EVSC10001 · The Global Environment
Plate Tectonics & Geological Time
This chapter is the solid-Earth core of The Global Environment: how Earth's rigid lithosphere is broken into drifting plates whose three boundary types build almost every earthquake, volcano and mountain belt, and how geologists read deep time by ordering rocks (relative dating) and putting numbers on them (radiometric dating). It matters because plate boundaries and dating are recurrent Part-A short-answer topics, and every Part-A answer is scored on a clearly labelled diagram — so mastering the boundary cross-sections, the stratigraphic principles and the half-life clock is high-yield exam currency.
What this chapter covers
- 01Continental drift (Wegener) and its evidence
- 02Lithosphere vs asthenosphere
- 03The three plate boundaries: divergent, convergent, transform
- 04Sea-floor spreading evidence (magnetic stripes, Wadati-Benioff zone, crust age)
- 05Driving forces: slab pull and ridge push
- 06Relative dating: the stratigraphic principles
- 07Absolute dating: radioactive decay and half-lives
- 08The geological time scale (Eon > Era > Period > Epoch)
Part-A short-answer: name and explain a convergent boundary with a labelled diagram
- +1Identify the boundary type from the diagnostic clues: a deep-sea trench plus earthquakes that deepen inland (a Wadati-Benioff zone) plus a parallel volcanic chain means an oceanic plate is sinking beneath a continental plate. State it: convergent (ocean-continent subduction) boundary.
- +1Draw and label the cross-section. From left to right show: ocean - trench (where the plates meet) - dense oceanic plate bending DOWN beneath the lighter continental plate - the subducting slab descending into the asthenosphere - a volcanic arc on the overriding continent. This labelled sketch is the mark-bearing core of every Part-A answer.
- +1Mark the earthquake foci as dots lying ALONG the down-going slab, getting deeper away from the trench. Annotate this line as the Wadati-Benioff zone - it directly explains the deepening earthquake band in the question.
- +1Explain the volcanism: as the slab descends it releases water into the overlying mantle wedge, lowering its melting point (flux melting); the melt rises to feed the volcanic arc above. This links your diagram to the chain of volcanoes.
- +1Close with the conservation statement: at a convergent boundary lithosphere is DESTROYED (consumed by subduction), the counterpart to crust being CREATED at divergent ridges - so globally surface area stays constant.
Key terms
- Lithosphere
- Earth's cool, rigid, brittle outer shell - the crust plus the uppermost (rigid) mantle - that is broken into the moving plates. It is defined by mechanical strength, not chemical composition.
- Asthenosphere
- The hotter, weak, slowly-flowing layer of the upper mantle beneath the lithosphere, over which the rigid plates ride and move.
- Sea-floor spreading
- The creation of new oceanic crust at a divergent mid-ocean ridge, where decompression melting of rising asthenosphere makes basalt that records symmetric magnetic-polarity stripes as the plates move apart.
- Wadati-Benioff zone
- The inclined band of earthquake foci that deepens along a subducting slab at a convergent boundary, tracing the descent of oceanic lithosphere into the mantle.
- Half-life (t-half)
- The constant time required for half of the remaining radioactive parent atoms in a sample to decay to the stable daughter; after n half-lives the parent fraction is (1/2)^n, the basis of radiometric (absolute) dating.
- Superposition
- The stratigraphic principle that, in an undisturbed sequence of sedimentary layers, each bed is younger than the one beneath it and older than the one above - a foundation of relative dating.
Plate Tectonics & Geological Time FAQ
What are the three types of plate boundary and what does each one do to the crust?
Divergent boundaries (mid-ocean ridges) pull apart and CREATE new crust; convergent boundaries (subduction zones and collisions) come together and DESTROY crust; transform boundaries slide past sideways and CONSERVE crust (none made or lost). Globally creation balances destruction, so Earth's surface area stays constant.
What is the difference between relative and absolute dating?
Relative dating ORDERS events as older or younger from rock geometry using stratigraphic principles (superposition, cross-cutting, inclusions, unconformities, faunal succession) - no numbers. Absolute dating attaches an AGE IN YEARS, almost always by measuring the radioactive parent-to-daughter ratio in a mineral (radiometric dating).
How do you calculate a radiometric age from a half-life?
Count how many half-lives have passed from the surviving parent fraction: if the sample retains (1/2)^n of its parent, then n half-lives have elapsed, and the age is t = n times the half-life. For example a parent fraction of 1/8 = (1/2)^3 means 3 half-lives.
What evidence supported continental drift and plate tectonics?
Continental lines: the jigsaw fit of continental shelves, matching geology and Permian glacial striae across reassembled Gondwana, and shared fossils (Glossopteris, Mesosaurus). Sea-floor lines: symmetric magnetic stripes about ridges, oceanic crust that ages away from the ridge, and the Wadati-Benioff earthquake zone. No single line is conclusive - their convergence is the proof.
What is the order of the geological time scale hierarchy?
From largest to smallest: Eon > Era > Period > Epoch. The four eons are Hadean, Archean, Proterozoic and Phanerozoic; the Phanerozoic divides into the Paleozoic, Mesozoic and Cenozoic eras. Key boundary ages to know are roughly 541 Ma (base of the Cambrian), 252 Ma (end-Permian extinction) and 66 Ma (K-Pg).
Exam move
Treat this chapter as a diagram-first topic, because Part A scores you on a labelled sketch for every answer. Build a small deck of three cross-sections you can reproduce in under two minutes each: a divergent ridge (with mirror magnetic stripes and youngest-crust-at-axis), an ocean-continent convergent zone (trench, subducting slab, Wadati-Benioff dots, volcanic arc), and a transform fault (offset ridge segments, shallow quakes). For each, memorise the one-line conservation verdict (made / destroyed / conserved). For deep time, drill two reflexes: (1) order a cross-section by chanting superposition then cross-cutting then unconformities, and (2) date a sample by writing parent fraction = (1/2)^n to read off n, then t = n times the half-life - never age from the daughter amount alone. Finally, rehearse the evidence-synthesis answer as a paired list (one continental line + one sea-floor line) so you can answer a continental-drift Part A with both a Gondwana sketch and a ridge sketch.
