Topographic prominence

In mountaineering

Prominence is valued by mountaineers because it objectively correlates with a summit's subjective significance. Peaks with low prominence are either subsidiary tops of higher summits or relatively insignificant independent summits, while those with high prominence tend to be the highest points around and often offer extraordinary views. Only summits with sufficient prominence are regarded as independent mountains; for instance, K2 (height 8,611 m, prominence 4,017 m) is the world's second-highest mountain, whereas Mount Everest's South Summit (height 8,749 m, prominence 11 m) is not considered independent because it is a sub-summit of the main summit (height and prominence 8,848 m).

Many mountain lists use prominence as a cutoff: John and Anne Nuttall's The Mountains of England and Wales uses 15 m, Alan Dawson's list of Marilyns uses 150 m (limited to Britain and Ireland), the U.S. fourteeners list uses 300 ft (91 m), and 2,000 ft (610 m) informally indicates major stature. Lists with high cutoffs favor isolated peaks or massif high points; low cutoffs, like the Nuttalls', include many summits some view as insignificant. Prominence can also rank peaks directly, producing lists that emphasize isolated high peaks such as range or island high points and stratovolcanoes, with the advantage that no cutoff is needed.

Parent peak

A peak's parent is commonly defined as a particular peak in the higher terrain connected by the key col, though various definitions exist and are not necessarily based on geological or geomorphological factors. The parent relationship creates a hierarchy where some peaks are subpeaks of others. Encirclement parentage, also called prominence island parentage, defines the parent as the highest peak inside the contour that meets the key col's other contour.

For example, Mont Blanc's encirclement parent is Mount Everest, because its key col near Lake Onega (113 m elevation) is where contours encircling Mont Blanc and Everest meet. This demonstrates that the encirclement parent can be very distant when the key col is low, which often fails the intuitive expectation that a parent should be close to its child. In a schematic range, the encirclement parent of peak M might be H, whereas an intuitive view would suggest L; if col k were slightly lower, L would become the true encirclement parent.

Issues in choice of summit and key col

Human alteration of the landscape and water features can affect the choice of summit and col location and height. In Britain, extensive discussion has produced a protocol adopted by the main sources of prominence data there and in Ireland, while other sources commonly ignore human-made alterations, though this convention is not universally agreed upon; some authors discount modern structures but allow ancient ones. Mountaintop removal is another point of disagreement, though for high-prominence peaks and low-prominence subpeaks with intact summits, the difference between conventions is typically small. The key col and parent peak are often close to the sub-peak, but not always, especially when the key col is low.

For example, Denali's key col (56 m) is near Lake Nicaragua, and its encirclement parent is Aconcagua (6,960 m) in Argentina, with a prominence of 6,138 m. A small hill on the Alaska coast with elevation 100 m and key col 50 m would also have Aconcagua as its encirclement parent, despite many closer higher peaks. A hill in a low-lying area like the Netherlands often has Mount Everest as its direct child, with prominence nearly equal to its height.

Wet prominence and dry prominence

Two varieties of topographic prominence exist: wet prominence and dry prominence. Wet prominence, the standard type, assumes the earth's surface includes all permanent water, snow, and ice, so the wet prominence of an ocean island's highest summit always equals its elevation. Dry prominence ignores water, snow, and ice, assuming the surface is defined by the solid bottom of those features; it equals wet prominence unless the summit is the highest point of a landmass or island, or its key col is covered by snow or ice. If the highest surface col is on water, snow, or ice, dry prominence equals wet prominence plus the depth of the highest submerged col.

Mount Everest's dry prominence would be 19,772 m (its wet prominence of 8,848 m plus the depth of the Challenger Deep at 10,924 m). Aconcagua's dry prominence is about 7,000 m (wet prominence 6,960 m plus about 40 m depth of the Bering Strait's highest submerged col). Dry prominence is also useful for measuring submerged seamounts, which have a dry topographic prominence, topographic isolation, and negative topographic elevation.

Did You Know?

• The summit of Mount Everest is the parent peak of Aconcagua at a distance of 17,755 km, as well as the parent of the South Summit of Mount Everest at a distance of 360 m.
• The key col for Aconcagua, if sea level is disregarded, is the Bering Strait at a distance of 13,655 km.
• A way to visualize prominence is to imagine raising sea level so the parent peak and subject peak become separate islands, then lowering it until a tiny land bridge forms—the key col.