ellipsoid. Isostasy describes the gravitational equilibrium between surface topography and subsurface crustal roots, similar to an iceberg floating in water. Mountains have low-density crustal roots that balance the visible mass above. Three factors influence gravity measurements: altitude, visible mass, and hidden mass.
The Bouguer gravity anomaly corrects for altitude and visible mass to compare gravity as if standing at sea level. Mountains typically show negative Bouguer anomalies due to low-density roots, while ocean basins show positive anomalies due to thin crust and dense mantle. These anomalies reveal subsurface structures.
Earth’s spheres include the Geosphere (crust, mantle, core), Atmosphere (gases), Hydrosphere (liquid water), Cryosphere (ice), Biosphere (living organisms), Magnetosphere (magnetic field), and Anthroposphere (human impact). These spheres are strongly interconnected; changes in one trigger changes in others.
The atmosphere is layered by temperature: Troposphere (weather occurs here), Stratosphere (ozone layer), Mesosphere, Thermosphere, and Exosphere. The tropopause acts as a boundary that controls atmospheric circulation, contributing to Hadley cells and global wind systems.
Earth’s internal structure is defined by composition (Core: Fe-Ni; Mantle: silicate minerals; Crust: lighter silicates) and mechanical behavior (Lithosphere: rigid outer layer; Asthenosphere: weak, ductile layer).
Earth system interactions are profound. For example, the Himalayan orogeny altered atmospheric circulation, intensifying monsoons and increasing chemical weathering, which consumed atmospheric CO2 and contributed to the Cenozoic ice age. Similarly, the Ordovician–Silurian mass extinction was linked to glacial cycles, nutrient release, and ocean anoxia.
L2 – Formation, Differentiation, and Early Tectonics
The early solar system consisted of nebular gas and solid pebbles. Gas drag slowed pebbles, causing them to spiral inward until pressure bumps—caused by temperature changes or magnetic fields—trapped them, allowing planetesimals to form via gravitational collapse.
Planetesimals underwent planetary differentiation, where dense metals (Fe) sank to form a core and lighter silicates rose to form a mantle. The primary heat source was the radioactive decay of 26Al. Evidence for this is found in Calcium-Aluminum-rich inclusions (CAIs) in meteorites, which show excess 26Mg.
The Hf-W system is used to date core formation. Because Hf is lithophile and W is siderophile, the ratio of 182W in the mantle reveals when differentiation occurred. Earth’s core formed within approximately 30 million years of solar system formation.
Earth formed from the collision of proto-Earth and Theia, which also created the Moon. Early Earth was a global magma ocean that solidified bottom-up. Tectonic regimes evolved from vertical tectonics (sagduction) in a homogeneous mantle to horizontal plate tectonics as the crust cooled and became rigid.
L3 – Earth Interior and First Crust
Melting depends on temperature and pressure. Decompression melting occurs as mantle rises, while flux melting (involving water) lowers the melting point of minerals. The first stable continental crust formed as TTG (Tonalite-Trondhjemite-Granodiorite) through the partial melting of hydrated basalt (amphibolite).
Seismic waves probe the interior: P-waves travel through solids and liquids, while S-waves travel only through solids. Velocity changes at the Moho (crust-mantle boundary) and the core-mantle boundary reveal Earth’s layered structure. The outer core is liquid, as evidenced by the S-wave shadow zone.
L5 – Tectonic Settings: MOR, Iceland, and Subduction
Mid-Ocean Ridges (MOR) form new crust in three layers: pillow basalts, sheeted dikes, and gabbro. As plates move away, the lithosphere thickens through cooling. Iceland is an anomaly where the MOR is above sea level due to a mantle plume providing excess heat and magma.
Subduction zones are driven by slab pull. Flux melting in the mantle wedge produces arc magmas. Viscosity dictates eruption style: mafic (basaltic) magmas are runny and effusive, while felsic magmas are viscous and explosive.
L6 – Volcanism, Earthquakes, and Collision Tectonics
Volcanic hazards include pyroclastic flows, lahars, and ash clouds. Sector collapse, as seen at Mt. St. Helens, can trigger explosive eruptions by rapidly reducing confining pressure. Megathrust earthquakes occur at subduction zones due to elastic rebound, often generating tsunamis.
Continental collisions, such as the India-Eurasia collision, result in mountain building (orogeny), crustal thickening, and extensive faulting. Ophiolites—fragments of oceanic lithosphere thrust onto continents—provide direct evidence of ancient oceanic crust.