How do You Know...?

How do you know that Easter Island wasn't wiped out by a tsunami?
asks Rae Ann in this comment.

There are many similar questions. How do we know that the Universe is expanding? How do we know that the Earth is more than four billion years old? How do we know that Finland was covered with ice 25,000 years ago? How do we know that human activity is increasing the amount of CO2 in the atmosphere? How do we know that Africa and South America were joined two hundred million years or so ago? How do we know what the sea level was 500 years ago or how much CO2 was in the atmosphere 200,000 years ago. How do we know, for that matter, that the Earth is round and that the Moon isn't made of green cheese?

In each case, even the last, there is a chain of evidence that leads to the deduction. In the case of Easter Island, and Rae Ann's question, the answer is easy. The great part of Easter Island is more than 50 meters above sea level. Portions are more than 400 meters above sea level. A tsunami on a scale to destroy all the trees of the island would have left a huge amount of evidence, and not just on that one island. The occasional giant tsunamis produced by the volcanic Hawaiian islands are of that scale, and they have left profound evidence hundreds of thousands of years later. A tsunami of that scale in the last 800 or so years could not have escaped leaving much more obvious evidence.

This kind of evidentiary chain is the basis of everything we know about the past. Some deductions about the past are easy, like the one above. Others, less so.

It must be fifty years since I last read Tarzan and the Jewels of Opar, but I still pretty clearly remember the high-priestess saying to Tarzan: "the more you know, the less you believe." She was talking about her religion (which demanded Tarzan as a human sacrifice), but so too it is with geology - if you know nothing, you are free to believe any damn fool thing you like. More knowledge constrains your options. That is the burden of those of us who would live in the reality based community.

In the case of Rapa Nui, AKA Easter Island, there is solid evidence of forests of large palms when humans arrived, and quite solid evidence that they were all gone when the first Europeans arrived.

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