Geology For Dummies. Alecia M. Spooner

you may find yourself overwhelmed by all the new words that you have to learn. Indeed, geologists have their own language for describing rocks, earth processes, and geologic features. But after you get the hang of what all the different words indicate, reading about geology is much less intimidating.

      Lamination vs. foliation: Similar outcomes from different processes

      Geologists describe characteristics of rocks with the intention of understanding the processes that formed those characteristics. For this reason, an observed characteristic, such as “layers,” will have different terms indicating what kind of process resulted in those layers. Here’s an example of what I mean:

       A rock with layers may be described as laminated. Laminations are thin layers formed by the accumulation of tiny particles that settle through standing water (such as at the bottom of a lake or pond). This layered (laminated) rock is a sedimentary rock.

       A rock with layers may also be described as foliated. Foliations are thin layers or sheets of minerals that are created by intense amounts of pressure and heat deep within the earth’s crust. This layered (foliated) rock is a metamorphic rock.

      

The layered characteristic of these rocks may seem similar at first glance but is actually the result of very different processes that occur under very different conditions on the earth. Closer inspection of the rocks (perhaps with a microscope) will reveal that the layers made of particles are different in appearance than the layers made of mineral sheets.

      Gabbro vs. basalt: Different outcomes from similar processes

      Another defining characteristic of rocks is their composition, or what minerals they are made of. However, rocks with the same mineral composition may have different names. Why? Geologists want to categorize rocks according to both composition and formation process. An example is the distinction between the rocks called gabbro and basalt.

      Both gabbro and basalt are dark-colored rocks with the same mineral composition. They both are formed by the cooling of liquid rock (magma or lava) into a solid. Gabbro is formed when the liquid rock cools underground, slowly, over a long period of time. Basalt is formed when liquid rock cools very quickly, at or near the surface of the earth where it is exposed to air or water.

The different formation processes create observable differences in the rocks — gabbro has large, visible mineral crystals, while basalt does not — but the composition of the rocks is still the same. By giving them different names, geologists can categorize a rock with a term that includes information about both composition and formation characteristics.

      

When studying geology it is very helpful to have a dictionary of geologic terms or dictionary of earth science handy to help tackle the immense amount of new vocabulary that you will encounter!

Chapter 3

      From Here to Eternity: The Past, Present, and Future of Geologic Thought

      IN THIS CHAPTER

      

Using catastrophe to explain geologic phenomena

      

Proposing origins for Earth’s rocks

      

Arriving at modern ideas about the earth

      

Using today’s processes to understand the past

      

Unifying theories with plate tectonics

      

Continuing to ask questions

      For many sciences, the foundations of modern thought were laid during Europe’s Scientific Revolution of the sixteenth and seventeenth centuries. During this time great thinkers began to redefine how they examined and understood the world around them. Although important advances were made in astronomy, mathematics, anatomy, and other sciences during this time, advancement of geologic science was constrained by a widely held belief that the Bible described an accurate age of the earth at only a few thousand years.

As a result, the road to modern geologic theories did not begin to be paved until later in the eighteenth and nineteenth centuries. In fact, significant insights into the earth’s systems are still occurring today. In this chapter, I describe the important theories presented along the way to our current understanding of the earth and its systems. I also describe the sequence of important hypotheses that led to a unifying theory of the earth (called plate tectonics theory), as well as some of the exciting areas of current geologic research.

Catastrophe Strikes Again and Again

      When early geologists looked at the mountains, valleys, and seas around them, they realized that something dramatic must have occurred to create what they saw. Because people believed that the earth was only a few thousand years old, the only way to explain what they saw was by assuming the occurrence of occasional catastrophic events, such as massive floods, volcanic eruptions, and earthquakes.

      

This early belief that earth’s features were created by a series of catastrophic events is called catastrophism.

      Geologic explanations involving dramatic, worldwide, catastrophic events were in sync with stories from the Bible, such as the great flood. In this way, catastrophism reconciled strong biblical beliefs with explanations of geologic processes that scientists now know occur over many hundreds of thousands (or even millions) of years.

Early Thoughts on the Origin of Rocks

      While catastrophism attempted to explain the creation of earth’s features, questions about the origin of earth’s rocks remained. Where did the rocks on the crust of the earth come from before they were subjected to the catastrophes that shaped and shifted them?

      

Two theories dominated early thoughts on the origin of rocks: Neptunism and Plutonism.

       Neptunists proposed oceanic origins. The Neptunist theory of rock origins proposed that all the rocks on Earth were created from sea water, having crystallized from the earth’s first oceans. (The theory is named after the Roman god of the sea, Neptune.)

       Plutonists proposed volcanic origins. The Plutonists believed that all Earth’s rocks originated from volcanoes and were then changed by pressure and heat into other rocks. (The theory is named after the Roman god of the underworld, Pluto.)

      

While neither of these theories accurately explains how all rocks are formed, each contains partial truths. Some rocks do precipitate from ocean water, some rocks do form from volcanoes, and many rocks are changed into other rocks through heat and pressure, as you discover in Chapter 7.

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