The oceans get saltier due to the rivers that flow into them. Water from the ocean evaporates, and then rains over land and forms rivers. As the rivers flow over the land, things like salt dissolve into the river and are carried out to the sea. Since the amount of water in the ocean is more or less constant, the sea keeps picking up more and more salt and doesn't have any place to get rid of it. In certain places, like the Salton Sea in California, the Great Salt Lake in Utah, or the Dead Sea in Israel, rivers flow into lakes that also evaporate - and the salt is concentrated more and more over time until the salt concentration is much higher than in the ocean - and nothing much can live in these lakes.

The oceans formed about 4 to 4.5 billion !!!! years ago by the eruption of volcanoes that brought gases up to the earth's surface. Some of these gases condensed (went from gas to liquid state) and formed "water " to fill the low lying areas called ocean basins. The salt of the ocean is present because rain on the continents, or areas that stand above sea level, dissolves rocks a little bit as it travels eventually through the hydrologic cycle to the oceans. Another cycle is what happens in the middle of the ocean: there, undersea volcanoes erupt molten lava that reacts with sea water at high temperatures and induces changes in the composition of the waterduring the rock/water reaction at 400 to 500 deg C. WHY DONT YOU CALCULATE WHAT THE TEMP IS IN DEG F EQUIVALENT TO 400 DEG C...and then see if your oven can reach this value of 400 C.

At any rate, the net reaction is that Na, Ca and many other ions or "salts" are added to sea water.

This may seem surprising, but the salt from the ocean comes from the "fresh" water in the rivers. In fact "fresh" water is not totally fresh (there is even a little bit of salt in your tap water!). The amount of salt in the rivers is very low, but when that river water dumps into the ocean the water evaporates and turns to clouds and rain, but the salt stays in the water. In this way, over hundreds of millions of years the ocean has been gradually becoming more salty.

Now, the second part of the answer has to do with how salt gets into the rivers. It comes from disintegrating rock, like the granite that makes up the Sierra Nevada, and from volcanic eruptions. Certain minerals in granite contain small amounts of sodium (Na). When mountains get worn down by rivers and glaciers that sodium gets into the rivers. Volcanic eruptions spew out a bunch of water vapor and carbon dioxide and other gasses like chlorine. The chlorine dissolves into the ocean and then combines with the sodium to get sodium chloride, or table salt.

My answer to your question has two parts. First - It is thought that originally the only water present on the earth was in the form of water vapor in the atmosphere. When the atmosphere cooled down the water vapor condensed on atmospheric particles many of which were in the form of salts. At a certain point these water droplets become too heavy to remain in the atmosphere and fall to the earth in the form of rain. In this manner the salt particles were leached from the atmosphere as rain and deposited on the surface of earth. Second- As the rain washed over the Earth's terrestrial surfaces salts were leached from the soils where they traveled
down streams to form the oceans. What do you think is the primary mechanism that puts salts into earth today? After you think about that you can read on for the answer. Presently the main input of new salts is from volcanic activity that puts
salt laden materials into the atmosphere or on the surface of the earth where the salts are removed in the same manner as above or by the scouring of sea.

Good question. Our oceans have been salty for a very long time (millions of years), and it seems a little mysterious where all that salt came from. Part of the answer lies in the nearest little stream. Each time it rains water runs over rocks, and over time the water dissolves bits of the rocks away. The water carries these dissolved bits ("salts") down to the ocean. Although each river does not carry
very much salt at any one time, over the years all the bits add up. Eventually, the slow erosion of rocks and minerals around the world has left us with a salty ocean. You can find out more about this process by looking up subjects like the "water cycle" and "erosion" in the library.

To answer your question we need to go way back in Earth's history, over 4 billion years age, that's 4,000,000,000 years. The early earth was a very inhospitable place, it was likely very hot and had lots of volcanoes. During this time, the elements that make up the earth were differentiating (moving around). The heavy elements like iron and nickel moved to the center of the earth due to the gravitational pull of the earth. The lighter elements like silica, oxygen, nitrogen, hydrogen and many others moves towards the outside of the earth. Some of these elements were gases so they became the atmosphere.
The volcanic activity played an important role in delivering these gases from deep in the earth to the surface and into the atmosphere. One of those gases was water vapor. Eventually the earth cooled and the water condensed and rained down on the earth.. The water filled up the low places and became the oceans. The salts that are in the oceans came from the weathering and eroding of the the land while it was raining so much. All of the elements like sodium, chloride, magnesium, sulfate, silica, calcium and potassium got into the oceans this way. This process is still going on today but at a much slower rate. We got a good look at it this winter with all of the erosion that occurred in our area. Another way elements get into the seawater is through underwater volcanoes. These volcanoes have vents that squirt out hot water and elements dissolved in the water.
The reason our oceans remain salty is that some elements like sodium (Na+) and chloride (Cl-) remain in the oceans a very long time. This is called a residence time. When sodium gets put in the ocean from rivers it takes over 200 million years before it gets put in to sediments or evaporite deposits (salt deposits). An eliment like silica has a residence time of about 8,000 years, not very long. This is because silica is used by tiny organisms called diatoms to make their shells. When they die, they settle to the bottom of the oceans and become sediments.
I hope this answers your question. Good luck with your science adventures!

Water washes lands and soils before to arrive to the oceans. That wash carries salts from the different soils and the ocean is an accumulation basin, where over time (geological times), evaporation concentrates these salts.