Atlantic Ocean

Long ago, the Atlantic was only a thin strip of water.

The floor of the ocean slowly spreads apart in the middle. So the Atlantic gets a tiny bit wider every year.

Down the middle of the Atlantic runs the Mid-Atlantic Ridge, an underwater mountain range. Along the ridge, two plates of Earth's crust are moving apart, and melted rock rises from below to make new ocean floor.

The ridge spreads about 2.5 centimeters a year. That sounds slow, but over 100 to 200 million years it has turned a tiny inlet between Europe, Africa and the Americas into the wide ocean of today. Iceland sits right on top of the ridge.

The Mid-Atlantic Ridge is a divergent plate boundary: plates move apart and new oceanic crust forms as magma rises from the mantle. USGS describes it as a submerged mountain range extending from the Arctic Ocean to beyond the southern tip of Africa, one segment of the global mid-ocean ridge system.

Spreading averages about 2.5 cm/yr, or 25 km in a million years. Over the past 100 to 200 million years it has widened the Atlantic from a tiny inlet between Europe, Africa and the Americas into the present ocean. Iceland straddles the ridge, splitting along the spreading center between the North American and Eurasian Plates as North America moves west relative to Eurasia, which lets scientists study ridge processes on land.

Some water in the Atlantic moves like a giant river.

It is called the Gulf Stream. It carries warm water up from the south.

The Gulf Stream is a strong, warm current in the western North Atlantic. It flows north along the coast of Florida, then turns east off North Carolina and heads northeast across the ocean.

Its average speed is 6.4 kilometers an hour, and near the surface it can reach about 9 kilometers an hour. NOAA says the Gulf Stream carries more water than all the world's rivers put together.

The Gulf Stream is an intense, warm ocean current in the western North Atlantic, described by NOAA as the main conveyor of heat from south to north in the Atlantic. It runs north along Florida, turns east off North Carolina and flows northeast across the ocean.

Its velocity is greatest near the surface, typically peaking near 9 km/h; the average is 6.4 km/h, slowing to about 1.6 km/h as it widens to the north. It transports more water than all the world's rivers combined. The Gulf Stream also forms the western edge of the Sargasso Sea, which is bounded on its other sides by the North Atlantic Current, the Canary Current and the North Atlantic Equatorial Current.

In the middle of the Atlantic, brown seaweed floats on the water.

Baby turtles hide in it. Eels lay their eggs there, and whales swim through.

The Sargasso Sea is a patch of the Atlantic with no coast at all: ocean currents form its edges instead of land. It is named after sargassum, a brown seaweed that floats freely and even grows and spreads while floating on the open ocean.

The floating mats are a home for many animals. Young sea turtles find food and shelter there, and shrimp, crabs and fish live among the weed. Threatened eels lay their eggs in the Sargasso Sea, and humpback whales pass through every year.

NOAA calls the Sargasso Sea the only sea without a land boundary: it lies within the North Atlantic Subtropical Gyre and is defined by currents, so its borders shift, roughly following the Azores High. Its sargassum is holopelagic, floating freely and reproducing vegetatively on the high seas, while other seaweeds begin life on the seafloor.

The mats form a highly productive floating ecosystem: nurseries for turtle hatchlings and habitat for shrimp, crabs and fish adapted to the algae. The Sargasso Sea is a spawning site for threatened and endangered eels, white marlin, porbeagle sharks and dolphinfish; humpback whales migrate through it each year, and tuna and birds depend on it for food on their migrations.

The beach is fun, but the sea is strong.

Only swim where there are lifeguards. Always swim with a grown-up. Never turn your back on the waves.

Before swimming at an Atlantic beach, check the beach forecast and talk to the lifeguard. The US National Weather Service says to swim only at beaches with lifeguards and never to turn your back on the ocean.

Rip currents are powerful, narrow channels of fast-moving water, and they often form on calm, sunny days. If one pulls you, don't fight it: swim parallel to the shore, then back to land at an angle.

The US National Weather Service advises checking the local beach forecast, talking to lifeguards, swimming only at beaches with lifeguards and never turning your back on the ocean; good beach weather does not always mean safe swimming, because rip currents often form on calm, sunny days.

NOAA's National Ocean Service describes rip currents as powerful, narrow channels of fast-moving water, common along the US East, Gulf and West coasts and the Great Lakes, moving at up to 8 feet (about 2.4 m) per second, faster than an Olympic swimmer. Swimmers who try to swim straight back against one tire; the advice is not to fight it, but to swim parallel to the shore and return at an angle.