HSC Year 11 Earth and Environmental Science practice exams with worked answers

Earth formed about 4.54 billion years ago (4.54 Ga) as dust, rock fragments and planetesimals orbiting the young Sun collided and stuck together, a process called accretion. Kinetic energy from impacts, gravitational compression and heat released by radioactive decay raised the temperature until much of the planet melted. While it was molten, material separated by density (differentiation): dense iron and nickel sank to form the core, and less dense silicates rose to form the mantle and, later, a thin crust.

A giant collision early in Earth’s history is widely accepted as the origin of the Moon and added more heat.

The early atmosphere formed mainly by outgassing: volcanoes released water vapour, carbon dioxide, nitrogen and sulfur gases from the interior, while the first, hydrogen-rich atmosphere was largely driven off by impacts and lost to space. As the surface and atmosphere cooled, water vapour condensed and fell as rain, filling basins to form the first hydrosphere; comets and water-rich asteroids may have added more water. Zircon chemistry suggests liquid water existed by about 4.4 Ga.

Free oxygen built up much later, once photosynthesising cyanobacteria were widespread; banded iron formations record oxygen reacting with dissolved iron, and a major rise in atmospheric oxygen, the Great Oxidation Event, began about 2.4 Ga. These changes show that Earth’s four systems are interrelated: the biosphere changed the atmosphere, which changed weathering of the geosphere and the chemistry of the hydrosphere.

Technology-based evidence for internal structure. No drill hole has passed through the crust, so the interior is inferred from indirect evidence. Seismometers record earthquake waves. P waves (compressional) travel through solids and liquids; S waves (shear) travel only through solids. Wave speed depends on the rigidity and density of the material, so sudden changes in velocity reveal boundaries. The Mohorovičić discontinuity (Moho) is a sharp velocity increase at the base of the crust.

At about 2900 km depth S waves stop and P waves slow abruptly (the Gutenberg discontinuity), showing that the outer core is liquid. This creates an S-wave shadow zone beyond about 103° from the earthquake and a P-wave shadow zone from about 103° to about 150° (sources give slightly different limits), caused by refraction at the core–mantle boundary.

Faint P waves inside the shadow zone led Inge Lehmann (1936) to propose a solid inner core, whose boundary is now placed at about 5150 km depth.

Meteorites give a second line of evidence: iron meteorites, made mostly of iron–nickel alloy, are interpreted as fragments of the metallic cores of shattered ancient asteroids, supporting an iron–nickel core, while stony chondrites, rich in the silicate minerals olivine and pyroxene with some iron–nickel metal, come from bodies that never melted or differentiated and so show the kind of raw material the planets formed from.

Earth’s average density (about 5.5 g/cm³) is roughly double that of crustal rock (2.7–3.0 g/cm³), so the interior must contain much denser material.

The lithosphere is defined by mechanical behaviour, not composition: it is the rigid crust plus uppermost mantle, typically about 100 km thick (thinnest beneath oceans and volcanically active regions, and about 150–200 km beneath much of the continental crust).

Below it the asthenosphere, extending to roughly 250 km (estimates of its base vary), is hot and weak enough to flow slowly; seismic waves slow down in it because it is close to its melting point and may be partially molten in places.

Evidence for Earth’s age. Zircon (ZrSiO₄) crystallises from magma and takes uranium into its structure but excludes lead, so nearly all lead in a zircon formed by radioactive decay. Zircon is hard and chemically resistant, so it survives weathering and recycling. Detrital zircons from the Jack Hills, Western Australia, give uranium–lead ages up to about 4.4 Ga, the oldest known Earth materials.

Because plate movement and erosion recycle the surface, no rocks survive from Earth’s formation, so meteorites are used instead: they formed with the solar system and have not been reprocessed. Lead isotope ratios in meteorites (used by Clair Patterson, who published an age of 4.55 billion years in 1956) and radiometric ages of more than 70 meteorites, which lie between 4.53 and 4.58 Ga, date the formation of the solar system.

These independent lines of evidence agree on an Earth age of about 4.54 billion years.