Course Review

Into the Sunset

See caption.

A picture of a beautiful Sunset
Credit: R.B. Alley

We’ve been through a lot—a tour of some of the finest places in the world, and a “tour” of the science of the Earth. What should you have learned from all of this?

First, science is a successful human activity that allows us to do things we want by first learning how things work. Scientists come up with ideas, and then try very hard to prove those ideas wrong. Situations are found in which a new idea makes predictions that differ from those of old ideas. These situations are then created (experiments), and the outcomes show which idea predicts better. An idea that makes wrong predictions many times is discarded; an idea that makes correct predictions many times is accepted tentatively. It may be right, or just a good approximation, or just lucky. If we behave as if the ideas that make accurate predictions are in fact true, we are able to do things successfully. Many people believe that this shows that the successful ideas are in fact true, or at least close to being true, but there is no way to prove this.

Geology is the science of the Earth. Geologists study the materials that make up the Earth, and the processes that happen to them or have happened to them in the past. Much of geology is highly practical: finding oil, clean water, metals, and other valuable things in the Earth; warning people of geological hazards such as landslides, earthquakes, and volcanoes. Some of geology will become important; geologists are helping in writing the “operator’s manual for the Earth” to help us and the Earth live in harmony in the future. Some of geology is just plain fun—learning about the dinosaurs, for example. Much of geology is fundamental, and contributes to all of these.

The Earth formed about 4.6 billion years ago. It has a layered structure, with a mostly-iron core (which is solid in the middle but liquid outside of that), with a silicate mantle (but, as silicates go, with relatively much iron, magnesium and calcium and relatively low silica), and a thin silicate crust on top that is rich in silica, sodium, potassium, and aluminum.

Crudely, the heat of the Earth drives processes that build mountains, and the heat of the sun drives processes that tear the mountains down. Inside the Earth, radioactive decay makes heat, with a little help from other processes. Heating rocks softens them and reduces their density. The heating of rocks from below causes convection cells. Columns of hot rock rise from deep in the Earth, perhaps from the core-mantle boundary, to feed hot spots that erupt basaltic lava (higher in silica than the mantle) and produce volcanic chains such as Hawaii. Where a hot spot exists under a continent, the basaltic lava may melt some continental rock to become silica-rich and explosive, as at Yellowstone.

Most of the heat loss from the Earth is through roll-shaped convection cells higher in the Earth’s mantle. These reach into the upper mantle; the very upper mantle and the crust make the cold and brittle lithosphere, which floats on the softer rocks deeper in the mantle. Where the convection cells rise and spread, the rocks above are raised high and split apart. Basaltic lava leaks up through the cracks and hardens, making sea floor at spreading ridges in the oceans. Where a spreading ridge passes under a continent, the ridge splits the continent to form fault-block, pull-apart mountains separated by deep valleys, including the Sierra Nevada and Death Valley.

Ocean lithosphere initially is warm, but cools and becomes more dense with time. Eventually, it sinks back into the mantle at a subduction zone. The downbending at a subduction zone makes a deep trench in the ocean. Sediments may almost completely fill a trench near a continent, but trenches lacking abundant sediments are the deepest places in the oceans. Sediments scraped off a downgoing slab may pile up to make coastal mountains, such as those of Olympic National Park. Some sediments go down with the subducting slab, but are heated as they go down, and eventually melt to make andesitic lavas, higher in silica and lower in density than the basaltic sea floor. These feed explosive, steep stratovolcanoes such as Mt. St. Helens and Mt. Rainier. Sometimes, such stratovolcanoes form offshore rather than on the land, giving island arcs. When such an island arc, or another continent, reaches a subduction zone, the silica-rich rocks are too low-density to sink. The collision of an island arc or a continent with another continent is called obduction, and forms folded and thrust-faulted mountains such as the Appalachians, including the Smoky Mountains and Mt. Nittany.

