# Craters by radiation/Laboratory

This laboratory is an activity for you to create or analyze a cratering. While it is part of the radiation astronomy course principles of radiation astronomy, it is also independent.
Some suggested types of cratering to consider include a lightning strike, a bullet shot into some material, a water droplet hitting the surface of a beaker of water, a subterranean explosion, a sand vortex, or a meteorite impact.
More importantly, there is your cratering idea. And, yes, you can crater a peanut butter and jelly sandwich if you wish to.

## Evaluation

Okay, this is an astronomy cratering laboratory, but you may create what a crater is. Another example is a volcanic crater.
I will provide an example of a cratering experiment. The rest is up to you.
Questions, if any, are best placed on the discussion page.

## Control groups

For my cratering laboratory example, I will compose a control group. You will need at least one too.
Control group (circle):
* As a first approximation, a crater in the horizontal plane of a rocky-object's surface is a circle for an object dropping, or falling vertically, or for rocky matter ejected vertically.
* The lower the angle of impact, falling, or ejection, from vertical (90°), the more elongated and ellipsoidal the crater is in the direction of impact, fall, or ejection.
From crater astronomy, the sources of a circular crater are many:
* a volcanic bomb falling nearly vertically from above,
* a rocky meteor falling vertically from above,
* a volcanic eruption from an approximate point source below the center of the circle,
* an ejection from an electric arc above the ground arcing at the center of the circle,
* an explosion above or below the center of the circle, like a volcano,
* a subsidence or falling below the center of the circle into the ground below, or
* a lightning strike directly from above leaves a circle and a hole where missing rocky matter either melted into less volume or was ejected like other explosions.
Special conditions:
* angularity away from a circle, when symmetric about the center is likely due to the structure of the rocky surface and beneath it.

## Evaluations

To assess your cratering experiment, include your justification, analysis and discussion. I will provide such an assessment of my example.

## Hudson Bay

The Nastapoka arc is a geological feature located on the southeastern shore of Hudson Bay, Canada.

### Justification

It is a near-perfect circular arc, covering more than 160° of a 450 km diameter circle.
"C. S. Beals (1968) suggested that the Hudson Bay arc is the remnant rim of a giant impact crater nearly 300 miles across, or comparable to Mare Crisium in size."
"Although extensive clean rock exposures abound, no shatter coning was observed."
"[S]uevite-type or other unusual melt rocks, pseudotachylite or mylonite, radial faults or fractures, unusual injection breccias, and other possible shock metamorphic effects [were searched for, but] [n]one was found."
Deitz' "negative results, however, probably do not disprove an impact origin for the arc, as even shatter coning, which is the lowest level shock indicator, still requires over-pressures from 20 to 50 kbars."
"[A]n Archean impact might be forever buried beneath the Proterozoic sedimentary cover which filled the entire basin inside the postulated rim."

## Hudson Bay-Evaluation

From the high degree of circularity of the Nastapoka arc for more than 160°, the area of Hudson Bay where the arc occurs is part of a crater. But, what happened to the western 55 % of the crater? And, of course, what is the origin of the crater?

### Analysis

Looking at the image above right, where Quebec is in red, there are the Belcher Islands, also in the image at lower left, almost within a chord connecting the two ends of the arc. Putting a circle holometer over the arc shows that the center of the circle is slightly north-north west of the Belcher Islands and does not touch them.
With respect to the paleoproterozoic orogens, all of the arc just beyond the chord connecting the upper and lower latitude tips of the arc is considered part of the continental margin for the Trans-Hudson orogen to the west.
The bay just north of that which is the bay of the Nastapoka arc is less circular and only about 90° of a circle. Up the same coast, the third inlet is also circular and about 80°.
The rock layers composing the Belcher Islands are not horizontal but may be dipping near vertically. If a cratering event occurred after the rock beds were tilted nearly on edge, the damage to the rock strata may have been more absorbed with less actual crater depth.

### Discussion

As at least half of each circle on the western half is missing, it may be that the craters are dipping downward to the west. The amount of dip is likely less than 10° so as to keep the craters from being ellipsoidal.
Craters initially circles dipping to the west result in a horizontal shortening of the W-E axis, while the N-S axis remains unchanged. The mountains of the eastern rim of the crater appear to decrease in relative elevation trending northward or southward around the arc from almost due east.
The more likely possibility is that the western halves have fallen or been depressed into the Earth approximately vertically. This would have the effect of maintaining circularity while causing a loss of the western half of each crater.

### Conclusions

The origin of the residual craters is concluded to be subsidence of the rocky surface, perhaps under the weight of glacial ice.
An origin for the craters themselves is considered as unknown due to a lack of evidence associated with causes.

## Hypotheses

* Many of the craters traditionally assigned to impact craters may be electric arc or discharge craters.