Underground nuclear and conventional bomb tests. Giant volcanic eruptions. The sudden opening of yawning sinkholes that seem to have no bottom.
These events all leave strange, circular depressions and holes in Earth's crust. Yet despite the many different things that can trigger the land beneath our feet to give way, collapse structures all look remarkably similar. Why is that? What can we learn about the dangerous processes that cause them? Can studying giant holes in the planet even make us better at finding valuable minerals?
Keith Howard of the United States Geological Survey has dedicated his life to peering into the abyss in search of answers.
In a new paper in the journal GSA Today, he outlines a few general rules about why collapse structures look so much the same -- in short, it's all down to rocks' strength (or lack thereof).
When an explosion goes off underground, or a large volcanic event leaves behind an empty magma chamber, the rock simply can't stand up to gravity, and faults quickly spread through the walls and ceiling of the empty space. The hollow roof will collapse straight down, piston-like, forming a sagging structure like that pictured on the volcanic Genovesa Island in the Galapagos, above. The same is true of smaller collapses with very weak rocks, like still-warm and soft lava flows.
In cases like the loose fill beneath Guatemala City, the collapse itself is small, and the rock formation is relatively strong compared to the weight of the material collapsing. Steep-walled holes with rubble at the bottom are typical of this style of failure.
Howard's work is more than just a single-minded quest to understand mysterious holes. He told Discovery News:
For collapse calderas, we want to be able to better understand how a volcano works in order to better evaluate volcanic hazards as well as to study the placement of mineral endowments that are associated with some volcanic structures.
Mineral deposits often occur in places where magma-heated water circulates through rocks and leaches out useful resource metals, then deposits them in fractures -- like the faults that form during collapses. Knowing where to expect them could increase prospectors' chances of a rich strike.
Understanding collapses can also help gauge hazards posed from abandoned mines and old bomb test craters, like those in the Nevada desert pictured above. Howard writes:
[There are] numerous applications to non-volcanic hazard analysis too, for instance in understanding the width, shape, and internal structure of hazardous collapse into active and abandoned mines for coal or metals, and of sinkholes developed in limestone or salt terrains because of dissolution below. Some of the old nuclear-test sinks have continued to subside years and decades after the explosions.
Images: Keith Howard; Department of Energy
Tags: Everyday Science, Geology, Geophysics, Mining, Volcanic Eruptions
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