🌋 Earth’s Mantle

It’s 1,800 miles thick and makes up 84% of Earth’s volume. There is more water in our mantle than in our oceans.

We have never drilled deep enough to sample the mantle directly. But on an island in the aptly named Newfoundland, you can find Earth’s mantle transported to the surface.

I was given this fine sample at a recent Tesla event by someone who thought I might like it, and as I researched it, I found a newfound awe at how special the Earth’s inner dynamics are compared to other planets and moons.

First, the mantle is not solid rock, but a viscous fluid that circulates from the convection of heat from our molten core… circulating once per billion years! This circulation is the fundamental force driving plate tectonics and its side effects: earthquakes, volcanoes, and the formation of mountains.

Normally, the tectonic plates of Earth’s crust are subducted down deep into the mantle, recycling the surface of the planet. In the rare case of the Canadian Tablelands, the mantle was instead thrust up onto continental crust about 500 million years ago when continents collided.

This mantle sample is mostly the ultramafic rock peridotite, weathered here to a yellowish rusty color. Inside, the unaltered rock is a dark green olivine. It also embeds shiny flakes of mica.

The basalt that comes up from the mantle cools and slides over to the subduction zone over ~ 150 million years. Cool enough to start sinking, it is still lighter than the mantle, and for it to cycle back down, it has to become much more dense, and it does, 30 miles down, becoming eclogite which then sinks further into the mantle, which itself circulates as a viscous fluid down to our molten core, on a billion year roundtrip to circle back to the crust again. Without the materials marvel of eclogite, our plate-tectonic system would grind to a halt.

Only Earth has developed the habit of subduction, which has helped keep the planet on an even keel for eons. Subduction recirculates not only the solid ocean crust but also volatiles like water and carbon dioxide vented by volcanos back to the mantle. In contrast, other planets, like Mars with its fossil river valleys, have simply lost their volatiles to space over time, without a circulating reserve in its mantle.

Subduction circulates water down to the magma, forming granite (which accumulates in the continental crust), and lowering the viscosity of the mantle, allowing it to flow convectively, keeping the plates in motion. Granite is only found on Earth among the planets in our solar system.

Mars, lacks the movement of tectonic plates, enabling the largest volcanoes of our solar system to stay in one place erupting continuously for billions of years. In contrast, a volcano range, like the Hawaiian islands, leaves a chain of former cones sinking into the seabed all the way back to Japan.