Why is the Ocean Salty?

Activator: Show What You Know- Using the white board and the dry erase marker at your table, and starting with seat number 1, take turns answering the following questions. You may confer with the members of your group before writing your answer.

1. The_________ Ocean surrounds the North Pole, and borders northern Europe, Asia and North America. It is the smallest ocean.

2. The ___________ Ocean is bordered on the east by North and South America, on the west by Europe and Africa, on the north by Greenland and Iceland, and on the south by Antarctica. It is the second largest ocean.

3. The __________ Ocean, the third largest, lies between the west coasts of Indonesia and Australia and the east coast of Africa. To its north lies India and Pakistan, and to the south lies Antarctica.

4. The ________ Ocean is composed of the waters surrounding Antarctica. It includes the water south of 60 ̊S latitude. This fifth ocean was recognized by the International Hydrographic Organization in the spring of 2000.

5. The ___________ Ocean is the largest ocean. It is bordered on the east by North and South America, on the west by Asia and Australia, on the north by Alaska and Siberia, and on the south by Antarctica.

Now, stand up and do “the wave” if your group did an amazing job answering those questions!

You already know that water covers almost three-fourths (3/4) of the Earth’s surface. Why, do you suppose, astronauts refer to Earth as the “blue marble?”

You should also recall that most of Earth’s water, 97%, in fact, is saline (salty). The main substance dissolved in ocean water is sodium chloride, ordinary table salt.

By some estimates, if the salt in the ocean could be removed and spread evenly over the Earth's land surface it would form a layer more than 500 feet (166 meters) thick. That’s about the height of a 40-story office building!

But, where did all this salt come from?

Go, Ask, and Tell: Get up and go for a little walk. Ask two different students if they know the sources of salt in the oceans. Then come back to your group and tell what you’ve learned. You have five minutes to complete this task.

The answer is really very simple. Salt in the ocean comes from rocks on land, from under the ocean floor, and from underwater volcanoes.

Here’s how it works: The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes rainwater to be slightly acidic due to carbonic acid.

The rain physically erodes the rock and the acids chemically break down the rocks. The rainwater carries salts and minerals along in a dissolved state as ions. The ions in the runoff are carried to the streams and rivers and then to the ocean.

Satellite view of La Plata River discharge to the Atlantic Ocean. The outflow from rivers drains the landscape of minerals and salts and deposits them into the oceans, thus causing the oceans to be salty.

Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not used up and are left for long periods of time. This makes their concentrations increase over time.

The two ions that are present most often in seawater are chlorine (Cl) and sodium (Na). These two make up over 90% of all dissolved ions in ocean water.

BRAIN BREAK!!!

Now, just to get ourselves back on task, ones turn to twos, threes turn to fours, and tell what common chemical compound forms when sodium and chlorine ions bond. Twos and Fours tell your partners on what you most recently used that compound. I had some on popcorn at the movie Saturday night!

The concentration of salt in seawater, or its salinity, is about 35 parts per thousand. In other words, 3.5% of the weight of seawater comes from dissolved salts.

Salt from below the ocean floor – Rivers and surface runoff are not the only source of dissolved salts. Hydrothermal vents contribute dissolved minerals to the oceans. Do you remember what the prefix “hydro-” means? What about the meaning of the root word, “therm?”

So, if hydro means “water,” and therm refers to heat, what do you think exits from these hydrothermal vents on the ocean floor?

Hydrothermal vents are the exit point on the ocean floor from which sea water that has seeped into the rocks of the oceanic crust has become hotter. The super-hot water dissolves some of the minerals from the rock of the oceanic crust (basalt), and then flows back into the ocean. With the hot water comes large amounts of carbon dioxide and minerals.

Submarine Volcanism – A final process that provides salts to the oceans is submarine volcanism, the eruption of volcanoes under water. This is similar to the previous process in that seawater is reacting with hot rock and dissolving some of the minerals.

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Summarizer: “Slap and Go” On the Post-It® note provided, answer the following questions: 1) What is the main substance dissolved in ocean water? 2) From what three sources does this substance originate?

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