Ocean Water Ocean Floors and Ocean Currents Earth

Ocean Water, Ocean Floors, and Ocean Currents Earth Science 11

World Oceans • Cover more than 70% of Earth’s surface • In order of size: – Pacific Ocean is the largest – Atlantic Ocean is the 2 nd largest – Indian Ocean is the 3 rd largest – Arctic Ocean is the smallest

Salinity • Is a measure of the dissolved solids in sea water • In 1 000 g of sea water: – 965 g of water – 35 g of salts • In polar regions freezing of fresh water can increase ocean salinity. • In hotter areas, evaporation of water can increase ocean salinity.

Ocean Layers • Water has a high specific heat capacity. This means that oceans require huge amounts of sunlight to heat their surface layers by only a few degrees. • Most of ocean water is below 5°C even at the equator! • Only the top km of water is, on average, slightly warmer.

Ocean Layers • A thermocline exists as temperature rapidly drops from the bottom of the surface layer to the deep water zone. • The surface layer is also called the mixed layer because winds and waves stir it up.

Studying the Ocean Floor

Studying the Ocean Floor • Is largely done using echo sounders – Sound waves are sent downward – They are reflected up from the ocean floor – Knowing the speed of sound in water ( 1 500 m / s) allows us to calculate depth

A profile of the ocean floor can then be made.

As a result: • Three major areas are recognized: – Continental margins – Deep-ocean basins (includes the abyssal plains) – mid-ocean ridges

Not in notes. A profile from New England to the coast of North Africa.

Continental Margins • Consist of: – The continental shelf, – The continental slope, and – The continental rise.

Continental Shelf • Is a gently sloping, submerged extension of the continents • Can be almost nonexistent or can extend outward as far as 1 500 km • On average it is 80 km wide and 130 m deep (seaward edge). • It drops about 2 m every km.

Continental Slope • Marks the boundary between continental crust and oceanic crust. • It is steep and relatively narrow (20 km).

Continental Rise: • Drops between 4 to 8 m per km. • Can be hundreds of km wide. • Has a thick accumulation of sediment (moved from the continental shelf). • Contains the occasional submarine canyon or submarine volcano.

Passive Continental Margins • Such as along the Atlantic Ocean: – Are continuously being built from the accumulation of eroded continental sediment

Active Continental Margins: • Such as along the Pacific Ocean: – Have very little continental shelf. – Have continental slopes that merge with deep ocean trenches. – Result from the subduction of one plate underneath another.

Active and Passive Continental Margins

Formation of New Continental Margins Sediment the erosion of continental inland, As Upwelling the twofrom land of magma masses causes separate, cooling causes continental crust the growth of the subsidence crust to fracture. of the causes continental margins. continental shelf. Not in notes.

Submarine Canyons and Turbidity Currents

Submarine Canyons • Are deep, steep-sided valleys that begin on the continental slope. • Can be as deep as 3 km. • Are most likely formed by turbidity currents.

Turbidity Currents: • Are down-slope movements of dense, sediment-laden water that gouge the sea floor. • Are created when sediment on the continental shelf is dislodged (earthquake) and thrown into suspension. • As it flows downward it gains in sediment and speed.

In 1929 an earthquake off the coast of Newfoundland occurred. A turbidity current formed and, as a result, telephone cables broke. Turbidity current produced in a lab tank. The times are too long after the earthquake to have been caused by the seismic waves.

Turbidity Currents: • Usually originate along the continental slope. • Continue across the continental rise still cutting channels. • As the slope lessens, their speed lessens and sediment begins to settle out coarsest first and finest last (graded bedding). These deposits are called turbidites.

Not in notes.

Features of the Deep Ocean Basins

Deep Ocean Basin • Lies between the continental margin and the mid-ocean ridge. • Located here are: – Ocean Trenches – Abyssal plains – Seamounts

Ocean Trenches: • • • Are long and narrow. Form the deepest part of the oceans. Some exceed 10 km in depth! Are located at subduction boundaries. Trap sediment from turbidity currents The continental slope merges with a trench. A very narrow continental shelf is formed. Why?

Ocean Trenches: • In the open ocean are paralleled by volcanic island arcs.

Ocean Trenches: • Near the land are bordered by volcanic mountains.

Abyssal Plains • Are incredibly flat! • Have accumulated lots of sediment from turbidity currents which has buried its rugged features.

Seamounts • Are submerged, steep-sided volcanoes • Form near mid-ocean ridges or above hot spots. • Can sometimes emerge as islands. • Can have coral reefs.

Coral Reefs • Form in the warm, clear, sunlit waters of the Pacific and Indian Oceans • Since light cannot penetrate very deep, reefs only descend about 45 m • Sudden temperature changes or exposure to cold water can kill a reef. • Special reefs called atolls can form.

Coral Bleaching – this reef is dead.

Atolls: As the island gradually sinks, its reef grows upward. Eventually the island will be completely submerged and only the reef (called an atoll) will touch the surface. These reefs, because they are continually growing upward, can reach depths of thousands of metres!

