SCNC1112 Chap.6 Planet Earth: The Geosphere and Deep Time
Planet Earth: The Geosphere and Deep Time
Module 2 opens with a methodological claim rather than a fact: we can learn about things we cannot touch. Nobody has sampled the mantle, let alone the core, and the structure of the Earth's interior is nevertheless known in detail. Understanding how that is possible matters more than memorising the layers. Earthquakes supply the light source.
A quake sends pressure waves, which push and pull along their direction of travel and pass through both solids and liquids, and shear waves, which move material sideways and cannot pass through a liquid at all. Seismometers worldwide therefore record a shadow zone where no shear waves arrive, and the geometry of that shadow fixes the depth of a liquid outer core. Changes in wave speed at other depths mark further boundaries.
Two independent constraints support the picture: the mean density of the planet, obtained from its mass and volume, is far higher than that of surface rocks, and meteorites formed from the same early material contain iron and nickel alloys of about the right density.
Heat left from formation and heat released by radioactive decay keep the mantle slowly convecting, and that convection drags the rigid plates of the outer shell across the planet. This is why earthquakes, volcanoes and mountain belts are concentrated along lines rather than scattered: a map of earthquake locations is already a map of plate boundaries. The second half of the chapter is about time.
Unstable nuclei decay at a rate set by the nucleus itself, unaffected by temperature, pressure or chemistry, which is what makes them usable as clocks buried inside minerals. After one half-life half the original atoms remain, after two a quarter, after three an eighth. Carbon-14, with a half-life near 5,700 years, dates material that was once alive and runs out of usable signal beyond roughly fifty thousand years.
Uranium-lead, with a half-life measured in billions of years, dates the crystallisation of minerals and is the basis of the ages quoted for the oldest rocks and for the solar system. Two assumptions sit behind every such date and a good answer names them: the decay rate must have been constant, and the starting composition must be known or recoverable.
Long before any of this was measurable, geologists could still order events. In undisturbed layers the lower one is older, and anything cutting across layers is younger than what it cuts. Sequence comes from geometry and a numerical age then pins that sequence to a calendar.
What this chapter covers
- 01
Seismic Waves and the Shadow Zone
- 02
Density Evidence From Mass and Meteorites
- 03
Mantle Convection and Plate Boundaries
- 04
Half-Lives and the Decay Law
- 05
Choosing a Clock for the Age
- 06
Superposition and Cross-Cutting Relations
Dating a hearth, and saying what the date belongs to
- 2Convert the surviving fraction into half-lives. 12.5 per cent is one eighth, and one eighth is one half raised to the third power, so three half-lives have elapsed.
- 2Multiply by the half-life. Three times about 5,700 years gives roughly 17,000 years.
- 2Name the dated event. The clock started when the tree stopped taking carbon from the atmosphere, so the date is the death of the wood, not the lighting of the fire and certainly not the occupation of the site.
- 1Give one way it could mislead. If the wood was old when burned, from a long-dead trunk or reused timber, the fire is younger than the date by the age of the wood at burning.
Key terms
- Shear Wave
- A seismic wave that displaces material sideways and cannot travel through a liquid, which is how a liquid outer core was detected.
- Half-Life
- The time for half the atoms of a radioactive isotope to decay, fixed by the nucleus and unaffected by temperature, pressure or chemistry.
- Radiometric Dating
- Estimating an age from the proportion of a radioactive isotope remaining, given a known decay rate and a recoverable starting composition.
- Superposition
- The principle that in an undisturbed sequence of layers the lower layer was deposited before the one above it.
- Mantle Convection
- Slow circulation of hot mantle material driven by internal heat, which drags the rigid surface plates and concentrates earthquakes along their boundaries.
Planet Earth: The Geosphere and Deep Time FAQ
How can anyone know what the core is made of without a sample?
By combining independent measurements that would each have to be wrong in the same direction to mislead. Shear waves fail to pass a liquid layer, so the shadow they leave fixes a boundary depth. The planet's mean density is far higher than surface rock, so something dense sits inside. Meteorites from the same early material contain iron and nickel of about the right density.
Why is carbon dating not used on rocks?
For two independent reasons. Rocks were never alive, so they never took carbon from the atmosphere and there is no starting point for the clock. And a half-life near 5,700 years is exhausted after roughly fifty thousand years, which is nothing on a geological scale, so a system with a half-life measured in billions of years is used instead.
Does a radiometric date prove when something happened?
It dates one specific event, usually the death of an organism or the crystallisation of a mineral. Whether that event is the one you care about is a separate argument, and stating the gap between the two is part of a complete answer rather than an optional caution.
Exam move
Practise the fraction-to-half-lives conversion until it is automatic, then rehearse naming the dated event separately from the event of interest for three different scenarios. For the interior, be able to state which observation would fail if the outer core were solid.
