Humberside Geologist No. 14
Humberside Geologist Online
... but where do we start?
By Cyril Dutton
It can be said that when starting geology you should first learn that rocks are sorted into the three general types. But in my experience as a tutor in adult education, this method was seldom employed.
Well, before I go on to explain why, let me first mention that the three main types of rocks are classified as sedimentary, igneous, metamorphic, Frequently, geologists like to draw triangles. In this case they put one of the three words at each corner. Then you are asked to mention all the rock names you can think of and classify them under the three headings.
When we did start in this way, we had some samples of each of these three rock types and then explained some of their characteristics. But you will be wondering why this method was not always used in my courses, like playing a gramophone record. Well, the most important reason was that more than 50% of the students from one year enrolled for the following year.
So each course had to offer something fresh -- that was necessary. Working in this way it was proved to my satisfaction that one need not slavishly follow a fixed rule. That you can start to learn geology at any of a number of points. And it was far better for me also to write new courses each year to keep myself fresh.
For example, one year I ran a course on Rivers. Both newcomers and experienced students enjoyed it very much. The whole year was devoted to rivers, glaciers, lakes, estuaries, and so on. Many field meetings were included and all related to the main subject. For one practical exercise we measured the rates of flow in different parts of the river and studied the flow patterns.
On a four-day field meeting we followed a river from its source upon an igneous mountain to its estuary. We observed the differences in the rock types as well as the changes in landscape at various points along the way. It became a practical demonstration of one side of the well-used triangle: how igneous rocks became sedimentary rocks. And how of course they were transported.
The triangle has its merits. By adding arrows on each side you can indicate in a classroom that igneous rocks change into sedimentary rocks and how sedimentary rocks are changed into metamorphic ones. All good basic stuff. But travelling from a mountain region along a river valley to the seashore we watched the changing panorama of the countryside, saw how the rocks changed from igneous rock to sedimentary and how the river shaped the valleys and the estuary.
Along the way the river passed through three glacial lakes. And we saw how each became a sediment trap. It seemed almost impossible for any sediment to reach the estuary, let alone the sea. At one spot we proved that certain rocks on the river-bed had tumbled down the hillside --- they had not been transported by the river.
And what was the proof we found?
In addition, we could say for sure that these pebbles and rock fragments had tumbled and were in place at least prior to 150 years ago! How did we do that? Later in the day, when we finally reached the estuary we began to wonder how so much silt had actually reached the sea from the mountains. And we found that some of the sand had been returned from the sea. Now, how did we prove that?
Of course, not all the sediment had been trapped by the lakes. Much had been carried by the glaciers and dumped in the estuary. Or even in what is now the sea. But it was not the sea then! Maybe I've identified enough for you to realize that such a course on Rivers could include some basic geology, and maybe do it in a more graphic way.
Other courses were based upon other topics. But naturally, we all have to learn about rocks one way or another, at one time or another. But what we proved was that a starter can start at any one of a number of places -- as long as it is simply taught.
While talking about rocks, which came first? And which came second and so on? Well, since all rocks had to derive from mantle rock, the first on the Earth's surface had to be igneous. As the mantle peridotite became partially melted, it produced basalt. And as basalt was partially melted, andesite, and then granite were produced.
As these were eroded into pebbles and then into sand, sedimentary rocks appeared. At first unconsolidated. Soft and wet layers which could be folded with case. Then de-watered and hardened. And maybe buried and heated in due course. The increased heat and pressure changed them into metamorphic rocks.
But igneous rocks do not need to be changed first into sedimentary rocks to become metamorphosed. An igneous rock (for example granite) can be metamorphosed directly without passing through an intermediate stage of being a sedimentary rock. It can be heated and pressurized just the same.
Many tutors like to illustrate this with the triangle on the blackboard. (Well, in many cases we have used a white board, writing with a black felt pen. One can learn just a quickly. And one does not inhale chalk dust!) An arrow on the side from igneous to metamorphic can indicate this route.
Space does not permit mentioning the other courses used for starters. But new ways of starting were demanded all the time since some students had done 15 to 20 courses with me, and the newcomers had to be welcomed as well. Such is the value of originality. Don't you agree? Away with gramophone records! Start anywhere!
(c) Hull Geological Society 1999 + 2007