Macrocosm Science Magbook: February 2008

If you look at the Moon, you will see how much it's surface is cratered. Some of these craters are caused by meteoroids and small asteroids that have impacted the surface. But the picture above is not of the Moon. It's somewhere closer to home. In fact, it is home.

Unknown to many people, the Earth has also been subject to the bombardment of space rocks in the past and most recently in Peru (more links at the end of this post) such that it also has many craters. But unlike on the Moon, which has no atmosphere, less erosion, and geological changes, the craters on Earth have gone through upheavals and changes that have left them diminished, except for a few like the Wolf crater in Arizona.

This diagram of Asia (left) shows the general distribution of craters made by asteroids that have impacted in the past. It is from the Earth Impact Database, which documents impact sites all over the world. It is interesting to note that most countries in Asia, including the Philippines are not marked. Does this mean there are no impact sites in those places? Possibly. But it is also possible that they haven't been found yet.

The Philippine archipelago does get it's share of meteorites, the most recent that's been authenticated is the one that went down in the Mountain Provice of Bontoc. This rare rock from space is now in the possession of amateur astronomer Allen Yu of the Astronomical League of the Philippines (ALP) and consultant to Pyxis Astronomy Education Services.

The Philippines also has numerous tektites scattered in what is called strewn fields. There is a number of ideas about the origin of tektites, and one of them is the asteroid-impact theory.

Philippine tektites have been associated to geological events as far away as the Zhamanshin crater in Kazakhstan (left). But this is only a theory. The source of the Philippine tektites, the dark, glassy rocks used as amulets by locals, is still pretty much a matter of debate. It is possible that they came from a much closer, even local, source. The latest studies point to an Indochina crater.

I tried using Google Earth to look for impact sites in the Philippines. I managed to find circular structures, but most of these have definite volcanic origins, like the perfectly round lakes in San Pablo, Laguna and Quezon province (below). However, there is one structure in the big island of Luzon that is sizable enough to be noticed and it looks like a crater (top).

This landscape ring structure is in the province of Pangasinan and while it appears to be relatively flat, it is nevertheless outlined by a river system much like the Manicouagan crater in Canada, shown here from a space shuttle photograph (left). However, this crater is much larger and has a broader waterway.

The Pangasinan Ring, though small, has tree-lined river banks and rice fields that define a circular structure, making it more distinct from above with a whitish outline. A smaller ring is at the upper left corner. If you look closely, you will also see a road that bisects the "parent" ring. In the middle is a town where a grove of trees south of the long building with the blue roof, mark the very center (below).

The strange landscape feature is very easy to miss at ground level, and may not even be apparent from an aircraft flying at 30,000 feet. But it is very visible from outer space and an astronaut in orbit would readily see it.

Is it an ancient dried up lake? Is it an extinct volcanic basin? Is it an impact site that's been missed all these years? Is it the source of the tektites (left, from the alvinwriter collection) that are plentiful in Bicol region, further south? Perhaps it's just an odd landscape formation with nothing to it but the quirky path of the rivers?

Rocel Pereira, a remote sensing specialist, says its possible that the area inside the ring is of a higher elevation than the land outside. "Notice, that the area inside the ring is well vegetated," she says. "But my inference is only based on Google Earth. My suggestion is to check out the elevation and contours in the area using topographic maps, which can be purchased from the National Mapping and Resource Information Agency (NAMRIA)."

The Earth Impact Database has set criteria for determining if a structure that resembles a crater is an asteroid impact site or not. Included here are shatter cones, multiple planar deformation features in minerals, the presence of high pressure mineral polymorphs, the existence of an impact melt sheet or dikes, and the presence of pseudotachylyte and breccias.

The Pangasinan Ring can only be classified as an impact site if many of the criteria for authenticating a crater-like geological structure are found there. The findings will also need to be verified by experts. While intriguing to think and speculate about, it is likely that this landscape feature is not an impact site, as suggested by Pereira's initial impressions. But if you have a personal opinion or a clue as to what this strange structure is, feel free to comment. It would be interesting to know what people think. If you like, you can even view and study the Pangasinan Ring yourself using Google Earth. You can download the program here.