Understanding evidence for craters that shaped Earth's Geology
Seeing cratering in Lineaments
To understand the geological history of our planet, we need to see and understand all of the evidence available to us. Part of that evidence is the recognition of lineaments in the landscape visible in satellite pictures available as Landsat images used by Google Earth. Gravity maps, such as Global Gravity Anomaly, downloaded from Scripp’s Institute of Oceanography as “Marine Gravity Anomaly,” an overlay for Google Earth, provide valuable additional information.
Figure 1: The Vredefort Impact Crater of South Africa is considered to be one of the largest and oldest recognized impact craters on the Earth’s surface. It makes a pattern of concentric circles that are hard to miss on a satellite image. These implied lines in the landscape are referred to as linears, and the partial circles they form in this image are called “circular” or “arcuate lineaments.”
Figure 2: Some of the other craters that show up in this Landsat view had a major effect on the visibility of the Vredefort crater. Blue circles are three rings of the Vredefort Dome crater. “D” is the dome and “C” is the collar structures. The first blue ring is drawn just outside the collar. Blue 3-ring shows the strongest with mountain arcs showing to both the west and the east. Yellow circles 1-4 are the primary reasons the collar terminates to the southeast. Yellow 5-ring is most likely an earlier, larger crater which maybe more responsible for defining the Witwatersrand than the Vredefort Dome since it came earlier and was apparently larger.
Figure 3: Many of the linears show differences of topographic features. When the topography is viewed from an airplane, even when the elevation has been exaggerated as in this image, the circular pattern is not near as evident, because much more than just large topography produces visual linears. Any true pattern in these small features may only be observed when we are directly overhead. Other patterns can only be perceived when our field of view is much larger.
Figure 4: But, with the Vredefort Structure, careful geologic study on the ground has verified a lithologic pattern that is very consistent with a circular impact origin. While the Karoo Supergroup covers the southern half to the crater it was not responsible for the lower elevation of the ring. Instead, the yellow 1-ring of Figure 2 swallowed the heat signature’s energy imprint never allowing the ridge to thrust-up.
Figure 5: This is a very different map of the Vredefort area. Known as Global Gravity Anomaly Map overlay for Google Earth it is based on slightly different strengths of gravity as measured by satellite altimetry. Generally referred to as Free-Air Gravity Anomaly, the red areas measure a little higher gravity and the blue areas measure a little lower gravity. The differences in the pull of gravity is thought to reflect differences in topography and thickness of crust, but as the visible pattern differs markedly from a simple topography map, there must be something else involved.
Figure 6: The three pairs of circles I have drawn here relate to three different guesses as to the original crater size for the Vredefort Crater. The inside red circle is the diameter (160km) recognized by the Earth Impact Database. The blue circle is the diameter (>300km) given by Wikipedia, while the white (500 km) given by this author as the most visible in the Gravity Anomaly readings. Yet, all three circles point out two good principles.
Figure 7: 1) The colored circles were located in Landsat images, where dark shadows of ravines are more pronounced than ridges at this resolution. These more pronounced circular lineaments correspond to low gravity readings. 2) The inner, thin white circular lineaments, correspond more with higher gravity readings on this map. Yellow arrows indicate points of pronounced gravity change consistent with a circular impact lineament.
Figure 8: If we apply these same principles to all of southern Africa we will note some interesting results. The southern half of Africa has a shape we all recognize in a standard satellite image. But, look at it in Global Gravity Anomaly…..
Figure 9: Can you pick out the continent shape in this Gravity map??? If this is a new tool for you, take a few minutes; flip back and forth between the pictures until you can see the outline of southern Africa.
Can you pick out a near circle of red to yellow in the southern Africa region? Let’s focus on that circular area.
Figure 10: I call it the Mabule Circle, named for a small town near its center. It is seen here in Free Air Gravity Anomaly map published by BGI, France. You may also notice the name Vredefort just below the word Mabule. The Vredefort Crater is located within the Mabule Circle. Based on the Vredefort Crater being an accepted crater, but its gravity pattern showing less distinct changes in the gravity readings, the red circle here shows a very strong circular lineament that could be a much larger circular crater.
Figure 11: While the red expression of the circle has been interrupted at the top and bottom, it has not been totally obscured at either place. This is partial occlusion at these points. It shows something has interfered or covered the expression in that area and not that this isn’t a complete circular expression. We will discuss partial occlusion more shortly.
Figure 12: Here is the Mabule circle with a much heavier line that might help you see it. When I am locating circles, I prefer a lighter line, as my eye is drawn to the heavier line, and I do not feel as able to objectively evaluate its presence. The Mabule circular lineament is one of the strongest on this planet. So, if we are going to start seeing these lineaments so we can understand them, this is a good place to start.
Figure 13: What can we really see? Looking at the same circle on a Terrane Map, the circle does correspond to many mountain ridges, gullies, rivers, and other topographic features. Take your time. Examine the circle carefully. You will only start to see these circles as you take the time to train your eyes.
Figure 14: Bouguer Gravity Anomaly is the Free Air Gravity Anomaly adjusted for the surface rock above sea level. It is as close to a gravity reading for only the crust that we can get. The Mabule and Vredefort crater and their centers are indicated with a light white circle for the crater rim and a place marker for the center. Neither show-up quite as well as in Free-Air Gravity Anomaly.
