Well, finally one I can answer! thinks I. “Well, Son, as water moves in streams and rivers, it disovles bits of minerals, including salt, from the stream and river beds. It carries this to the oceans, where it meets other very slighty salty water. As water in the ocean evaporates, it leaves salt behind, so over time, the water becomes saltier and saltier.”
“So, will all the land be disolved into the water someday? What happens when no more salt can be held in the ocean water?”
It already is. Minerals don’t just stay disolved in the ocean, they are also removed through various processes. Marine animals remove calcium to build shells, some minerals just precipitate to the bottom, some are removed when ocean water becomes trapped and evaporates leaving salt pans behind.
I believe that the rate of increase in the oceans’ salinity is so slight that it would take millions of years to get to the point where the salinity made them uninhabitable by most present-day lifeforms, much less precipitate the salt out.
Also, I think I once heard that if you removed all the salt from the oceans, you could cover the earth’s entire land surface with a crust of salt several feet thick. For some reason, this reminds me of a Yale Prom and marinara sauce. Hmmmmmmm.
The ocean actually isn’t all that far from saturation with calcium, and ocean water concentrated to about a third its volume will precipitate calcium sulfate. But the other ions present are extremely far from saturated; the ocean could dissolve many tens of times as much salt as it does.
It might be mentioned that, besides erosion, additional sources of salt and other minerals in ocean waters are underwater volcanic eruptions, and the superheated water with dissolved minerals that comes from black and white “smokers” in rift zones.
So the extra salt gets deposited on the ocean floor, leaving the water above it unsaturated, and able to accept more salt, right? In addition, other minerals, like calcium, are used by sea creatures to make their skeletons or shells.
So could one assume that before all the dry land is dissolved into the sea, the sea floor wil rise up due to mineral deposits and become new dry land, so that we’ll never be a totally water covered planet?
That’s not exactly what happens. The sea floor and its sediments are consumed in oceanic trenches where one tectonic plate collides with another. New sea floor is produced at the mid-oceanic spreading centers. Some of the old sea floor melts as it rides under the adjacent plate, and the molton rock erupts as volcanoes. All sea floor is relatively young compared with the age of the planet; IIRC the oldest is only about 80 million years. (This finding was one of the key discoveries leading to modern plate tectonics.) Actually, the amount of continental land compared to sea floor has been gradually expanding over geologica time.
Once the Earth cools sufficiently, billions of years from now, the processes of plate tectonics and mountain building will cease. Erosion will dominate, and the continents will eventually be worn down to sea level. Offhand, I am not sure if it is thought this will happen before the sun expands to fry the Earth to a crisp and boil off the oceans.
OK, so for the purposes of explaining this to a 12 year old who hadn’t studied plate tectonics in depth (but has studied types of rocks and volcanoes, and I think knows that land floats around as big chunks of rock over liquid rock), how does this sound?:
*So the extra salt gets deposited on the ocean floor, leaving the water above it unsaturated, and able to accept more salt. In addition, other minerals, like calcium, are used by sea creatures to make their skeletons or shells.
The salt lying on the sea floor gets pushed under the earth’s crust at certain parts of the ocean. It, along with the rest of the sea floor at that spot, goes into hot spots and is melted and mixed up with all sorts of other minerals to make magma. Sometimes magma is shot out of volcanoes as lava, and new land is made.
Then the whole thing starts over again, with water redissolving the salt in the new land, and carrying it out to sea.*
(I think I’ll leave out the earth cooling off and flattening out vs. the earth being swallowed by the sun and immanent death. One step at a time!)
Interesting how we spent so much time every year at school studying the water cycle, but somehow completely skipped over the “rock cycle”.
Small nit: the mantle, over which the plates move, is solid rock (this is not an attempt to revive glss flws). The outer core, between the inner core and mantle, is liquid.
Some good answers, but everyone has overlooked the most important and most ‘true’ answer. The fact is that the oceans aren’t getting any saltier any more. They stopped getting saltier many hundreds of millions of years ago and are now at equilibrium. The amount of salt being added to the land directly from the oceans in any year is identical to the amount that is being washed away. And I mean every year, I’m not talking about geological timescales. Salt is lost directly from the oceans to the land without any tectonic pocesses being involved.
Most water that leaves the ocean does so as as aerosol droplets that are torn from the surfaces of waves. That’s quite different to the highly simplified myth taught in schools that water evaporates directly from the oceans. In fact if it werent’t for the aerosolisation of water from wave tips there would never be enough water to make rain.
Now that’s important because it means that ocean water becomes airborne salt and all. It’s not the pure water levaing the ocean that we all got taught in school. The airborne water droplets themselves do usually evaporate and leave the salt behind, but that all occurs after the stuff is already floating in the breeze.
And of course when the water vaporises the salt is still left behind, and it’s still floating around in the breeze. Imagine the entire area over the ocean contains billions of microscopic dust motes comprised of pure salt crystals and you’ve got a pretty good picture of reality. Two thrids of the planet is covered in air laden with salt dust.
Much of that salt dust ends up floating back down into the ocean, but an awful lot doesn’t. it gets blown around in the air and eventualy finds its way into the land.
IIRC the amount of salt dust falling onto land from the oceans amounts to about one tablespoon of salt being added to every acre of land surface every single day. And that amount of deposition is, as far as anyone can tell, exactly the same as the amount being eroded into the oceans.
The entire salt cycle has settled down. What is lost from the land equals what is gained from the sea and vice cersa. Just as with water or carbon or any of number of other nutrient cyclessalt moves between land and sea directly, but in proportional amounts.
That might be abit hard to explain tot a child. It’s hard for some adults to grapple with the idea that water evaporates not directly from the ocean but from slat water droplets floating above the ocean. It’s even harder to ‘believe’ that a few grains of salt are spead evenly over your yard (and roof) yard every day so thinly that they are invisible.
But there you have it. The salt from the land washes into the oceans and the salt from the oceans tuns onto salt dust and gets blown straight back onto the land.
