Printed from “The Discovery of Hydrothermal Vents - 25th Anniversary CD-ROM” ©2002 Woods Hole Oceanographic Institution

EARLY CLUES: Evidence

Introduction

The plate tectonics revolution that culminated in the late 1960s fundamentally rearranged our understanding about how our planet works. It launched new ways of thinking about Earth’s history and opened new lines of inquiry—including the idea of seafloor hot springs.

Here’s the logic: The surface of the Earth is broken into rigid plates that move in different directions. In places where plate boundaries are moving apart, columns of molten rock from the mantle rise toward Earth’s surface. Lava erupts onto the seafloor and is quenched to form new oceanic crust. This new crust creates volcanic submarine mountain ranges called the mid-ocean ridges.

Here’s the key: Newly made seafloor would likely have cracks in it, because hot rocks contract when they cool. Cold seawater could seep into those cracks, where it would come in contact with hot rocks deeper down. The water would heat up again, rise back to the surface, and discharge into the ocean. Geysers on land, such as those in Yellowstone National Park, work in a similar way.

As recently as 1977, no one had seen a seafloor hot spring. They remained solely within the realm of possibility. But the idea that seafloor hot springs might exist grew more intriguing, especially as circumstantial evidence accumulated. The evidence included:

• Red Sea “hot brines”
• Metal-rich sediments
• Seafloor rocks with curious chemistry
• Ophiolites—slivers of seafloor on land
• The “missing heat”

In this way, vents literally “ventilate” the mid-ocean ridges in a process called “convection.” The same process occurs in a pot of boiling water. Heated water rises to the top of the pot and discharges heat to the air above it. The cooler, and now denser, water then sinks to the bottom of the pot to be heated again.

So, by the early 1970s, scientists had predicted that there might be hydrothermal vents at mid-ocean ridges. But until 1977, no human had ever seen hydrothermal vents on the seafloor, nor could they prove they really existed.