Dating geological events

What AS 91412 asks of you

• AS 91412 is a 4-credit internal. You investigate one geological event or a sequence of events, often a New Zealand one such as the Taupō eruption, Alpine Fault ruptures or the formation of a local landscape, and how they have been dated.
• Achieved: explain the event, and explain a range of evidence used to date it.
• Merit: explain in detail how each key piece of evidence helps us know when the event happened.
• Excellence: justify why each key piece of evidence is trustworthy and useful, and explain how matching up different kinds of evidence (cross-correlation) strengthens the dates.

Relative dating: putting events in order

• Relative dating puts events in order without giving an age in years.
• Superposition: in undisturbed layers, each layer is younger than the one below it. Original horizontality: sediments settle in flat layers, so tilted or folded layers were disturbed later.
• Cross-cutting relationships: a dike, fault or erosion surface is younger than every layer it cuts. Inclusions: pieces of one rock inside another are older than the rock that holds them.
• An unconformity is a buried erosion surface: a gap in the record where rock was removed or none was laid down, so time is missing between the layers above and below it.

Try it: which principle?

Fossils and faunal succession

• Species appear, live for a time, and go extinct, always in the same order, so the fossils in a layer tell you its place in the sequence (faunal succession).
• The best index fossils lived for a short time but were widespread and common, so finding one dates a layer to a narrow window anywhere it occurs.
• New Zealand's geological timescale is divided into local stages defined by fossils, many of them foraminifera (tiny shelled plankton), which let geologists match marine layers from Northland to Southland.
• Fossils give relative ages; the ages in years come from radiometric dates on volcanic layers above and below the fossil-bearing beds.

Radiometric dating and half-life

• A radioactive parent isotope decays to a stable daughter at a fixed rate; which atom decays next is random, but half of any large sample decays every half-life, whatever the temperature or pressure.
• The clock starts when a mineral crystallises from magma and traps the parent but not the daughter, or when an organism dies and stops taking in carbon-14.
• Choose an isotope whose half-life suits the age: carbon-14 for the last 50 000 years or so, potassium-argon or argon-argon for volcanic rocks thousands to billions of years old, uranium-lead in zircon crystals for the oldest rocks.
• The age comes from N = N₀ × (½)^(t ÷ T½), so t = T½ × log₂(N₀ ÷ N), or with a daughter-to-parent ratio D ÷ P, t = T½ × log₂(1 + D ÷ P).
• Most sedimentary rocks cannot be dated directly, because their grains crystallised long before they were deposited; instead, date volcanic layers above and below them.

Try it: radioactive decay

Tephra layers: New Zealand's time markers

• Tephra is the ash and pumice blasted out by an eruption. It falls over a huge area within days, so one tephra layer marks the same moment in time everywhere it is found: an isochron.
• Each eruption's glass has its own chemistry and its crystals their own minerals, so a tephra found in a lake core, a peat bog or a road cutting can be matched to its eruption by analysing it.
• Once one tephra is dated (by radiocarbon on charcoal or plant material, or by counting tree rings), that age applies to every site where the layer is found.
• The central North Island volcanoes have spread dozens of dated tephras over the North Island, so they are used to date lake sediments, archaeological sites and fault movements.

Cross-correlation: one timeline from many methods

• Each method has weaknesses: relative dating gives order but no years, radiocarbon needs calibration and only reaches about 50 000 years, and a reworked tephra or contaminated sample can give a false age.
• Confidence comes from independent methods agreeing. The Taupō eruption is dated by radiocarbon on charred trees, by matching tree-ring patterns in buried logs (wiggle-matching, giving 232 ± 5 CE), and by the eruption's place in the tephra sequence.
• Other records can join the timeline: sediment and ice cores, tree rings, stable oxygen isotopes, magnetism locked into lava and the rate of weathering and erosion.
• For Excellence, show how one line of evidence fixes a weakness in another: the tephra links distant sites, radiocarbon gives years, and superposition proves the order.

What Achieved, Merit and Excellence look like

• The same evidence, the Taupō tephra, written about at each grade.