Highest mountain, as we all know, is Mount Everest (8848 meters above mean sea level) in the Himalayas. However, there are several different ways to measure the height of a mountain. One alternative way is to measure the farthest point of the Earth’s surface from the center of the Earth. Even Kilimanjaro, the highest mountain of Africa which stands only 5895 meters above mean sea level, is higher than is Mount Everest if measured that way. And the record holder is Chimborazo (6268 meters above sea level) in Ecuador.
These mountains stand so high from the center of the Earth because they are located almost at the equator. Earth is slightly flattened sphere which means that areas at the equator are farther from the center than are polar areas. Hence, equatorial mountains have an unfair advantage over the summits at higher latitudes.
Measuring the height of a mountain from the center of the Earth is understandably very theoretical and perhaps even useless approach. I have never heard that anyone seriously thinks that Chimborazo is the highest mountain on Earth. But the list of possible ways to measure the might and height of a mountain does not end here. Another way to do it is to measure the height from the base of the mountain to its summit. Such an approach could be attractive to geologists because in some cases it gives really much better representation of the real height of a mountain.
It does not make much sense to measure the height of oceanic islands from the mean sea level. Sea level is no permanent marker anyway. It fluctuates in geological time and island volcanoes really do not care about the sea level. They start growing usually from the abyssal plain which is some 5000 meters below the sea level. It doesn’t do much justice to kilometers high and very voluminous oceanic island if we say that it is just a mere 100 meter high rock in the sea.
Oceanic islands are indeed the highest or perhaps I should say tallest mountains in the world. Highest and most voluminous oceanic island is Hawaii which is composed of several volcanoes, highest of them are Mauna Kea and Mauna Loa. Mauna Loa is the most voluminous, but Mauna Kea reaches few tens of meters higher. Here is the summit of Mauna Kea which stands 4205 meters above sea level, but its height from its base is more than 10 kilometers!
The summit of Mauna Kea is a cinder cone. Its appearance and the angle of slopes may be somewhat misleading because the bulk of the massive island is composed of basaltic lava, not pyroclasts. And its slopes are way more gentle because basalt flows easily and composes mountain with a very large base. I climbed the summit which was quite easy because you actually have to climb less than 100 meters. Mauna Kea has one of the best conditions for land-based astronomical observatories. There are lots of telescopes very close to the summit and obviously there is a road leading to them which means that one can almost drive to the summit.