Figure 15: Yet, several incomplete circles show up in the Bouguer Gravity Anomaly. There are three marked with several black dots each. One overlapping the Vredefort circle to the northeast, a second overlapping to the southeast, and a third completely separate to the west.
Figure 16: Could you find all three of these yellow rings? Take time to flip back and forth between these images to see if you can recognize the indicators I used to identify these circles. All 4 of these smaller craters would have much greater differences in deep crustal lithology which the Bouguer map reflects rather then mountain peaks that the Terrane map reflects for the Mabule crater. There are also several other circles that can be seen here. Are all of these circular lineaments?? The next image will show some additional circles. As each of these different maps show different things, this is a strong reminder that we need to look for circular lineaments in any map we can see them in, as they find expression in different forms under differing conditions.
Figure 17: Check out the orange circles. Flip back and forth, again. Does knowing where to look help you see them?? This still is not all of the circles that can be recognized. In someways, eliminating some of the peaks leave the remaining curves more evident.
Figure 18: Other rocky bodies in our Solar System seem to be covered in craters, why don’t we see them on the Earth?? One of the excuses given is the large part of our planet is covered by water. But, with Google Earth’s inclusion od NOAA data, this is no longer a problem. This is the Eastern Pacific; if you look closely, maybe you can just make out some linear patterns running east and west through the dark blue ocean. While these aren’t circular craters, maybe they are still related to impacts.
Figure 19: Here are some of the more visible lines/ linears. Linear patterns are something we can see all over our globe. Some are circular, but many appear to be straight or nearly straight lineaments.
Figure 20: Here is the same area after converted to black and white and having the contrast raised. We can see those same linears so much clearer. We must also learn to ignore the myriad of diagonal linears going every which way. These “tank tracks” are a factor of extra data gathered in standard shipping lanes.
The most obvious lines are thousands of kilometers long. Of course the question arises, “Where did they come from?? What is their meaning?” Plate Tectonics has explanations for a few of these lines, suggesting they are where the plate’s movement slowed down and stalled. But, they are too regular for such a random explanation. S.P. Gay, in an address to the American Association of Petroleum Geologist in 2012 said, “To not attempt to understand lineaments is to ignore one of the most common and basic features in geology.” My goal here is to help you start to see circular and straight lineaments, and hopefully as we see more of them; the patterns will emerge and we can start to understand where they come from. Where are their points of shear?
Figure 21: This is the Global Gravity Anomaly Map of the exact same portion of our globe. In gravity maps, blue represents lower gravity readings and red higher readings. Notice while the myriad of diagonal lines all vanish, yet, the east west lines become very defined as dark blue lines. We will notice one more thing, details do not generally show up very well on Gravity maps. Gravity maps used here are lower resolution then Google Earth Landsat images. Actually, the detail possible in Landsat images is pretty remarkable. We will look at this again later. Be aware, I don’t believe any source for these linears can be found that explains them as well as impacts. But, the basic problem for many people is seeing them in the first place.
Figure 23: In seeing lines, we are dealing with a science called “Spatial Perception.” Is there a pattern in the words in this image?? Is it random or planned?? Does there seem to be a purpose to it?? Can you identify that possible purpose?? Just because you do not know a purpose, it does not mean we should ignore that a pattern is there. Because you can not yet answer the final question does not mean that we can ignore the previous questions. Any non random pattern reflects some cause or purpose. Energy was expended to make any pattern. Complex patterns do not happen randomly.
Figure 23: Look at this small white area. There are two short lines or linears like we might see on a map in the form of a stream or a rock outcrop. Look closely, can you see a pattern in them?? Are you maybe not sure??
Figure 24: If we add this additional linear, does it help?
Figure 25: How about a little more information. Maybe now we are starting to see a pattern. Maybe the top three are a line. But, then what about the two towards the left bottom. Sometimes we cannot see a pattern because our field of view is not large enough.
Figure 26: Sometimes just a little more information will give a much clearer idea of the pattern, if there is one. We are starting to see three lines. Maybe this is a triangle. But, then we must ask the question, is this the entire pattern?
Figure 27: But, sometimes it takes a whole lot of additional information before we can apply Spatial Perception with confidence.
But, just because we can see a pattern, the importance is not in what we can see. The importance is in the purpose of the pattern.
Figure 28: Spatial Perception allows us to mentally fill in and complete patterns. There may be multiple solutions, and we may have to follow each one through to its logical end to test whether it is likely or not. Using Spatial Perception we interpret the lineaments into the pattern we think most likely, and share that interpretation with others.
Figure 29: This is my interpretation of this pattern, colored to make it more visible. Seeing a nonrandom pattern gives us assurance that a purpose does exist. We may not understand the purpose. We may have all kinds of questions about this figure. Is there a white triangle in the pattern?? Is there a black triangle?? Are there three black circles or are they “Packman” figures?? If we do not admit to the white triangle, then the black triangle and black circles can not exist, and we are looking at 6 repetitive, but strange geometric shapes. They would still not be random. There is repetition of elements in a consistent arrangement. There is a type of radial symmetry at three places which gives us repeated elements. And, there are parallel elements that show a common purpose exclusive to this arrangement. All of these are interpretations about the pattern, but first we needed to determine that it existed in the sketchy linears we started with.