The Ocean Floor and Coastlines

We have better maps of Mars and the Moon than of our own ocean floor. Light can't reach the deep sea, so we can't just take pictures. Instead, scientists use sonar: a ship sends a sound pulse down, it bounces off the bottom, and the time it takes to return tells the depth. Sound travels about 1,500 meters per second in water. If an echo takes 4 seconds to return, the sound went down and back, so the depth is about 3,000 meters.

The science of measuring ocean depth is called bathymetry. Today, scientists also use satellites, robot subs, and deep-diving submersibles like Alvin.

Walk off a beach and keep going, and here's what you'd find:

• Continental shelf: the shallow, gently sloping edge of the continent, underwater. Usually less than 200 meters deep. It's packed with sea life and is where most fishing and offshore oil drilling happens.
• Continental slope: where the shelf drops off steeply toward the deep ocean. Think of it as the real edge of the continent.
• Continental rise: a gentle pile of sediment at the bottom of the slope.

Cutting through the shelf and slope are submarine canyons, some as big as the Grand Canyon.

• Abyssal plains: huge, flat areas of the deep sea floor covered in thick, fine sediment. Some of the flattest places on Earth.
• Mid-ocean ridges: the longest mountain chain on Earth, about 65,000 km (40,000 miles), running through every ocean. New crust forms here (remember sea-floor spreading?).
• Trenches: deep, narrow valleys where one plate subducts under another. The Mariana Trench reaches almost 11,000 meters. If you dropped Mount Everest in it, its peak would still be more than 2 km underwater.
• Seamounts: underwater volcanoes that don't reach the surface. Flat-topped ones, worn down by waves long ago and then sunk, are called guyots.
• Hydrothermal vents: hot springs on the sea floor near ridges, blasting out mineral-rich water as hot as 400 °C. Amazingly, they support whole ecosystems of giant tube worms, crabs, and bacteria that live without sunlight, getting energy from chemicals instead.

The ocean floor is covered in sediment, from sand and mud washed off the land to the shells of tiny dead sea creatures. These shells pile up into thick layers of ooze. Over millions of years, some become rocks like limestone and chalk. The famous White Cliffs of Dover in England are made of ancient chalk ooze.

Most ocean waves are made by wind blowing across the water. Here's the surprising part: the water itself doesn't travel with the wave. Energy moves forward, but the water mostly moves in circles, like a crowd doing "the wave" at a football game.

Wave words:

• Crest: the top. Trough: the bottom.
• Wavelength: distance from one crest to the next.
• Wave height: distance from trough to crest.

As a wave reaches shallow water, the bottom drags, the wave slows and gets taller, and it tips over as a breaker. That's where surfers want to be.

Tides are the daily rise and fall of sea level, caused mainly by the Moon's gravity pulling on Earth's oceans (the Sun helps a little). Most coasts get two high tides and two low tides each day.

• Spring tides: extra-high highs and extra-low lows, when the Sun, Moon, and Earth line up (at new and full moon). Nothing to do with the season spring.
• Neap tides: smaller tides, when the Sun and Moon are at right angles (at quarter moons).

Waves are constantly carving and building coastlines.

Erosion features: pounding waves cut sea cliffs, carve sea caves, punch through to make sea arches, and leave behind tall pillars of rock called sea stacks when arches collapse. The Oregon coast has amazing examples.

Deposition features: waves usually hit the beach at an angle, which moves sand along the shore in a zigzag. This longshore current builds:

• Beaches: strips of sand and pebbles. Beach sand is constantly on the move.
• Spits: sandbars that stick out from the shore like a finger.
• Barrier islands: long, narrow sandy islands parallel to the coast that protect the mainland from storms.

People try to protect beaches with jetties, groins, and seawalls. But blocking sand in one spot often causes erosion somewhere else down the coast.