The following is Admiral Howe’s second (and last) guest post on the series. Originally posted on the Stellaris forums.
Hello again! Admiral Howe here for a repeat performance. Thanks once more to The Beautiful Void for being gracious enough to offer me a turn in the rotation for his awesome series. He, single handedly, has made this period of super-hype go far more easily. Especially for my hype-addled mind.
So, last time I left you with doom, gloom, and a whole ‘die in fire’ motif. It wasn’t my intention to be such a gloom-bearer so, today, I’ll balance my firebending with a bit of waterbending.
Yes. Free surface water makes Earth the crown jewel of Sol’s system – a fact we all raise a drink to I believe. This gleaming azure orb spins through space draped in a mantle of 1,400,509,000 cubic kilometers of liquid water. That’s enough water to mantle Mars in oceans averaging 10km deep. (Take that Kim Stanley Robinson!)
But that water is here and it’s vital to the biology we all rely on (and really, it would suck to try to move it). This information is nothing shocking to moderately well informed folks. Yet, like with volcanoes, that’s only a small portion of the story…for water plays a vital role in Earth’s geology. And that role is likely repeated on any water-bearing terrestrial planet.
You see, water runs this show.
Take these two winning folks here:
First you have anhydrite, also known as calcium sulfate, or CaSO4. Second you have gypsum, also known as calcium sulfate dehydrate. Yes, CaSO4*2H2O. Same chemistry as anhydrite but with a tiny amount of water added. In fact, anhydrite will spontaneously change to gypsum when exposed to water in normal circumstances – something we exploit in plaster of Paris.
But, more importantly here, the two crystals are different size – gypsum is considerably larger because of the absorbed water. The difference is enough that adding water to even the hardest rock rich in anhydrite will cause the rock to disintegrate in minutes as the gypsum crystals push apart the rock’s matrix.
Now imagine siting an important piece of infrastructure on bedrock containing anhydrite. Thank goodness no one would ever make such a mistake.
Mineralogical cycles and processes change dramatically when water is added in a vast number of contexts. Venus may be our twin in mass and density but the rocks and processes we find on her surface will never look like Earth. And Mars may have once been much warmer and wetter but in its desiccation even the rocks themselves changed as their water bled away.
Where this is most dramatic, and most impactful for us, is with the igneous rock known as basalt. I touched on these before when I mentioned the Siberian Traps as those Large Igneous Provinces are uniformly basalt and thus can flow for great distances until they cool.
Once they do that water begins facilitating rapid weathering as the iron and magnesium rich basalt will react with most chemistries mobilized by water – especially if the planet also has oxygen available. In fact, the longer basalt is exposed to water – and water gets a chance to percolate through the cracks and fissures in the cooling rock – the more it can weather and alter the basalt into new minerals. And this is vitally important because the seafloor is uniformly basalt.
This map shows the seafloor with the ages marked (key is on the bottom left). You can clearly see the globe-spanning Mid Ocean Ridge, the single greatest mountain range in the solar system, and you’ll notice the seafloor is brand new right at the ridge.
We know these ages of the seafloor because of yet another mineral, an iron mineral named magnetite. This mineral earned its modern name because it is naturally magnetic (historically it is known as lodestone) and when it crystallizes it does so in alignment with the Earth’s magnetic field.
Every reversal is recorded, world-wide, precisely on the seafloor because of this and – when combined with land-based research – lets us pin down the ages of most of the floor. It’s actually amazing that once we discovered this (another surprise from WW2) we not only rapidly established plate tectonics as our working theory of Earth’s crust, developed a magnetic-field calendar for earth to let us age-date large portions of the world, and also could establish spreading rates for the seafloor current and past (and find they’re not uniform).
While this seems like a neat aside, bear with me a bit longer…because water is responsible for this too. If you look back up three images and take a look at the ages of the seafloor something should jump out at you.
Go ahead, take a moment.
I’ll still be here.
Really, where would I go before Monday?
Yeah, you should have noticed it…on our 4.6 Gy planet the ocean floor is no older than about 200 million years. You’d think losing 4.4 Gy of ocean floor is embarrassing but, it’s not. It was a revelation.
What happens is after hundreds of millions of years of cooling and mineralogical alteration by water basalt reaches a point where it is now denser than the mantle beneath it. When this threshold is reached, even if there isn’t any plate collision pushing against the basalt, the seafloor falls away and begins sinking back into the Earth.
This is known as subduction and the pull of subduction is what powers the whole tectonic engine, from driving the continents across the globe to pulling open the Mid-Ocean Ridges and leading to new crust formation.
Just that by itself would be enough to keep me fascinated for a whole career…cold, wet seafloor diving into the abyss while causing new seafloor to be born thousands of kilometers away. A whole planet’s face constantly rearranging simply in response to this endless conveyor belt.
But wait, there’s more.
One more bit about water here and that’s what happens when the old seafloor begins its death dive. Increasing pressure and heat begins to force the water in the seafloor plate out and into the mantle. And there, like the anhydrite->gypsum reaction above, dramatic changes in the minerals take place.
The first effect, and perhaps the more obvious one, is the water entering the mantle lowers the melting point of the surrounding rock. That melted rock is, naturally, more buoyant than the solid rock around it and it rises only to erupt in long chains of volcanic mountains mirroring the subduction off shore. It’s why the Ring of Fire is where it is and why, someday, a new one will form around the Atlantic when it’s that ocean’s turn to disappear again.
The second effect is the more profound one. As the melted magma rises it carries with it the water from the subducting plate’s water. This water changes the nature of the melt and thus the rocks being formed by the magma as it reaches the surface. Instead of basalt erupting the new melt – a combination of water, mantle, and melted crustal rocks passed through on the way to the surface – you get andesite and rhyolite. Or, more commonly, continental rocks.
Oceanic crust never gets more than 5-10km thick, as shown above, but continents can grow over 70km thick – with parts of Eurasia potentially 120km thick. The reason for this is buoyancy once again. Continental crust is significantly less dense than the mantle and cannot be subducted baring incredibly specialized circumstances. Once you form continental rocks they remain for the rest of the planet’s life gradually accreting into larger and larger continents. And there lies what I think is the the biggest magic water has on the Earth…without the plate tectonic system we wouldn’t have the very continents as we understand them. Their diverse terrains, soils, climates, and resources are a byproduct of the tectonic system pushing and pulling them all over the globe.
All because a little water got into the system.
And somewhere out there, maybe Kepler 186f, this same process is building and reshaping the continental faces of unfamiliar worlds our descendants may one day lovingly call home.
Thanks for reading.
I would like to thank The Beautiful Void one more time for giving this rambling author a chance to guest spot his wonderful series.