Inverted Topography: How Lava That Filled Valleys Ends Up on Top of Mesas

Lava flows, and the ash flows that make tuff, follow the low ground: they run down river valleys and sometimes fill them. But volcanic rock is often harder than the rock around it. As erosion goes on, the softer valley sides wear away faster than the lava in the valley floor, until the old valley stands above its surroundings as a mesa, and the former high ground becomes the new valleys. This reversal is inverted topography, a classic landform of the American Southwest, with its young volcanic fields and fast desert erosion.

At Petroglyph National Monument, a lava flow that once ran along low ground now stands above it. National Park Service.

How it forms: lava fills a valley (top), protects the rock beneath while the surroundings erode (middle), and ends as a mesa above new valleys (bottom). After Muehlberger and others, 2005. National Park Service.

A clock in the landscape

Inverted topography can show the relative ages of volcanic deposits: where there are many flows of different ages, the oldest stand highest, and younger ones sit closer to today's surface. Most examples are capped by basalt flows, but valleys filled by ash-flow tuff or lava domes can be inverted too, since those rocks are also often harder than their surroundings.

In the national parks

At least ten units of the National Park System are in or near volcanic fields with inverted topography.

• Capulin Volcano (New Mexico): the Raton-Clayton field has had three phases of activity over 9 million years. Its older flows stand above the present ground; the youngest, such as Capulin's, form today's surface.
• Grand Canyon (Arizona): basalt flows cap mesas and buttes around the canyon. Dating a flow and measuring how far it stands above the surrounding land gives the rate of erosion, and so constrains the age of the landscape. Red Butte, near the South Rim, is capped by a basalt that once lay on low ground.
• Petrified Forest (Arizona): the Hopi Buttes field, mostly north of the park but reaching into it, is mainly maar and diatreme volcanoes with basalt flows, and shows inverted relief.
• Petroglyph (New Mexico): West Mesa is held up by resistant flows erupted about 200,000 years ago.

Red Butte, near the Grand Canyon’s South Rim. U.S. Geological Survey.

Why it is valued

Volcanic inverted topography is recognized for its geoheritage value: such landscapes are easy to see and understand, and they show the passage of geological time and change directly.

Sources

• National Park Service, "Volcanic Inverted Topography"; rewritten in hubnx's own words. Photographs and diagram: National Park Service and U.S. Geological Survey. A figure from a journal article is not reproduced here.