By dating the rocks in Earth's ever-changing crust, as well as rocks from Earth's neighbors, such as the moon and visiting meteorites, scientists have calculated that our planet is a little over 4.5 billion years old.

Pangaea was a supercontinent that formed from earlier continental units approximately 335 million years ago and began to break apart about 175 million years ago. Pangaea was centered on the Equator and surrounded by the superocean Panthalassa.

Trilobites (meaning "three lobes") were among the most successful of all early animals, existing in Earth's oceans for almost 300 million years.

The Triassic period, from 252 million to 200 million years ago, saw the rise of reptiles and the first dinosaurs. The Jurassic period, from about 200 million to 145 million years ago, ushered in birds and mammals.

Oxygen makes up 46% of the Earth's crust, followed by silicon (28%), aluminium (8.3%), and iron (5.6%). On Earth and in rocky planets in general, silicon and oxygen are far more common than their cosmic abundance because they combine to form silicate minerals, while other cosmically-common elements such as hydrogen, carbon and nitrogen form volatile compounds such as ammonia and methane that easily boil away into space from the heat of planetary formation.

Most people don't realize that ruby and sapphire are both gems of the mineral corundum. Both of these gemstones have the same chemical composition and the same mineral structure. Trace amounts of impurities determine if a gem corundum will be a brilliant red ruby or a beautiful blue sapphire.

The first eon was the Hadean, starting with the formation of the Earth and lasting over 600 million years until the Archean, when the Earth had cooled enough for continents to form. This period was named after Hades, the Greek god of the underworld, because of the hellish conditions then prevailing on Earth: the planet had just formed and was still very hot owing to its recent accretion, the abundance of short-lived radioactive elements, and frequent collisions with other Solar System bodies.

California's sleeping giant, the San Andreas Fault, marks the slippery yet sticky boundary between two of Earth's tectonic plates. It is responsible for the biggest earthquakes in California, up to at least magnitude 8.1.

Since 1900, Alaska has had one magnitude 7 or 8 earthquake per year, 45 earthquakes of magnitude 5 or 6, and 10,000 quakes overall annually. In fact, Alaska has 11% of the world's earthquakes, and 3 of the six largest in recorded history were located there.

Pumice is the only rock that floats, but only because it "cheats" by having air pockets inside it. If water fills these air pockets, the rock will sink.