geology

Ocean water is salty primarily because rain and rivers slowly weather rocks on land, dissolving ions like sodium and chloride and carrying them to the sea. When seawater evaporates, water leaves but salts remain and accumulate over geological time. Hydrothermal vents and volcanic activity add dissolved minerals. The ocean sits near steady state, with salt being removed about as fast as it arrives.

Mars looks red because iron in its dust and surface rocks has oxidized into rust-colored minerals. A 2025 study argues the main pigment is ferrihydrite, a water-bearing iron mineral, and that planet-wide winds spread the fine powder that gives Mars its color.

No one has drilled to Earth's core. Scientists infer the interior from seismic waves, density, magnetism, meteorites, and high-pressure experiments.

Earth's age is not guessed from one rock. This guide explains radiometric dating, meteorites, zircon crystals, half-lives, and why 4.54 billion years is robust.

Volcano prediction uses earthquakes, gas, ground deformation, heat, and history. This guide explains what scientists can forecast and what remains uncertain.

The biggest mines on Earth expose how copper, iron, coal, gold, and diamonds reach the global economy, and why size can mean depth, output, or area.

Deep in the Vietnamese jungle is a cave so large it has its own weather. Hang Sơn Đoòng — the world's largest cave passage — contains a forest, a river, and a microclimate that forms clouds beneath its ceiling, which soars high enough to fit a 40-storey skyscraper. It was only fully explored in 2009. The largest caves on Earth are among the last great unmapped spaces on the planet, carved by nothing more than slightly acidic water and a great deal of time.

Mauna Loa in Hawaii rises 4,170 metres above the sea — but that's only the tip. Measured from its true base on the ocean floor, it stands roughly 17 kilometres tall, more than twice Everest's height above sea level, and is so heavy it has pushed the seabed downward beneath it. It may not even be the largest volcano on Earth by volume. And lurking beneath Yellowstone is something that isn't a mountain at all: a supervolcano whose eruptions are measured not in metres but in continents.

The Andes stretch 7,000 kilometres down the spine of South America — the longest mountain range on land, and a wall so high it creates deserts on one side and rainforests on the other. But it's not even close to the longest range on the planet. That title belongs to a mountain chain ten times longer, winding 65,000 kilometres around the globe entirely beneath the oceans, where most of Earth's volcanic activity quietly happens.

Venezuela's Angel Falls drops 979 meters — so far that much of the water turns to mist before it ever reaches the ground, especially in the dry season. It is the tallest waterfall on land, plunging off a flat-topped mountain built from some of the oldest rock on Earth. But the single tallest waterfall on the planet is not on land at all: it is underwater, a cataract beneath the ocean that dwarfs every waterfall you can see.

On 22 May 1960, the ground beneath southern Chile ruptured along a 1,000-kilometre fault, releasing a magnitude 9.5 earthquake — the most powerful ever recorded by instruments. It was so violent it shifted Earth's mass enough to measurably change the length of a day and set the entire planet ringing like a bell for weeks. Because the magnitude scale is logarithmic, this single quake released more energy than every other recorded earthquake of the 20th century combined.