## Q&A: Earth’s Tallest Mountain

Question: I read that Mt. Everest isn’t really the tallest mountain on Earth. If you count the underwater part of Mauna Kea (on the Big Island of Hawaii) it’s actually taller than Everest. My question is: Do mountains on land also have extensions below the surface that should be added to their visible height?

Answer: Yes and no. Mountains are not discrete geological entities resting on the ground like a large boulder. Mountains built by volcanos, such as Kilimanjaro in Tanzania, have “roots” erupted from lava chimneys at various levels below the surface. Mountains built by plate tectonics, such as Denali in Alaska, are themselves above-ground extensions of the underground plates that created them.

But it gets way too complicated if you try to include the underground parts. For convenience and consistency, geographers use the altitude of the summit above sea level (ASL) to compare mountains. The main graphic shows how the numbers add up for Mauna Kea and Everest. So yes, Mauna Kea is taller if you count the part that’s below sea level (BSL).

You do raise a valid point that bears on the definition of mountain and height. If our oceans evaporated (as some planetary scientists believe happened on Venus and Mars), peaks that were previously partly or totally submerged would now be “mountains” in their entirety by definition. [On planets without seas, mountain heights are expressed relative to other global features.] So why shouldn’t Mauna Kea be claimed as Earth’s tallest mountain?

Interestingly, if the summit’s distance from the center of Earth (COE) is used as the criterion for height, then Mount Chimborazo in Ecuador would top the list. Earth is not a perfect sphere — it has an oblateness of 0.0034, with a polar radius of 6357 km and an equatorial radius of 6378 km. Chimborazo is near the equator with latitude 01°28′09″S. Everest is at latitude 27°59′17″N, which puts it 5 km closer to the COE:

FYI, you can get Earth’s radius for any latitude with this handy calculator: https://planetcalc.com/7721/. Check your latitude and see how far you are from the COE.

When you run all the numbers the summits compare as follows:

Chimborazo: 6378 km + 6.263 km = 6384.263 km
Everest: 6373 km + 8.849 km = 6381.849 km

So the summit of Chimborazo is 6384.263 km − 6381.849 km = 2.414 km farther from the COE than Everest.

There’s at least three ways one can define the “height” of a mountain. The convention is to measure the summit’s height above sea level, in which case Everest rules. But comparing the alternative measures provides interesting insights into the  physical structure of our planet.

Next Week in Sky Lights ⇒ Limits to the Growth of Mt. Everest