The background for this example is in Ehrlich's book, pages 103-110. These seven pages have a lot of information and data. One of the difficulties in doing this kind of analysis is deciding what the initial observation is and then figuring out what data is relevant to that initial observation. I will attempt to summarize these pages.The history of the earth is contained in layers of rock that have been deposited over long periods of time. Each layer represents a different time period and is consistent over the whole earth. Boundaries between these layers signify a change from one set of conditions to another. An interesting boundary is the K-T boundary which marks the end of the dinosaurs. There is a great 2017 PBS video that shows that there were dinosaurs before the boundary, but none have been found after the boundary. The PBS video also backs up the data given in our book.
Many theories have been proposed to explain why the dinosaurs disappeared. These include inability to adapt to climate change, a meteorite or a volcano causing dust, darkness, and general destruction all over the earth, disease, asteroids, etc. One thing to consider is whether the extinction was gradual or abrupt. It seems to have been abrupt, so we will only consider two theories for our example: was it a meteroite or was it a volcano?
There is a one centimeter layer of clay at the K-T boundary that has no fossils. An analysis of the clay showed that it contained iridium. Iridium is slowly deposited from space, but is very rare on the earth's surface. It was found that the concentration of iridium is much higher than expected if it is slowly deposited from space. It was also observed that the iridium was concentrated in the clay layer and not above or below it. This clay layer was found all over the earth and the high concentration of iridium was found in all samples; it was not specific to the initial place of study.
Iridium is much more common in meteorites than on the surface of the Earth. Determination of the amount of iridium in the clay layer showed that a meteorite with a 10 kilometer diameter could deposit that amount of iridium. A meteorite of this size would cause such a tremendous explosion that most of it would be vaporized and could then be carried around the earth and explain the uniform distribution in the clay layer.
Other rare elements were found to have abundances in the clay layer that agree with meteorites (and not with volcanoes).
This suggested that the dinosaurs were killed due to a large meteorite hitting the earth, but most scientists didn't believe it. Many explained the high iridium level as the result of volcanic action which would bring the heavier iridium (it had sunk toward the center of the earth due to its higher density) back to the surface of the earth.
In 1989, scientists found a crater that has the characteristics associated with the impact of a large meteorite. There are clear signs of high temperature and pressure from when it was formed, such as a unique crystalline structures. The crater has a raised ring in the middle, which is also characteristic of large metetorite impacts.
The thickness of the K-T layer is greater near the crater. This would mean that if a volcano was responsible for the high iridium in the clay layer, it would have had to have been a gigantic volcano at the site of the crater.
Since the critical observation is that there is an increased concentration of iridium in the clay layer, this will be used as the initial observation. The theories will explain why there is a higher concentration of iridium in the clay layer and not above or below it.
There are two theories:
- A meteorite caused the high iridium concentrations. (A1)
- A volcano caused the high iridium concentrations. (A2)
Preliminary Condition #1: Both of these theories satisfy over half of the criteria for a scientific theory and have merit as is shown by the following tables (the criteria is given in a previous section).
Criteria Condition
Met?Rationale for Meteorite Theory #1 yes The theory is not complicated and has no inconsistencies #2 yes No fundamental principles are violated. #3 yes This theory is consistent with the initial observation. #4 yes The theory applies to the area of investigation and does not go far beyond. #5 yes A competent, well-known researcher put forward the theory. #6 yes The proposer was aware of other theories and disclosed all methods. #7 yes The math used is appropriate. #8 yes No outside agenda is known.
Criteria Condition
Met?Rationale for Volcano Theory #1 yes The theory is not complicated and has no inconsistencies #2 yes No fundamental principles are violated. #3 yes This theory is consistent with the initial observation. #4 yes The theory applies to the area of investigation and does not go far beyond. #5 yes Competent, well-known researchers put forward the theory. #6 yes The proposers were aware of other theories and disclosed all methods. #7 yes The math used is appropriate. #8 yes No outside agenda is known. Preliminary Condition #2: Either theory could explain the higher iridium concentrations because both theories involve putting a lot of particles into the air (major meteorite collision or large volcanic eruption) which provides a means to spread the iridium around the earth and deposit it in the clay layer.
- Other rare elements match meteorite. (D1)
- Clear signs of high temperature, high pressure, and unique crystalline structure at the crater consistent with an impact. (D2)
- Center ring in crater. (D3)
- Thickness of layer increases closer to crater. (D4)
Meteorite Volcano Rare Elements yes no High Temp & Pres yes no Center Ring yes no Thickness yes yes
Since the meteorite theory leads us to expect four pieces of data and the volcano theory leads us to expect one piece of data, the meteorite theory is the best choice between the two theories.