The Megaflood that Created the Mediterranean

Hi! At the beginning of the year, y’all told me some things you’d like to see in Uncharted Territories. At the top of the list was seeing Uncharted Territories articles as YouTube videos, so I’m taking a stab at it! Here’s the first one.

I’m just starting, so please let me know everything that works or doesn’t. I’m here to improve! Alright, let’s go to this week’s article, it’s on a fascinating topic!

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The Mediterranean Sea didn't exist 6M years ago. It had dried up into a series of salty lakes. Then a megaflood filled it in a matter of months, killing everything in its path.

But it wasn’t just one megaflood. It was a series of floods.

Why did the Mediterranean dry up?
How do we know?
What happened then?
Why did it fill up so brutally fast?
What would it have been like to be there?

This is what we’re going to answer today¹!

1. The Messinian Salinity Crisis

Let’s rewind. Hundreds of millions of years ago, there was an ocean between Eurasia and Africa, Tethys.

Over time, it evolved into the Indian Ocean and seas like the Mediterranean and the Red Sea, which appeared only a few tens of millions of years ago. Since then, the African and European tectonic plates have been colliding, forming the mountains of southern Europe over millions of years.

We can also see it in the bathymetry (underwater topography) of the Mediterranean. Can you see how the seafloor generally slopes gently downward from Africa towards Europe? 

Light blue is higher, dark blue is lower. Look at all the dark blue close to the European coasts, and usually lighter blue or a more gentle gradient along the coasts of Africa.

That’s because the African Plate is going slightly below the Eurasian Plate, forming a deeper sea where that subduction happens².

We can see the complex tectonic plates of the Mediterranean here:

Logically, with all this pushing, it started closing. 

First, water kept flowing from the Atlantic to the Mediterranean, through what’s today the Guadalquivir River Valley³.

Eventually, the path from Atlantic to Mediterranean completely closed.

As it did, evaporation quickly dried up the sea. It supposedly took only 1,000 years.

How do we know this happened? This is heavily debated today, but there are a couple of factors.

Underwater Rivers

Look at the bottom of the Mediterranean Sea now. Can you see the riverbeds carved into the seabed? That only happens when water flows on ground above the sea.

Scientists believe most of these riverbeds were carved during the Messinian Salinity Crisis, when the water levels dropped by over 1 km. If you think about it, rivers can only carve their riverbeds thanks to gravity above the sea. Afterwards, it mixes with the seawater and can’t keep eroding the ground⁴. 

Here’s a still:

You can see that rivers have carved their beds into what would have been dry land at the time. Note that the Mediterranean water levels have gone up and down together with world sea levels, so some of this carving might have also happened outside of this period. This is just the period when the sea levels were lowest, and hence when the most extensive and deep carving happened.

This tells you the approximate levels of the remaining sea at the time: where the carving disappears.

There are other such canyons carved into the ground. The biggest one is, obviously, at the Strait of Gibraltar, formed during the Zanclean Flood.

The Strait of Gibraltar is on the left. All the action happens on the right, the Alboran Sea. The orange and red lines show water paths. The colors show underwater altitude: red is shallow water close to the coast, and as it becomes bluer, it’s deeper. You can see the depths of the Alboran Sea in dark and light blue in the middle.

The flood created a canyon 200 km long, 8 km wide, and hundreds of meters deep⁵.

Similarly, in rivers like the Rhône in France or the Nile in Egypt, the fact that sea waters dropped so drastically meant that the levels of the rivers also fell. They carved the ground much more aggressively, creating very deep canyons hundreds of meters deep⁶.

There’s another way we can tell: salt.

Salt

When so much water evaporates, its salt accumulates. This happens in the Dead Sea today—which is 1km deep⁷.

Layers of salt accumulate at the Dead Sea, as water arrives with salt and dries up. The Dead Sea is one of the lowest points on Earth, 400m below sea level.

In an event like this, we should see a massive amount of salt in the deepest parts of the Mediterranean. We do!

Halite is rock salt. Carbonates and sulfates are also salts. Source

Salt reaches a depth of over 1.5 km in the Mediterranean seafloor!

Messinian salt mines in the Realmonte mine in Sicily. Source.

The salt mines are so big and deep that we could be mining them for thousands of years and never run out.

A chapel made of salt in a salt mine. Source.

Today, we know that this layer trapped about 10% of the oceans' salt during the Messinian times!

