Physical geography essentials: plate tectonics, atmosphere, rocks

Physical geography questions reward precision over general familiarity. Three clusters come up again and again: the evidence behind continental drift, how the atmosphere actually behaves, and rock properties that sound backwards until you understand the mechanism.

The evidence for continental drift

Alfred Wegener's theory rests on evidence you can list, not just assert:

• Matching coastlines and rock formations: the Brazilian and West African coastlines fit together like puzzle pieces, and the rock strata along them match.
• Matching gold-bearing strata: similar mineral-bearing rock layers appear in Ghana and Brazil, continents now separated by an ocean.
• Gondwana-age glacial sediments: matching glacial deposits and scratch marks (striations) appear on continents now widely separated (South America, Africa, India, Australia, Antarctica), evidence they were once joined and glaciated together, then drifted apart.

All three point to the same conclusion: today's separated continents were once a single landmass.

Isotherms: why they bend the way they do

An isotherm joins places of equal temperature. In January (Northern Hemisphere winter), isotherms bend equatorward as they cross large landmasses like Asia and North America. The mechanism: land has much lower specific heat capacity than water, so it cools faster than the ocean in winter, making the interior of a continent colder than the sea at the same latitude, which pulls the isotherm line southward as it crosses land.

Atmospheric dust: more land than ocean, more mid-latitude than either extreme

Dust particles are overwhelmingly land-sourced, from dry, sparsely vegetated regions and human/industrial activity, so they are far less abundant over open ocean than over continents. Within land areas, dust is most concentrated over the sub-tropics and temperate latitudes (deserts, industrial belts), not the moist equatorial zone or the ice-covered poles.

Chalk vs clay: the counter-intuitive pairing

This is a favourite "which is which" trap:

Because clay is porous (holds water) but impermeable (won't let it pass through), clay layers commonly act as an aquiclude, a barrier that traps groundwater in the aquifer above or below it. This is why clay-lined reservoirs hold water while sandy or chalky ground lets it drain away.

The greenhouse effect, in three facts

1. The atmosphere absorbs and re-radiates outgoing longwave (infrared) radiation from the Earth's surface, keeping it warmer than it would otherwise be.
2. Water vapour and carbon dioxide are among the gases most responsible for this effect.
3. Without an atmosphere, Earth's average surface temperature would be dramatically lower, closer to the Moon's extremes.