Sea-Floor Spreading
Think back to the ridge volcano that has formed in the Gulf of California south of Death Valley. If you follow this ridge volcano south, you will find that it runs into the open Pacific Ocean and then wraps around the globe through all of the world’s oceans, like the seam on a baseball. The ridge runs right up through the middle of the Atlantic (coming to the surface in Iceland). (Because the ridge is in the middle of the Atlantic, the middle of the Gulf of Mexico, and the middles of some other ocean basins, it is often called a mid-oceanic ridge, although in some places the ridge is far from the middle of the ocean.) Study the seafloor, and you will see that it is made of rocks that are young near the ridge—they hardened as the lava cooled recently - with older rocks farther away, that hardened as the lava cooled a longer time ago. We will discuss how we learn the ages of rocks later; for now, please accept that the rocks, and the sediments just above the rocks, are progressively older with increasing distance from the ridge.

Everywhere we meet the ridge, it is relatively shallow and hot. The rocks that form at its volcano slowly cool as they move away, contracting and sinking. The ridge is a bizarre place in many ways—water circulates through cracks in the hot rocks, dissolves many chemicals in the rocks, and emerges at “black smokers” where the minerals reprecipitate or “rainout” in the colder ocean as a sort of black dust or as a cone around the emerging water. Many valuable ore deposits were originally formed from the chemical precipitates around these black smokers. An entire ecosystem of tube worms, clams, etc., lives on bacteria that use these chemicals for a fuel source, rather than relying on the sun for energy, as do most living things.
Video: Hydrothermal Vents: 2016 Deepwater Exploration of the Marianas (1:05)
So, right now we are at a depth of 3,275 meters. Looking at an incredible black smoker hydrothermal chimney. that black fluid coming out there is likely between 300 and 400 degrees centigrade. These ecosystems, literally islands, or oases, amongst what is - I mean it's really cliche, and the deep sea is definitely not a desert - but relatively, islands of abundant fauna, islands of life, in what is usually a desert-like deep sea floor. Oh, this is so cool. What is that 27 meters high? 30 meters high.
Text on screen: 2016 Deepwater Exploration of the Marianas, NOAA Office of Ocean Exploration and Research, Okeanos Explorer, May 1, 2016.