Overview

The Earth is a giant rock. But it’s not a static rock; it’s a chemical factory. Volcanoes spew gas, rain dissolves mountains, and oceans precipitate salt. Geochemistry tracks the movement of elements through the planet. It tells us how the Earth formed, how it works, and how to find gold.

Core Idea

The core idea is Elemental Cycling. Atoms don’t disappear; they just move around. A carbon atom might be in a volcano today, in a tree tomorrow, and in a diamond a million years from now.

Formal Definition

The study of the chemical composition of the Earth and other planets, and the chemical processes and reactions that govern the composition of rocks and soils.

Intuition

  • Geology: “Here is a mountain.”
  • Geochemistry: “This mountain is made of Calcium Carbonate. It used to be seashells in an ocean 100 million years ago. Acid rain is dissolving it and sending the Calcium back to the sea.”

Examples

  • The Carbon Cycle: The thermostat of the planet. Volcanoes emit CO2 (warming). Rocks weather and absorb CO2 (cooling). Life (plants/plankton) buries CO2 as coal/oil. We are currently digging up that buried coal and burning it, breaking the cycle.
  • Gold Veins: Gold doesn’t like to mix with other elements. When magma cools, the gold stays liquid until the very end, getting squeezed into cracks with hot water (hydrothermal fluids). That’s why gold is found in quartz veins.
  • Age of the Earth: We measure the ratio of Uranium to Lead in zircon crystals. Since we know the decay rate (half-life), we can calculate the age of the rock. (4.5 billion years).

Common Misconceptions

  • Rare Earth Elements: They aren’t actually rare. Cerium is more common than Copper. They are just “dispersed”—hard to find in concentrated deposits that are profitable to mine.
  • Isotope Geochemistry: Using “chemical fingerprints” (isotopes of Oxygen) to figure out the temperature of the ocean 100 million years ago (Paleoclimate).
  • Cosmochemistry: Analyzing meteorites to see what the solar system was made of before Earth formed.

Applications

  • Mining: Finding ore deposits.
  • Environmental Science: Tracking pollution. If we find lead in the water, isotope analysis can tell us if it came from a factory or natural rocks.

Criticism / Limitations

  • Deep Earth: We can’t drill to the core. We have to guess what’s down there based on meteorites and earthquake waves.

Further Reading

  • White, William. Geochemistry.
  • McSween, Harry. Geochemistry: Pathways and Processes.