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Glossary

The words, in plain English

36 terms used on this site, each in one sentence.

A

Asthenosphere
The soft, slowly flowing part of the upper mantle that the plates slide over. More

B

Basalt
A dark volcanic rock with about half its weight in silica. As magma it is hot and runny. More
Bridgmanite
The mineral that makes up most of the lower mantle, and so most of the Earth by volume. More

C

Core
The iron and nickel centre of the Earth: a liquid outer core and a solid inner core. More
Crust
The thin rocky skin of the Earth: about 30 km thick under continents and 5 to 10 km under oceans. More
Cryogenian
The period from about 720 to 635 million years ago, named for its global ice ages. More

D

Deep time
The idea of geological time: thousands of millions of years, far beyond human history. More

E

Eon
The largest unit of geological time. There are four: Hadean, Archean, Proterozoic and Phanerozoic. More
Epicentre
The point on the surface directly above where an earthquake starts. More
Era
A division of an eon. The Phanerozoic has three: Paleozoic, Mesozoic and Cenozoic. More

F

Fault
A crack in rock where the two sides have moved past each other. More
Focus (hypocentre)
The point underground where an earthquake starts. More

G

Gneiss
A banded rock made when older rock is squeezed and heated deep underground. Say it "nice". More

H

Half-life
The time it takes for half of a batch of radioactive atoms to decay. More
Holocene
The epoch we live in, since the end of the last ice age 11,700 years ago. More

I

International Chronostratigraphic Chart
The official chart of geological time, kept by the International Commission on Stratigraphy. More
Isotope
A version of an element with a different number of neutrons. Some isotopes are unstable and decay. More

L

Lava
Magma that has reached the surface. More
Lithosphere
The rigid outer shell, crust plus the top of the mantle, broken into plates. More

M

Magma
Molten rock underground, usually with crystals and dissolved gas in it. More
Magnitude
A measure of the energy an earthquake releases. Each whole step is about 32 times more energy. More
Mantle
The thick layer of hot, solid rock between the crust and the core. It flows very slowly. More
Moho
The boundary between the crust and the mantle, named after Andrija Mohorovičić. More

P

P-wave
The fastest earthquake wave. It squeezes and stretches rock and can pass through liquid. More
Period
A division of an era, such as the Jurassic. Periods are the names most people know. More
PREM
Preliminary Reference Earth Model (1981): the standard table of density and pressure at each depth. More

R

Radiometric dating
Finding a rock’s age from how much of a radioactive element has decayed inside it. More
Rhyolite
A pale volcanic rock with a lot of silica. As magma it is cooler, sticky and often explosive. More

S

S-wave
A slower earthquake wave that shakes rock side to side. It cannot pass through liquid. More
Shadow zone
The band of the Earth, about 104° to 140° from an earthquake, where no direct P-waves arrive. More
Silica
Silicon dioxide, the main ingredient of most rocks. More silica makes magma stickier. More
Subduction
When one plate sinks under another into the mantle. More
Suture
The line where two continents have joined after the ocean between them closed. More

T

Tectonic plate
One of a dozen or so large slabs of lithosphere that move slowly over the mantle. More

V

VEI
Volcanic Explosivity Index: a scale from 0 to 8 for how much an eruption throws out. More

Z

Zircon
A tough mineral that takes in uranium but not lead as it forms, which makes it an ideal clock. More