Where was the rock found? Was it close by? Are there other volcanic rocks in the area? Or did someone bring it into the area?
By itself a piece of volcanic rock is not evidence of a volcano. The rock has to be placed in context for it to be evidence of a volcano nearby.
This idea is easy to understand, but sometimes very hard to apply. Here I will give a few real examples of observations that when taken out of context can be considered evidence for a particular conclusion, but when put back into context do not support the conclusion.
In an article entitled Paleoindian ochre mines in the submerged caves of the Yucatán Peninsula, Quintana Roo, Mexico published recently in Science Advances, the authors were describing their work in a system of caves in Mexico. They found evidence of humans using the caves to mine ochre for pigment and paint. One of the things they had to determine was how long ago humans were using the caves.
They found charcoal in the caves near where the mining had taken place. If the charcoal was left there by the people who were mining the ochre then all they would have to do is use carbon dating to determine the age of the charcoal. The age of the charcoal should tell us when the mining took place. It would be easy to assume that the charcoal came from the people who were mining the ochre, but the full context must be taken into account before we can accept that conclusion. The age of the charcoal may not be evidence for the time when there was mining.
As the authors noted,
"Charcoal is a difficult medium for dating in the submerged caves of Quintana Roo because it may be produced by forest fires, then deposited by wind and rain, and remobilized repeatedly by floods during major storm events or, ultimately, by rising sea level. Archaeologists have often interpreted instances where charcoal concentrates in small catchment basins and litters cave floors as prima facie evidence of human activity. However, the mere presence of charcoal concentrations is insufficient to make this inference. Before submerged-cave charcoal can be interpreted as anthropogenic, it is necessary to establish that the sample materials are artifacts, that is, that they are representative of human activity and distinct from the products of natural processes."In other words, the presence of charcoal is not automatically evidence for human activity. The charcoal must be considered in context.
In this case the authors could argue that the charcoal most likely came from human activity, and human activity at the time the ochre was being mined. To make this case they considered the broader context to see if there were other ways that the charcoal could get there, or if the cave formations showed that the charcoal had been there for a long time. Some of the charcoal was covered over by flowstone, which allowed the authors to get a rough date for when it was left there. This dating agreed with the carbon dating.
Only after all this could they use the carbon dating as evidence for when the mining took place. Before it was evidence for their conclusion they had to consider the evidence in context.
Now a second example. In a recent meeting at work we were discussing ways of detecting starburst driven galactic outflows using X-ray observations of galaxies. These outflows should produce strong X-rays which are easy to detect. The problem is that the things we were looking for are not thew only things that produce X-rays in galaxies. Just detecting a strong X-ray source is not evidence of a galactic outflow.
Before we could consider it to be evidence for what we were looking for we had to look at the context and see if other things could produce the X-rays and rule those out first. Only then we could use the X-ray detections as evidence for our conclusions. Just like the archaeologists with the charcoal in the caves, we had to consider the context.
Now a final example. In a discussion I had about the age of the earth, the person I was talking to brought up polonium halos as evidence of a young earth.
Polonium is a radioactive element and if polonium is mixed with melted rock it will collect inside micro-zircon crystals inside the rock. As the polonium decays the released radiation will "burn" the rock around it. This leaves a "halo" of scorched rock around the zircon crystal that held the polonium. These halos are very small and can only be seen under a microscope.
Young earth creationists argue that these scorched halos around zircon crystals are evidence of the rapid formation of the rock instead of the rock slowly cooling to its present state over thousands or millions of years. Their reasoning is that polonium has a very short half life (138 days) so the only way it can be in the zircon crystals is if the rock formed and cooled into its solid state in a matter of days. This fast formation would allow for the polonium to last long enough to freeze in the rock, and then burn the halo as it decayed.
There are a few problems with this, and all involve the context of these halos. First, assuming the source of these scorched halos is polonium, that would only show that the rock formed quickly, but it would not tell you how old the rock was. In order for the halos to be used as evidence for a young earth they have to show that the rocks are young, not just that they formed quickly.
Second, while polonium is highly radioactive, it isn't the only radioactive element. There are other elements with much longer half lives that can still do just as much damage to the surrounding rock. Most notably, uranium. Uranium is also found in zircon crystals. There is no proof that the scorched halos around zircon crystals were caused exclusively by polonium.
Third, even if the halos were caused exclusively by polonium there is more than one way to get polonium in the crystals without rapid formation. It turns out that polonium is a daughter product of uranium. In fact the only source of naturally occurring polonium is in rocks and ores with uranium content. So if the damage was done by polonium it could still have been done over millions and billions of years as uranium slowly decayed into polonium, and polonium quickly decayed into lead.
Young earth creationists argue that these halos around zircon crystals are evidence of a young earth. But when considered in context they cannot be used as evidence of rapid formation or especially young rock ages. In this it fails to be evidence for a young earth.
In these three real examples, before something can be considered evidence for a certain conclusion, it must be considered in context and scientists must ask the questions,
- How did it get here?
- Is there another possible source for it?
- Does its presence make sense in its environment?
- Does it actually support my conclusion?
If we do not ask these basic questions then we cannot claim that something is evidence for our conclusions.