Mid-ocean Ridges

Mid-ocean Ridges • Are like the seams on a baseball. • Can be 500 to 5 000 km wide! • Might occupy as much as one half of the total ocean floor. • Are divergent boundaries. • Become wider from sea-floor spreading.

• Are the longest topographic feature on Earth (65 000 km). • Rise 2 – 3 km above the ocean basin (but are still submerged under 2. 5 km of water!) • Are made of basalt.

Recall: Rocks & Magnetism • When rocks form in the presence of Earth’s magnetic field, they are influenced by the field. • In cooling lava or magma, the iron rich atoms are aligned with Earth’s field, and then frozen into the rock. Not in notes.

Sea Floor Spreading • As the rock cooled, it inherited the polarity of Earth’s magnetic field, and then the sea floor spread apart like a conveyor belt. Not in notes.

Mid-ocean Ridges • Are characterized by: • • Hydrothermal systems Deep sea vents (black smokers) Pillow basalts Living organisms !

Hydrothermal System Cold seawater enters the ridge, is heated, and exits as a plume of hot water.

Deep Sea Vents (Smokers)

Deep Sea Vents (Smokers) • The heated saltwater can dissolve • One of the minerals. gases released • is. When it gushes hydrogen from the sulphide. as • seafloor Some particle-filled organisms plumesby of using water survive it isgas called a this instead “black smoker”. of oxygen!

Pillow Basalts • When lava directly encounters water it is super-cooled into pillow basalt.

Pillow Basalt Trinity Bay, Newfoundland

Life at Deep Sea Vents Not in notes.

Ocean Layers • Four main zones: – Epipelagic (0 – 120 m) – Mesopelagic (120 – 1 200 m) – Bathypelagic (1 200 – 3000 m) – Abyssal ( 3 000 m to bottom or 6 000 m) Not in notes.

Obstacles to Life in the Deep Ocean • Pressure • Varying Oxygen Levels • Increasing Carbon Dioxide • Diminishing Light • Decreasing Plankton

Very Deep!

Very Deep!

Very, Very Deep!

Bottom

Bottom

Bottom

Ocean Currents

An Ocean Current: • Is a continuous flow of water along a broad path in the ocean • Flows at any depth or along the surface • Can move vertically or horizontally

World Surface Ocean Currents Notice that they turn clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

An Ocean Current: • That flows away from the equator carries warmer water • That flows toward the equator carries colder water • The direction a current circles is caused by Earth’s rotation

Two Types of Ocean Currents • Surface Currents – Upper 400 m – 10% of all ocean water • Deep Ocean Currents – 90% of all ocean water

Forces Creating Ocean Currents: • Solar Heating • Winds • Gravity • Rotation of Earth

Solar Heating: • Causes water to expand – Water is actually higher (8 cm) at the equator than anywhere else! – This creates a slight slope and water, under gravity’s influence, flows down slope

Winds Help Drive Surface Ocean Currents A mound of water, called a gyre, is created. Within a gyre large circular currents are produced. Not in notes.

Winds Help Drive Surface Ocean Currents Two Surface Seasonal setscurrents of changes winds are in caused are wind responsible: direction by wind. do not Friction • normally Trade winds atcause the ocean surface • changes Westerly causes inwinds current water to These direction. be dragged winds occur along. in belts Monsoons around arethe an world. exception to this rule.

Monsoons • Winds blow from one direction in summer and a different direction in winter • Changing wind direction changes current direction

Summer Monsoon • Air over land heats and rises drawing moist air in from tropical oceans

Winter Monsoon • Air over land cools and sinks drawing dry air in over the tropical oceans

Density Currents • Turbidity currents are an example – Because the water is laden with sediment, it is more dense and therefore sinks • Changes in density cause water to move vertically

Density Currents • These changes are caused by: – Salinity changes (higher salinity means denser water) • Evaporation increases salinity • Freezing increases salinity – Temperature changes • Cooler water is more dense

Ocean Layers • Because colder water is denser, water from the polar regions will sink and move beneath the other oceans creating deep ocean currents. • As it sinks, it carries with it the oxygen added to it in the surface layer from waves and plankton. – This is the only source of oxygen for deep ocean life!

Deep Water Masses • Form from variations in temperature and salinity. This causes layering of deep ocean water.

Upwelling • Occurs when cold, deep ocean water comes to the surface • Sometimes caused by winds along the coast. This is called coastal upwelling. • Sometimes caused by winds in the open ocean. • Sometimes caused by density currents.

Coastal Upwelling • Occurs when: – Winds blow parallel to the shore – Surface water is pushed away from the coast – Deep water rises up to replace water that has been moved out to sea

Coastal Upwelling Not in notes.

Equatorial Upwelling • Occurs as surface water spreads out causing sea level to fall. This causes the surface layer to thin. • Cold water moves up to thicken the reduced surface layer.

Upwelling is Important Because: • Water rising brings with it important plankton nutrients • These nutrients increase plankton numbers and thereby increase fish stocks.

Ocean Water Circulates • As a result of currents and upwelling, deep ocean water is recirculated around the globe every 1 000 years. • This means that contaminants released into the ocean in one spot can be found everywhere given enough time.