Earth Science: Strata and Fossils, Weather and Atmosphere, Celestial Bodies and Earth's Motion
Strata and fossils
The exam tests relative dating and Earth's movements. Sedimentary rocks form in layers, and in undisturbed sequences the youngest layer is on top (superposition). A fault or igneous intrusion cutting across layers is younger than the layers it cuts. Fossils appear in a recognisable order; index fossils are widespread and short-lived, so they correlate rock layers across regions. An unconformity marks erosion or a gap in deposition.
Weather and atmosphere
The atmosphere is mostly nitrogen and oxygen, and weather occurs in the troposphere. Air pressure falls with altitude. Warm air rises, expands and cools, and condensing water vapour forms clouds. Low pressure brings unsettled, rainy weather; high pressure brings clear, dry weather. A cold front brings heavy, short rain and a temperature drop; a warm front brings steady rain. Land heats and cools faster than sea, producing sea breezes.
Celestial bodies and Earth's motion
Earth rotates west to east once in about 24 hours, causing day and night and making the Sun appear to move east to west. Earth orbits the Sun in 365.25 days, and its axis is tilted about 23.4°, so the hemispheres receive different sunlight angles through the year. Seasons come from the tilt, not from distance from the Sun. The Moon orbits in about 29.5 days and shows phases; eclipses need the Sun, Earth and Moon aligned.
Worked example
In June the Northern Hemisphere is tilted toward the Sun: sunlight strikes more directly and days are longer, so it is summer there - even though Earth is near its farthest point from the Sun. In December the same hemisphere is tilted away, giving winter.
Traps
Do not say distance causes seasons, or that Moon phases are Earth's shadow - that shadow causes a lunar eclipse. The Moon rotates once per orbit, so the same face always points at Earth. Folding or faulting can overturn superposition, so check cross-cutting relationships first.