Domain A · General and Field Geology · 3 min read
Earth Systems and Geologic Time
Geology treats the Earth as a set of interacting systems: the geosphere (solid Earth), hydrosphere, atmosphere, biosphere, and cryosphere. Energy from two sources drives them. Solar energy powers the hydrologic cycle, weathering, and atmospheric circulation; internal heat from radioactive decay and primordial heat drives mantle convection, plate tectonics, magmatism, and metamorphism.
Earth's Layered Structure
| Layer | Depth (approx.) | Key characteristics |
|---|---|---|
| Continental crust | 0–35 km (up to 70 km under mountains) | Granitic (felsic) average composition; density ≈ 2.7 g/cm³ |
| Oceanic crust | 0–7 km | Basalt and gabbro; density ≈ 3.0 g/cm³; younger than about 200 million years |
| Lithosphere | Crust + uppermost mantle, ~100 km | Rigid; broken into tectonic plates |
| Asthenosphere | ~100–350 km | Weak, ductile; includes the low-velocity zone |
| Mantle | to 2,900 km | Peridotite (olivine + pyroxene); transition zone at 410–660 km |
| Outer core | 2,900–5,150 km | Liquid iron-nickel; generates the magnetic field; S waves do not pass |
| Inner core | 5,150–6,371 km | Solid iron-nickel |
The Mohorovičić discontinuity (Moho) separates crust from mantle and is marked by a jump in P-wave velocity from about 6–7 km/s to about 8 km/s. The Gutenberg discontinuity at the core–mantle boundary produces the S-wave shadow zone (beyond about 104° from an epicenter) that proves the outer core is liquid.
Earth Cycles
The Geologic Time Scale
The geologic time scale divides Earth history into eons, eras, periods, and epochs, defined by Global Boundary Stratotype Sections and Points (GSSPs) and calibrated by radiometric dates. Learn the order of the periods and the major boundary events, because sequence-of-events and biostratigraphy questions depend on them.
| Era | Period | Begins (Ma) | Key events |
|---|---|---|---|
| Cenozoic | Quaternary | 2.58 | Pleistocene glaciations; Homo |
| Cenozoic | Neogene | 23 | Grasslands spread; Himalayan uplift |
| Cenozoic | Paleogene | 66 | Mammal radiation after K–Pg extinction |
| Mesozoic | Cretaceous | 145 | Flowering plants; ends with K–Pg impact extinction |
| Mesozoic | Jurassic | 201 | Dinosaurs dominant; Pangea breaks up |
| Mesozoic | Triassic | 252 | First dinosaurs and mammals; ends with mass extinction |
| Paleozoic | Permian | 299 | Pangea assembled; ends with largest mass extinction |
| Paleozoic | Carboniferous | 359 | Coal swamps; first reptiles (Mississippian and Pennsylvanian in North America) |
| Paleozoic | Devonian | 419 | Age of Fishes; first forests and amphibians |
| Paleozoic | Silurian | 444 | First vascular land plants |
| Paleozoic | Ordovician | 485 | Great biodiversification; ends with glaciation and extinction |
| Paleozoic | Cambrian | 539 | Rapid appearance of shelled animals; trilobites |
Sources of Geologic Information
Before fieldwork, a geologist compiles existing data. Primary sources include the U.S. Geological Survey (topographic quadrangles, geologic maps, the National Geologic Map Database, water data, earthquake catalogs), state geological surveys, the Natural Resources Conservation Service (soil surveys), the Federal Emergency Management Agency (flood maps), state environmental agencies (well logs and contaminated-site files), and the peer-reviewed literature. Historical aerial photographs and Sanborn fire-insurance maps are standard sources for past land use in environmental site assessments.
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