[SOUND] [MUSIC] If you look up at the night sky and look at the moon, you're looking at a landscape that's been there for billions of years. Each crater, each ridge, each sea on the surface of the moon is a feature that formed back when the Earth was very, very young. But if you look at the Earth, the surface that you're seeing is something that's a much younger feature. It's a dynamic place. The evidence of that is all around. The volcanoes that erupt, the earthquakes that shake the surface of the Earth, the mountain belts that build. All of these features are a consequence of the fact that our planet is dynamic. Geologists have realized that the Earth is in motion for a long time, but had no way of understanding what caused those motions or how those motions operated. Now there are a number of older ideas that people tossed around for many years. For example, before the 18th century many cultures envisioned that supernatural beings, or supernatural processes were responsible for earth motions. For example, in ancient Japanese culture, it was thought that there was a gigantic catfish under Japan, who's wriggles and wiggles shook the ground and created earthquakes and uplifted mountains. Another point of view was that the Earth with all of its dynamic features formed the way that we see it. With mountain belts in place, continents in place, all the various features that we see today in place, and that they've simply been active for the last few thousand years. But in the early 19th century, late 18th century, geology started to come to be as a science and one of the earliest realizations of the earliest geologist was that physical processes that we observe today can be use to explain geologic features at form in the past. In other words, the present is the key to the past and this simple statement has been turned effectively into a principle known now as the Principle of Uniformitarianism. It was first proposed by James Hutton, the first geologist in many people's minds, who lived in Scotland during the Age of Enlightenment. Now, if you keep the Principle of Uniformitarianism in mind, then processes that we see today are responsible for the uplift of mountains, for the motion of faults and for the activity of volcanoes. So when people realize this fact, they began to question, or ask questions, about how and when did individual geological features come to be. When did the mountains form? When were volcanoes active in the past? And there came to be a realization that the Earth had a history. That processes happening over long periods of time were responsible for what we could see today. So the next question to ask is, what drove these processes? And this became the subject of a discipline in geology called tectonics. And that word come from the Greek word tecton, which means to build. So, when we talk about tectonics, we're talking about the processes responsible for building regional scale to continental scale geological features. Until the 20th century, geologists threw around many ideas about tectonic processes, none of which actually turned out to be correct. Three of the most popular ideas that were held, prior to our modern way of thinking, was first of all, that the earth had mountain belts. Because since it's beginning, there have been long-lived cracks, or weaknesses, in the crust. And, at times, molten rock pushed its way up through these cracks and uplifted mountain belts at the surface. Another idea that was popular for a long period of time was the concept of earth contraction. People envisioned that the earth was like a hot baked apple, just taken out of the oven. Which, when it undergoes cooling, shrinks. And as it shrinks, it's skin wrinkles up. And those wrinkles were thought to be analogous to mountain belts. That's called the Earth contraction hypothesis. Finally, another school of thought, recognized that, in many cases, mountain belts were associated with places that prior to the formation of the mountain belt, had been regions where sediments had accumulated to great thicknesses. So it was thought that there was some kind of process where a deep, deep trough, called a geosynclines formed, and that somehow, after awhile, this geosyncline began to contract horizontally, and began to melt at it's base. And as a consequence, molten rock rose up to the surface. And the sides of the geosyncline squeezed together. And the result was to form a long welt, which was considered to be a mountain belt. One common thread in all these ideas was what's now referred to as the Fixist point of view. The idea that the map of the Earth is pretty much fixed over geologic time. That continents have always been where they are and that oceans have always been where they are. As we'll see, we now know that this idea is fundamentally wrong. [MUSIC]
