Earth's internal structure was discovered through seismic wave analysis – P waves (compressional) and S waves (shear) travel at different speeds through different materials. The crust has two types: continental crust (granitic, less dense, 25-70 km thick) and oceanic crust (basaltic, denser, 5-10 km thick). The mantle contains the asthenosphere (partially molten, ductile layer that allows plate movement) and the lower mantle (solid but capable of slow convection). The outer core's liquid state is why Earth has a magnetic field (dynamo effect) – convection of molten iron generates electrical currents that produce the magnetosphere, which protects Earth from solar wind. The inner core is solid despite extreme heat because of immense pressure (over 3 million atmospheres). Earth's average density is 5.5 g/cm³, while surface rocks average only 2.7 g/cm³, indicating a dense core. Temperature increases about 25-30°C per kilometer in the upper crust (geothermal gradient).

Plate boundaries are of three types: divergent (plates move apart – mid-ocean ridges like the Mid-Atlantic Ridge, creating new oceanic crust), convergent (plates collide – subduction zones where one plate descends beneath another, creating volcanoes, trenches, and mountain ranges like the Himalayas from India-Asia collision), and transform (plates slide past horizontally – San Andreas Fault in California, causing earthquakes). Plate movement is driven by mantle convection (heat from core causing mantle circulation), ridge push (gravity pushing newly formed crust away from ridges), and slab pull (gravity pulling cold, dense subducting plates downward). Rates average 2-10 cm/year – about the speed fingernails grow. Evidence for plate tectonics includes matching fossils across oceans (Mesosaurus in South America and Africa), similar rock sequences and mountain ranges, paleoclimate data (glacial deposits in tropical locations), seafloor magnetic stripe patterns, and direct GPS measurements showing plate movement today.

Igneous rocks: Intrusive (plutonic) form when magma cools slowly underground, producing large crystals (granite). Extrusive (volcanic) form when lava cools quickly on the surface, producing small or no crystals (basalt, obsidian, pumice). Sedimentary rocks: Clastic from physical fragments (sandstone, shale, conglomerate), chemical from precipitation (rock salt, limestone), and organic from biological material (coal, some limestone from shells). Sedimentary rocks cover about 75% of Earth's surface but only about 5% of the crust by volume. Metamorphic rocks result from heat and pressure without melting – contact metamorphism (near magma intrusions, creating hornfels) and regional metamorphism (during mountain building, creating slate, schist, gneiss, marble, quartzite). Foliated metamorphic rocks have aligned minerals (slate, schist, gneiss); non-foliated do not (marble, quartzite). The rock cycle connects to plate tectonics: igneous rocks form at divergent boundaries and hotspots, metamorphic rocks form at convergent boundaries, and sedimentary rocks form in basins. Understanding the rock cycle is essential for geology, mining, and construction.

The troposphere contains about 80% of atmospheric mass and virtually all water vapor, clouds, and weather. Temperature decreases with altitude (about 6.5°C per km). The tropopause is the boundary to the stratosphere. The stratosphere's ozone layer (10-50 km) absorbs 95-99% of harmful UV radiation; CFCs (chlorofluorocarbons) damaged this layer, leading to the Montreal Protocol (1987), the first universally ratified UN treaty. Temperature increases with altitude in the stratosphere due to UV absorption. The mesosphere is where most meteors burn up – the coldest part of the atmosphere (-90°C at the mesopause). The thermosphere absorbs extreme UV and X-rays, causing high temperatures (up to 1500°C) but feels cold due to low density. Auroras (Northern and Southern Lights) occur when solar particles interact with Earth's magnetic field in the thermosphere. The Kármán line (100 km) is often considered the edge of space. Atmospheric pressure decreases exponentially – half at 5.5 km, half again at 16 km. Earth's atmosphere formed from volcanic outgassing and later oxygen from cyanobacteria (Great Oxidation Event, 2.4 billion years ago).

The Hadean Eon (4.54-4.0 billion years ago) – Earth formed from solar nebula, molten surface, moon formation from giant impact. No known rocks survive. The Archean Eon (4.0-2.5 billion years ago) – first continental crust, earliest life (prokaryotes, stromatolites), atmosphere had no oxygen. The Proterozoic Eon (2.5-0.541 billion years ago) – oxygen accumulation (Great Oxidation Event), first eukaryotes, first multicellular life (Ediacaran biota). The Phanerozoic Eon (541 million years ago to present) – visible life with hard parts. Paleozoic Era (541-252 million years ago) – Cambrian explosion (most major animal groups appear), colonization of land (plants, insects, amphibians), first reptiles, ended with Permian-Triassic extinction (worst mass extinction, 96% marine species lost). Mesozoic Era (252-66 million years ago) – age of reptiles (dinosaurs, pterosaurs, marine reptiles), first mammals and birds, breakup of Pangea, ended with Cretaceous-Paleogene extinction (asteroid impact killed non-avian dinosaurs). Cenozoic Era (66 million years ago to present) – age of mammals, evolution of humans (hominins appeared ~6 million years ago, Homo sapiens ~300,000 years ago). The geological time scale is crucial for understanding evolution, climate change, and Earth's history.

The water cycle is driven by solar energy and gravity. Processes: Evaporation (liquid to water vapor from oceans, lakes, soil) – oceans supply about 86% of atmospheric water. Transpiration (water vapor released from plants) – together with evaporation called evapotranspiration. Condensation (vapor to liquid forming clouds). Precipitation (rain, snow, sleet, hail) – returns water to Earth. Runoff (surface water flowing to oceans) and infiltration (water soaking into ground, becoming groundwater). Groundwater moves slowly through aquifers (porous rock layers) – about 30% of freshwater is groundwater. The residence time of water varies: atmosphere ~9 days, rivers ~2 weeks, soil moisture ~2-60 weeks, lakes ~10 years, groundwater ~200-10,000 years, glaciers ~1,000-100,000 years, oceans ~3,000 years. Human impacts include overpumping groundwater (causing subsidence), dam construction (altering flow), and climate change (affecting precipitation patterns). The water cycle connects all Earth systems – hydrosphere, atmosphere, geosphere, and biosphere. Understanding the water cycle is essential for water resource management, agriculture, and predicting droughts and floods.