And then 600,000 years later… BOOM!

2. The Zanclean Flood

If you didn’t click on the video above, you should click on it now!

Water furiously tore through the Strait of Gibraltar and started pouring in. Like 30,000 Niagara Falls gushing through. More than 100x all the water that rivers discharge into oceans worldwide.

We can see it in action in this experiment: 

At first water trickles in.
It carries some sediment.
Which widens the gap.
More water flows in.
Carrying more sediment.
Opening a wider hole…
Soon, you have a megaflood.

Can you imagine what that felt like?

Imagine how it felt to the animals or hominids⁸ living there⁹. Your species has been here 400k years.

Suddenly, water appears.
You run away.
You try to escape.
But water keeps rising.
10m per day.
If you’re lucky, you live on a future island.
The rest drowned.
Massive extinction event.

But look at the middle of the Mediterranean, at the altitude of the seabed (again, bathymetry):

Dark blue is deep, light blue is shallow. Source: @mappdoutofficial has lots of these maps on sale in his Etsy. You can find the links in the Instagram posts.

An underwater bridge exists between Africa, Malta, Sicily, and the Italian Boot. The seabed is elevated there. So initially only the West Mediterranean gathered water. As it poured in from the Atlantic, it filled in two weeks! Then…

Another flood!

The East Mediterranean Flood

How do we know? Because we have a good idea of where it happened.

Again, darker blue correlates to deeper water. Source.

Just at the mouth of the Noto Canyon, southeast of Sicily, there are massive amounts of deposits¹⁰ consistent with a brutal flood. Here, a huge waterfall formed, 1.5 km high, 50% higher than Angel Falls, the world’s tallest waterfall today¹¹.

It took less than two years for water to fill the Mediterranean basin¹².

And this is how the Zanclean Megaflood opened the Gibraltar Strait, flooded everything in what is today the Mediterranean, killed everything in its path, but created a new sea habitat—one that humans used 5M years later to thrive.

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Most of the research and visualizations in this email were performed by Garcia-Castellanos & Manolo Mantero, @CSIC-Geo3BCN. They’ve done all the hard work here. You can follow García-Castellanos’s YouTube channel here and Twitter here.

If you enjoyed this, you might enjoy this one I did about something similar that happened just a few thousand years ago between the UK and France¹³.

Did a similar megaflood happen in the Black Sea?
Could it be the one mentioned in religious texts around the world?
What caused the opening in Gibraltar? Was it tectonic plates? Erosion? Something else?
Did this cause... volcanoes to erupt?
What other megafloods have there been?
What happened to the local fauna?

I’ll cover these in this week’s premium article!

1. There are a bunch of short videos today (too long for gifs). But click on them! They’re worth it.

2. This is why the volcanoes around the Mediterranean are on the north side—Etna, Vesuvius, Santorini.

It’s also why Southern Europe has better deep water ports than North Africa, which is one of the reasons why it’s richer.

3. Fun fact: this large valley is much flatter than the rest of Spain, with a big river that reaches the Atlantic. It was the perfect port to be the capital of the colonization of America. This is why most ships sailed from and to this region—to the cities of Sevilla and Cádiz—and why the Spanish language in South America is spoken with the accent of the Guadalquivir region (“andaluz”).

4. If anything, freshwater stays on top, because it’s lighter than seawater due to salt.

5. So deep that submarines used it in WWII.

6. We didn’t know because they are now filled with sediment, but once we knew what to look for, we found it. For example, in the Nile, the canyon was discovered when Egypt built the Aswan Dam, and they started doing geological surveys when they realized the riverbed was much less stable than the surrounding bedrock.

7. If you think about it, this is unsurprising. If it’s 1 km deep, it means it’s closed from the ocean—otherwise, water would be flooding in and it wouldn’t be 1 km deep. So it makes sense that points that are very low on the Earth’s surface are also very saline. They accumulate water that evaporates, leaving its salts behind.

8. Hominids were roaming Africa back then. Maybe some were there too. 

9. Although it depends where. At the deepest points in the region, pressure was probably closer to two atmospheres than one, and temperature might have been unbearable. This parallels why we would want to settle in the deepest parts of Mars.

10. These are not just sediments, but the result of a massive water outburst.

11. This is debated, but you get the point.

12. It took an additional 26,000 years after the flood for the salt of the Mediterranean to reduce enough for marine life to thrive again.

13. In the premium part of the update, after the paywall