Topic #4 The Oceans

Topic #4 – The Oceans No matter where you live, you cannot escape the influence of the ocean. Oceans affect the weather, provide us with food and natural resources (oil, gas, minerals) and allow goods to be easily transported from other continents. Oceans cover 2/3 of the Earth’s surface

Salt Water On average, 1000g of seawater contains 35g of dissolved salts. The most common salt is sodium chloride, but there are other salts like sulphates, magnesium, calcium and potassium The measure of the amount of salts dissolved in a liquid is known as salinity.

Fraser River dumping into the Pacific

The oceans form the largest ecosystem on Earth The oceans form the largest ecosystem on Earth. Visible light does not penetrate beyond a depth of about 100m. On the ocean floor there are mountain ranges, steep valleys and vast plains – everything larger than on land.

Most features on the ocean floor are attributed to plate tectonics. Long undersea mountain chains called ocean ridges run along the centre of the oceans. These ridges are the youngest areas of sea floor and are still being formed by volcanic eruptions. Molten lava flows out, hardens quickly and pushed the tectonic plates apart

Along some margins of the sea floor, narrow, steep-sided canyons called trenches are formed when an oceanic plate pushed against the edge of a continental plate. The deepest trench, called the Marianas Trench, extends 11km below sea level. This is deep enough to submerge an object as tall as Mount Everest

Between the high mountain ranges at their centre and the deep trenches at their edges, you will find wide, open features called abyssal plains.

Ocean basins do not begin at the coastline, but may begin kms out to sea The area between the coast and the edge of the basin is called the continental shelf. From the edge of the shelf, the continental slope plunges to the sea floor.

Ocean Waves Ocean waves are just large ripples, set in motion by steady winds. Waves begin out in open sea and their height depends on how fast, how long and how far the winds blow over the water. Normal winds produce waves of 2-5m in height. Hurricane winds can create 30m waves. Even on a calm day there are smooth waves hitting the shore – called swells

Ocean Waves (cont.) When a wave reaches shore, it changes shape. As the trough touches the beach, it is slowed by friction, but the crest continues to move at the same speed. The wavelength shortens and the wave height increases causing the crest to outrun the trough and topple forward The ensuing tumble of water onshore is called a breaker.

Tides Ocean beaches are sometimes covered with water and sometimes not. They are uncovered and covered in regular daily cycles by the slow rise and fall of the ocean, called tides. Centuries ago, people realized that the cycle of tidal movement is caused by the Moon.

Tides (cont.) The largest tidal movements are called spring tides and occur when the Earth, Moon and Sun are in a line.

Tides (cont.) The smallest tidal movements are called neap tides and occur when the Sun and Moon are at right angles to each other. The difference between these two is called the tidal range.

Tides (cont.) The movement of water causes a low tide along the opposite coastline. As the Earth turns on its axis, different location on Earth’s surface face the moon and are affected by its gravitational pull.

Surface Currents Currents of water at the ocean surface are driven by winds. Most surface currents flow in the top 100-200m of water.

Surface Currents (cont.) Three factors influence the direction of winds and surface currents: uneven heating of the atmosphere, rotation of the Earth, and the continents. Warm air expands and moves upward Cool air moves into the area created when the warm air moves upward. The moving masses of warm and cool air produce winds.

Surface Currents (cont.) The rotation of the Earth produces a bending of moving currents. As the wind and water currents flow over the Earth’s surface, the planet turns beneath them from west to east. The overall result is that winds along the equator blow from the east, called trade winds. They push the ocean currents towards the west

Surface Currents (cont.) Toward the polar regions, westerly winds drive currents the opposite way, from west to east.