Explain xkcd: It's 'cause you're dumb.
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Title text: I just learned about the Slide Mountain Ocean, which I like because it's three nouns that sound like they can't possibly all refer to the same thing.
This comic shows what the landmasses of Pangaea were hypothesized to have looked like during the Triassic Period (roughly 200 million years ago) before continental drift, the process by which landmasses moving over the Earth's mantle collide and separate, brought them into the configuration we see today. It also shows the landmass Laurasia declaring, "Red rover, red rover, send India over!" as if the continents were playing the game Red Rover.
In the game of Red Rover, the aim is for an individual to charge into the opposing team who are holding hands, and attempt to cause a break in the human chain. If the individual succeeds, they take one of the opposing teams members back to their own team. If the chain doesn't break, the individual joins that team. In the game portrayed, an isolated landmass (India in contemporary geography), is the individual charging towards the Laurasian landmass, attempting to break through. We know of course that India failed in this attempt, and as per the games rules joined the Asian 'team'.
It is accepted that the Himalayas, the highest elevated mountain range on earth, formed by the collision of India into what is now Asia. For various reasons, the movement of the Indian plate from its location in Gondwana 90 million years ago to its impact point with the rest of Asia 50 million years ago was extremely rapid (as plate movements go) at about 20 cm per year. Needless to say, the idea that the landmasses on Earth are sentient and moving about in an incredibly slow game of Red Rover, with India's rapid movement being a result of being "called over", is not one which is scientifically accepted(citation needed).
The title text refers to the Slide Mountain Ocean, which was located between the Intermontane Islands and North America in the Triassic time beginning around 245 million years ago. The name interests Randall because oceans (bodies of water), mountains (land masses), and slides (playground equipment) are mutually exclusive concepts when using the most common definitions. In this case, however, "slide" is short for "landslide" which is a common feature of mountains. Slide Mountain is a particular mountain somewhere in British Columbia, the result of the remnant of the Slide Mountain microplate which accreted onto the continent, becoming the Slide Mountain terrane, as the majority of the microplate was subducted. "Slide Mountain Ocean" refers to the sea between the Slide Mountain microplate before it was subducted under what is now North America.
[Two depictions of Earth at different points in continental formation, one above the other]
[The top depiction shows a relatively earlier time period, "shortly" after the separation of Pangaea into Laurasia (northern supercontinent) and Gondwana (southern supercontinent), with the two supercontinents labeled]
Laurasia: Red Rover, Red Rover, send India over!
[The bottom depiction; the land mass that would become India is shown moving, with motion lines, toward the northern supercontinent]
How the Himalayas formed
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