Earth Science: Earth, Weather, Strata, and Celestial Bodies
Earth, weather, and the ground beneath
The Earth is layered: crust, mantle, outer core, inner core. The crust is broken into plates whose motion causes earthquakes and volcanoes. Magma cooling slowly underground gives coarse-grained rock such as granite, while rapid cooling at the surface gives fine-grained rock such as basalt. Strata record time: younger layers normally lie above older ones, and any fault or intrusion cutting across layers must be younger than those layers.
Weather depends on water vapour and pressure. Humidity compares the vapour present with the maximum the air can hold at that temperature; cooling air raises relative humidity toward 100 %, the dew point, and clouds form. Air moves from high to low pressure. A warm front brings prolonged steady rain as warm air rides over cold air, whereas a cold front brings brief heavy showers.
Sky and celestial bodies
Earth's rotation makes the Sun and stars appear to move east to west and causes day and night; revolution combined with a 23.4° axial tilt produces the seasons. The Moon shows phases because we see varying fractions of its sunlit half - not because of Earth's shadow - and a lunar eclipse occurs only at full moon.
Traps: blaming seasons on distance from the Sun; reversing the order in lunar versus solar eclipses; forgetting that P-waves arrive before S-waves; treating 100 % humidity as automatically meaning rain.
Worked example. S-waves arrive 6 s after P-waves. With P at 7 km/s and S at 3.5 km/s, d/3.5 − d/7 = d/7 = 6, so the epicentre is 42 km away.