Answering the Question "What makes the oceans salty?"

07 June 2010

This was a tricky question for us to research while we are out paddling. Luckily, our friend (and future Expedition Team member), Ellen Root, is a geologist and she had an answer for us. Here is what she said:

Here is one way to think about the answer to Amy’s question:

The Earth’s crust is made of rock.

Rocks are made of minerals. You might have heard of minerals such as quartz, garnet, muscovite, or gypsum. Minerals are made of elements like iron, aluminum, sodium, and 115 other known elements, including elements with silly names like ununoctium.

The elements within minerals are held together by chemical bonds.

When rocks are exposed at the Earth’s surface, they are exposed to things like sunlight, rain, wind, snow and ice, plants trying to grow on them, and many other forces.

These forces can act on exposed rocks and cause them to start breaking down. This process is called weathering. Rocks can experience physical weathering, when they break into pieces, and chemical weathering, when the bonds that hold elements together break and release the elements.

Chemical and physical weathering often happen at the same time.

Water is usually a part of weathering. It transports both physically weathered pieces of rocks (think stones in a streambed) and elements that have been freed from rocks.

Feldspar is the most abundant mineral in the Earth’s crust. It is estimated to form about 60% of all igneous rocks. The mineral feldspar contains the elements sodium, potassium, calcium, silicon, and aluminum in it. That means there are a lot of these elements in the Earth’s crust. Sodium is the element that is most important to answering our question, so let’s focus on elemental sodium that is freed from rocks through chemical weathering and transported by water to the world’s oceans.

The oceans are very large, with big currents that circulate and mix water from different areas together. This means that sodium that flows out of the Mississippi River will eventually mix with sodium from other rivers to make the oceans have a relatively homogeneous (or constant) sodium content.

The world’s oceans have a lot of surface area for water to evaporate from. When water evaporates, it leaves behind dissolved elements like sodium. This concentrates sodium in the world’s oceans, and is one explanation for why ocean water is salty.

To summarize: water dissolves elements like sodium from rocks on the Earth’s surface and transports these elements to the oceans. In the oceans, the water evaporates and leaves behind the sodium. This concentrated sodium is what makes ocean water taste salty.

This being said, the world’s oceans are very complex with many forces and factors that affect their composition. Can you add another level of complexity to this answer and help Amy learn even more about ocean salinity? Let us know what you find.