Above all of this activity, the Earth is wrapped in a thin layer of air and water and ice that erupted from volcanoes over time. The sun heats the air, causing convection cells. Convecting air cools by rising and expanding, or by radiation, and this cooling causes rain and snow.

Rocks at the surface of the Earth often formed in the Earth under different conditions and are unstable at the Earth’s surface. Physical processes break these rocks into smaller pieces, and chemical processes change the minerals in the rocks to other types. Silicate rocks release ions that wash to the sea, and clays, quartz sand and rust that mix with organic materials (worm poop, etc.) to form soil. If loose soil and rock are too steep and wet, they slide or creep downhill in mass movements.

Rain that reaches the surface of the Earth is mostly returned to the atmosphere after being used by plants, with most of the remainder soaking into spaces in the ground to form groundwater. Groundwater eventually returns to the surface to feed streams; we humans intercept this flow in the ground with wells. Pollution of this groundwater is a common problem, and very difficult to clean up.

The rocks and water that are moved across or beneath the surface of the Earth feed streams. The characteristics of streams—whether wide and braided or deep and meandering, for example—depend on the debris supply as well as the water supply. If humans interrupt the usual pattern of water and sediment transport, we usually cause other changes (erosion, sedimentation, subsidence, etc.) that cause problems for us.

Glaciers are really cold-weather streams. Glaciers move water and rock from place to place on the Earth, and typically are more effective at moving rocks than are other mechanisms of sediment transport such as mass movement, streams, and wind. Glaciers have grown and shrunk many times over the last million years, covering as much as 1/3 of the modern land surface. The Earth’s surface in those formerly-glaciated regions is still dominated by features left by the glaciers. Many of our most beautiful landscapes, including Glacier and Yosemite, center on the works of glaciers.

Mass movement, streams, and glaciers wear down the mountains. These processes also deposit sediments. The types of sediments, their fossils, and other features contain a history of events at the Earth’s surface. Reading this history, we find that the Earth is very old. Annual layers in the very youngest deposits, including trees and ice sheets, record more than 100,000 years. Rocks beneath the trees and ice sheets contain evidence of old seas and deserts and mountains, which early geologists realized had required more than about 100 million years to form. Radioactive clocks tell how long the time gaps were in the sedimentary record and how much time was required to make the oldest rocks, revealing that the Earth is about 4.6 billion years old.

The fossils in the rocks show that offspring differ from their parents slightly; these differences affect ability to survive to have offspring, and the genetic basis of these differences is passed on to offspring in greater or lesser degree. Over long times, accumulation of these differences has led to changes in the types of living things on Earth (evolution by natural selection). At some times, catastrophic events such as meteorite impacts have caused mass extinctions; at other times extinction has been a gradual process.

In the geologic record, recent times would look like a mass extinction. Humans are using more and more of the Earth’s resources. Because these resources are not available to other species, individuals and entire species have disappeared. Humans are also changing the climate, and spreading pollutants across the globe. These changes are not yet catastrophic, and may never be, but humans will need to reduce our impact in the future if we wish to prevent more extinctions and possible catastrophic harm to ourselves. This must be accomplished through some combination of controlling our numbers, or controlling our impact per person.

Science has proven, over and over again, to be the best tool humans have ever developed for learning enough about the world that we can keep ourselves sufficiently fed, clothed, free of disease, and otherwise healthy so that we can worry about the big questions of what we are here for and how we get along with each other and stay happy. Geology is an important piece of the big picture of science. National parks have proven, over and over again, to be beautiful, enjoyable preserves of our shared natural and human heritage. It is our fondest hope that you will have a happy and prosperous future, that you will appreciate and perhaps even contribute to the benefits of science, including geology, that you will enjoy using the benefits of science to contemplate big questions, and that you will do some of your contemplation in your national parks. Pleasant journeys!!!


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The Geology of North American National Parks Copyright © 2022 by Dr. Richard Alley, Evan Pugh, and Sridhar Anandakrishnan is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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