13.1 The Structure and Function of Earth Systems
Key Takeaways
- Earth has four compositional layers — crust, mantle, outer core, and inner core — and five mechanical layers — lithosphere, asthenosphere, mesosphere, outer core, and inner core — that explain plate motion and the magnetic field.
- Constructive processes (volcanism, tectonic uplift, deposition) build new landforms while destructive processes (weathering, erosion, mass wasting) break them down; topographic maps use contour lines to encode relief and slope for classroom analysis.
- Surface water organizes into watersheds (drainage basins) and subsurface water fills aquifers whose yield depends on porosity and permeability; the water table marks the upper surface of the saturated zone.
- The atmosphere is roughly 78% nitrogen, 21% oxygen, and 1% argon plus trace gases; its layered structure (troposphere, stratosphere, mesosphere, thermosphere) filters harmful radiation and holds gases that allow liquid water and life.
- Earth's four spheres — geosphere, hydrosphere, atmosphere, biosphere — exchange matter and energy continuously; solar radiation drives most transfers while geothermal energy powers plate motion and the rock cycle.
Earth's Layers: Composition and State
Earth is organized into compositional layers, defined by chemistry, and mechanical layers, defined by whether the rock is rigid or able to flow. A TExES candidate should know both classifications because the framework asks about each.
| Layer | Composition | State | Approx. thickness |
|---|---|---|---|
| Crust | Silicate rocks rich in oxygen and silicon; oceanic crust is denser (basalt, ~3 g/cm³), continental crust is lighter (granite, ~2.7 g/cm³) | Solid | 5-70 km |
| Mantle | Iron- and magnesium-rich silicates (peridotite) | Solid but with a plastic asthenosphere below the rigid lithosphere | ~2,900 km |
| Outer core | Molten iron and nickel | Liquid | ~2,200 km |
| Inner core | Solid iron-nickel alloy under extreme pressure | Solid | Radius ~1,220 km |
The lithosphere (crust + uppermost rigid mantle) is broken into tectonic plates that ride on the weaker asthenosphere. Convection currents in the mantle and the pull of subducting slabs drive plate motion. The liquid outer core generates Earth's magnetic field through the geodynamo effect, which shields the surface from solar wind and helps the atmosphere retain water.
Landforms and Constructive vs Destructive Processes
Constructive processes build new landforms: volcanism adds lava plateaus and shield volcanoes, tectonic uplift raises mountains, and deposition by rivers and waves forms deltas, beaches, and barrier islands. Destructive processes wear landforms down: physical and chemical weathering break bedrock into sediment, erosion moves that sediment by water, wind, or ice, and mass wasting (landslides, slumps) carries material downslope under gravity.
Plate boundaries set the stage for most large-scale landforms:
- Divergent boundaries — plates pull apart; new crust forms at mid-ocean ridges and continental rift valleys (East African Rift).
- Convergent boundaries — plates collide; subduction zones form ocean trenches and volcanic arcs (Andes, Cascades), and continent-continent collisions form folded mountains (Himalayas).
- Transform boundaries — plates slide past each other; the San Andreas Fault is the U.S. textbook example, storing strain that releases as earthquakes.
Texas classrooms can use the Balcones Escarpment and Edwards Plateau as local examples of gradual landform change, and the 1900 Galveston hurricane as a catastrophic example of how storm surge alters barrier islands in hours.
Topographic Maps and Satellite Imaging
A topographic map uses contour lines to show elevation. Rules a 4-8 teacher must articulate:
- Contour lines connect points of equal elevation.
- Relief is the difference in elevation between the highest and lowest points on the map; widely spaced lines indicate gentle slopes, closely spaced lines indicate steep slopes.
- Contour lines never split; they form V-shapes that point upstream where they cross a river.
- A bench mark marks a known elevation surveyed in the field.
Satellite imaging supplements ground surveys. Landsat and Sentinel sensors record reflected energy in multiple bands, allowing teachers to compare images across decades to track urban growth, deforestation, coastal erosion, and volcanic deposits without a site visit. In 2024 Texas teachers used drought-monitor satellite products to show reservoir changes during the summer dry season — a direct link from remote sensing to a local Earth-system issue.
Watersheds, stream processes, aquifers, karst, and the oceans are developed in "Surface Water, Groundwater, and the Oceans."
Atmospheric composition, the four layers, ozone, and the energy budget are developed in "Earth's Atmosphere: Composition, Structure, and Energy Transfer."
The Four Spheres and Their Interactions
Earth's matter cycles among four interacting spheres.
| Sphere | Includes | Interacts with |
|---|---|---|
| Geosphere | Crust, mantle, core; rocks, soil, landforms | Provides minerals to biosphere; volcanic gases enter atmosphere |
| Hydrosphere | Oceans, lakes, rivers, groundwater, ice | Erosion shapes geosphere; evaporation feeds atmosphere |
| Atmosphere | Gases surrounding Earth | Carries water vapor for hydrosphere; supplies O₂ and CO₂ to biosphere |
| Biosphere | All living things and dead organic matter | Respires gases into atmosphere; roots weather the geosphere |
Example interaction chain: solar energy evaporates ocean water (atmosphere-hydrosphere); water vapor condenses as clouds and falls as rain on a Texas hill country (atmosphere-geosphere); runoff carves a canyon and recharges the Edwards Aquifer (hydrosphere-geosphere); plants draw that water through roots and transpire it back to the air (biosphere-hydrosphere-atmosphere). A teacher who can trace one packet of matter through all four spheres demonstrates Competency 016 mastery.
Human Activity, Natural Processes, and Energy Transfer
Human activity and natural processes alter Earth systems on timescales from seconds to millennia. Gradual changes include erosion of coastlines, soil formation from weathering, groundwater depletion from overpumping, and climate change driven by greenhouse-gas emissions. Catastrophic changes include earthquakes, tsunamis triggered by undersea fault movement, volcanic eruptions that inject ash and sulfur dioxide into the stratosphere, and hurricanes that reshape barrier islands in a single day.
Energy that drives Earth's systems comes from two main sources and several secondary ones:
- Solar energy — the dominant driver; heats the atmosphere and oceans, powers the water cycle, and fuels photosynthesis.
- Geothermal energy — heat from Earth's interior; drives plate tectonics, volcanism, and the rock cycle.
- Secondary sources for human use include wind, hydroelectric, biofuels, and nuclear; these all trace their energy back to the sun or to radioactive decay in the geosphere.
Three mechanisms transfer energy through Earth systems: conduction (direct contact, e.g., warm ground heating air above it), convection (bulk movement of heated fluid, e.g., mantle convection, atmospheric circulation), and radiation (electromagnetic waves, e.g., sunlight reaching the surface). Recognizing which mechanism is operating in a given phenomenon is a common TExES multiple-choice task.
A teacher shows students a topographic map on which contour lines are very close together near a hillside. What does this spacing most directly indicate about that hillside?