Saturday, November 21, 2015

Not all holes in the ground are the same.

-Alison


Since I spend a lot of time thinking about holes in the ground, from the ones I make with dynamite, to volcanic craters, I have to spend some time thinking about other mechanisms, both human and natural, that make similar looking holes in the ground. If I want to say anything about volcanic holes in the ground, such as maar volcanoes, I need to know what makes them unique. If I want to recognize just one type of hole in the ground remotely on Earth, or other planets, I need to know more about holes in the ground in general.

Google Earth Image of Hole in the Ground Maar (left) and its neighbor Big Hole (right), these maars are located in Western Oregon.
Let's start with the largest holes in the solar system, impact craters. One of the most important processes for changing the surface of a planet, or any planetary body (moons, asteroids, etc.) is meteorite impact. There is a lot of junk flying around in space. This junk (rock, dust, ice) runs into other junk and in most cases form impact craters on the bigger piece of rock/ice. These impact craters are really common on planets with no atmosphere, biosphere, or plate tectonics like Mercury or our Moon. They are fairly circular holes in the ground with ejecta spread around them radially. In that simplified description they sound a lot like my explosion craters. In fact, I just went to the Geological Society of America Meeting in Baltimore in early November to talk about the ways my explosion experiments might be of interest to planetary geologists. I hung out a lot in talks about meteorite impacts. The session where I gave my presentation combined volcanic flows and impact processes. It had never been combined quite that way before, and people from both groups quickly learned how much we have in common, and what we don’t.
This enhanced color mosaic shows (from left to right) Munch (61 km/38 mi.), Sander (52 km/32 mi.), and Poe (81 km/50 mi.) craters, which lie in the northwest portion of the Caloris basin. That means these three impact craters are inside an even BIGGER impact crater. Image and links courtesy of NASA Photo Journal.
Meteorite impacts are found on Earth, but they tend to get modified pretty quickly by water, plants, and plate tectonics. One of the best examples of a well preserved meteorite impact is Barringer Crater in Arizona, also known as Meteor Crater. It was formed 50,000 years ago, and is fairly young as geologic processes go. The crater is about a kilometer wide and more than 200 m deep (that is more than one hundred 6 ft tall people standing on each other’s heads). I almost went there last week, but we only had about 20 minutes and other places to be, so it had to wait for another trip when I can spend more time to truly appreciate it. Meteorite impacts can be tiny impacts from micrometeorites (less than a millimeter), to 1,000 km across (Caloris Basin on Mercury). Earth has experienced some large impacts in the past, some are credited with impressive things like the formation of our moon and the extinction of the dinosaurs. If you want to know where the nearest impact crater is to your home you can check out this interactive map. And while news stories occasionally surface about asteroids heading for Earth, like when HM10 did a flyby this summer, it is highly unlikely that a large asteroid is heading for Earth any time soon.
Barringer Crater/Meteor Crater, Arizona (Image Wikimedia Commons).
Other holes in the ground can be formed by the release of methane (see these holes in Siberia, which are still being studied). The current leading theory is that these are formed from the sudden release of gas hydrates, or methane gas trapped in the pore spaces between sand grains. When the gas escapes, the ground above the release is disrupted (not unlike my explosions) and the sediment collapses downwards because the spaces between grains is now empty. What is left is a steep sided hole. These are, for now, much smaller - reaching 50-100 m in diameter.

A cenote, collapse of a cave roof, common in Mexico and important source of fresh water (image Wikimedia commons).
There are also less explosive ways of making holes in the ground. Karst topography, formed by the dissolution or erosion of rock (frequently limestone), is full of caves or other complicated shapes. There is quite a diversity of karst type topography on Earth, but most people will be familiar with caves and sink holes. I am interested in them because they can form small lakes or holes in flat ground and look a lot like maar craters. In areas of permafrost, where the ground is frozen all year, thermokarst (or the melting of that frozen soils) results in lots of roundish lakes in places like Alaska and Kamchatka. Some of these places are volcanically active, and since maars are formed by the interaction of rising magma and water it is common to have maars and thermokarst lakes side by side! Karst lakes and sink holes come in a wide range of sizes from a few meters across to a few kilometers diameter.  
Permafrost lakes in Alaska (Image Wikimedia commons).
Maar craters and permafrost lakes side by side on Seward Peninsula Alaska (image from Google Earth). The Devil Mountain lakes are two of the five maars in this image.
Kettle lakes, also a glacial feature, are formed by the melting of large chunks of ice that are trapped in the deposits left behind by a glacier. These features tend to be smaller, usually a few hundred meters or less in diameter.
Kettle lake from Isunngua, Greenland (image Wikimedia Commons).
There are also other volcanic features that have round craters, like scoria cones, rootless cones, and calderas. All these different features have similarities, but also differences. If the differences are not well preserved, or not visible from a satellite image it can make identifying what processes caused that particular hole in the ground very difficult.  One of the first traits to tell these features apart is size: maar craters vary from 100 – 5000 m across, with most around 700 m across. This means they are easy to tell apart from large impacts and large calderas, or tiny little kettle lakes and smaller sink holes, but that leave a whole bunch of overlap in the middle.  The best way to confirm how a hole in the ground formed is to look at the deposits and since I love field work, I don't mind this. Since I don't have enough time or money to check them all myself, I am grateful to the work done by previous geologists to describe these many processes ,and the resulting hole, in detail.  

Crater of Tambora Volcano, Indonesia is 6 km wide and over 1 km deep (Image Wikipedia Commons).

This list isn’t all inclusive because humans and animals have also found ways to produce round holes, but as they aren’t too common on other planets, I only have to worry about them when looking at Earth. Open pit mines make for very impressive holes in the ground. Although some of them are in kimberlite pipes, which have some similarities to maar volcanoes, the largest mines are for copper.
Lavender Open Pit Mine in Arizona. This is a common approach to extracting copper from the Earth's Surface. (Image Wikimedia Commons).
Kiberley Pipe in South Africa is a famous diamond mine and where Kimberlite gets its name (Image Wikimedia Commons).
Now as you look through images on Google Earth or fly over new terrain you can play the 'guess how that hole formed' game. I bet you didn’t know there were so many ways to make round holes in the ground!

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