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Reading Selection, Lesson 5

Density Creates Currents

Illustration of a convection cell in a room.

A convection cell can occur in a room.

How do changes in density move matter? This movement involves a process called convection. To understand how convection works, imagine a room in a house, like the one shown in the picture above.

One side of the room has a heater; on the opposite wall is a window. On a cold winter day, when the heat is on, air near the heater will warm up. What happens to hot air? It expands, becomes less dense, and rises. On reaching the ceiling, it is pushed along by more hot air rising behind it. The heated air starts to cool down the farther it drifts from the heater, and this process is speeded up when it meets the cold window. As the air cools, it becomes more dense, sinks to the floor, and eventually completes a circuit of the room. A circular convection current is set up. Circular currents like this are called convection cells.

Convection currents like this also take place in the atmosphere (see the picture below).

Illustration of a convection cell in the atmosphere.

A convection cell can also occur in the atmosphere.

We encounter these convection currents as wind. Where do you think the heat energy for these convection currents comes from? Real winds are more complex than what is shown in the picture, but all winds are created by changes in density brought about by temperature differences. How do you think the winds shown in the picture would be different at night?

Convection works in liquids as well as in gases. Ocean currents have several different causes, many of which are due to changes in density. Some ocean currents are convection currents (see the picture below)


The Gulf Stream shown off the east coast of the United States, from space.

The Gulf Stream is an ocean current that is driven, in part, by convection. It carries warm water from the tropics toward the North Pole.

Under the tropical sun, water at the equator warms up. At the cold poles, seawater cools down and sinks. Convection cells are set up with warm water moving along the surface to the poles and deep cold water flowing toward the equator. Changes in density, caused by changes in salinity (the amount of salt in the water), are also important in the formation of ocean currents. Ice formation near the poles leaves salt behind in the remaining water. This denser, more saline water sinks, creating its own density-driven currents. Surface winds also set surface currents into motion.

Moving and Making Mountains
Convection currents can move or split whole continents. Radioactive
substances deep within the Earth provide the heat that drives these currents (see the picture below illustrating how these convection cells work).

Earth's surface is made up of a series of giant plates that fit together like a moving spherical jigsaw. These plates can be made from two types of crustal material: dense oceanic crust and comparatively less dense continental crust. The hot rocks deep in the mantle behave like a soft plastic. These warm, less dense rocks move up, pushing aside rock that lies on the surface. These convection currents create some of the mountain ridges found on the ocean bed. The Mid-Atlantic Ridge is one example. Sometimes these ridges emerge at the surface of the ocean as islands. As plates expand, they push against other plates. When plates that consist of two pieces of continent push against one another, they may buckle up along their boundaries to form great fold mountains (see the photo of Mount Everest). They may also slide past one another, as at the famous San Andreas fault in California.

Illustration of a convection cell within the Earth.

Convection cells exist within the earth.

Earthquakes can occur as the plates slide past one another or build mountains.If a plate of more dense oceanic crust pushes against less dense continental crust, what do you think happens? The more dense ocean crust sinks down to create ocean quakes that are caused by all this activity.

Why is density important? Changes in density drive many of Earth's processes. Next time you climb a mountain or hear about an earthquake or a tornado, think about how density and density changes have an impact on human lives!

Surtsey island being formed by a volcanic eruption.

Convection currents in the Earth produce volcanoes, like this one near Iceland in the North Atlantic.





Surtsey island.

Eventually this volcano formed an island called Surtsey.





View of the Earth's undersea mountain ranges from space

Surtsey is part of a mountain system, formed as a result of convection currents, that extends under the Atlantic Ocean.




Mt. Everest

Great ranges of fold mountains are formed where plates collide. Mount Everest, the highest mountain in the world, is in the Himalayas of Nepal. These mountains were formed when the plate carrying India collided with the plate carrying Asia.







Big birds such as vultures and hawks can often be seen gliding around and around over big parking lots on sunny days, without even flapping their wings. Why?

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