Earth Systems, Weather, and Astronomy
This domain spans the solid Earth, the atmosphere, and the solar system. Items ask you to relate plate boundaries to earthquakes and volcanoes, interpret a weather map or front, explain the seasons, sequence moon phases, or apply a scaling law to an orbiting body.
Earth systems and tectonics
- Earth's layers: crust, mantle, outer core (liquid), inner core (solid). Convection in the mantle drives plate motion.
- Divergent boundaries create new crust (mid-ocean ridges); convergent boundaries produce subduction, volcanoes and mountain belts; transform boundaries produce strike-slip faults.
- Rock types: igneous (cooled magma), sedimentary (compacted deposits), metamorphic (heat and pressure). The rock cycle links them.
- P-waves (longitudinal) arrive first and travel through solids and liquids; S-waves (transverse) cannot pass through liquids.
Weather and climate
- Weather is short-term state; climate is long-term average. The atmosphere is layered troposphere, stratosphere (ozone), mesosphere, thermosphere.
- Air moves from high to low pressure; the Coriolis effect deflects it right in the Northern Hemisphere and left in the Southern.
- A cold front brings rapid uplift, heavy showers and a temperature drop; a warm front brings gradual cloud and steady rain.
- Seasons come from Earth's 23.5° axial tilt, not from distance from the Sun.
Worked example
An asteroid orbits the Sun at a mean distance of 4 AU. Estimate its orbital period.
Reasoning: use Kepler's third law in the form T² = a³, with T in years and a in AU. T² = 4³ = 64, so T = 8 years. The reasoning step is keeping units consistent - AU with years - which is why no conversion is needed, and taking the square root at the end rather than stopping at 64.
Traps that cost marks
- Explaining seasons by Earth's changing distance from the Sun.
- Confusing lunar phases (from orbital geometry) with eclipses (rare alignment).
- Reversing the Coriolis deflection between hemispheres.
- Treating a light-year as a unit of time.
- Confusing apparent magnitude with luminosity, or a star's colour with its distance.
- Assuming all planets orbit at the same speed; period rises steeply with distance.