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Unit 3.1 - Geology and Geomorphology

Timothy White (Pennsylvania State University)

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The basic concepts of geology will be considered to address the widely ranging textures and compositions of rocks and sediments formed in a wide range of environments. These variations in turn can affect soil formation and many related Critical Zone processes and architectures.

This unit requires substantial reading to cover basic concepts of geology: the rock cycle, plate tectonics, geologic time, erosion, weathering, and deposition, so that students have a firm grasp on how geology relates to and controls CZ processes. This background knowledge is accessed through a review of web sites and a scientific papers. An in-class activity uses the U.S. Geological Survey's National Geologic Map Database to identify resources for understanding and classifying the geology of a region.

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Learning Goals

By the end of this lesson students should be able to:

  • Recognize the three rock types and the processes by which they form.
  • Describe the rock cycle and plate tectonics.
  • Distinguish between weathering and erosion and their relationship to soils.
  • Recognize the Critical Zone as a very dynamic environment.
  • Recognize the differences between bedrock and surficial geologic maps and what each illustrates, and analyze site-specific information to evaluate the CZ at a study site.

Context for Use

This is the first of two units devoted to a study of the links between geology and geomorphology and the Critical Zone. It was designed to meet the needs of an upper-division undergraduate or graduate-level seminar class meeting twice a week for 75 minutes. Each unit should take two class periods.

Description and Teaching Materials


Parent material is obviously a critical component of the pedogenic system and the Critical Zone—all the other state factors of soil formation act upon the parent material to create soil, and thus the original, unweathered composition of the parent material plays a unique role in the weathered product.

The geology of a region controls the parent material available for pedogenesis. Parent material can be represented by a wide range of rock types of varying geochemical compositions, it can be lithified (hardened) or unconsolidated, and it can be relatively recent in age or billions of years old. The geologic setting of a region, past and present, also determines whether or not mountains or valleys exist, thereby exerting a first-order influence on the topographic setting in which CZ processes function and soils form. Furthermore, the geologic setting will determine whether parent material lies within an active setting, perhaps only recently exposed to pedogenic processes; or within a more stable setting in which soil formation has continued unabated for hundreds of millenia, subject only to the vagaries of climatic and biotic change.

The landforms and topography of a region control a variety of Critical Zone processes. The plate tectonic setting of a region plays a first-order role in determining the topography of a site and thus in determining whether soils develop and accumulate or are subject to erosion. The overall geologic setting determines other aspects of environmental control on the CZ. Consider the following questions throughout this module:

  • Can unique depositional environments consist of characteristic landforms? If so, what can those landforms show us about Critical Zone processes?
  • Do soils developed on unique landforms or landscape positions have unique characteristics?
  • What is the slope and aspect of a site and how do these relate to solar heat budgets, vegetation, and CZ processes?

Geologists use and create a variety of maps to display the results of our work. Among the simplest of these are maps of bedrock and surficial geology. Bedrock maps display the various rock types in a region. They are best understood if you can imagine stripping away all of the unconsolidated sediment, soil, and vegetation from the land surface down to solid rock. Bedrock maps do not indicate that XYZ rock type can be found at ABC locale; instead they show what exists in the subsurface beneath the soil and sediment and vegetation. In some places, particularly out west, rocks do exist at the surface! In that case, the bedrock map shows the rock type at the surface.

Surficial geologic maps differ from bedrock maps in that they display the sediments mantling bedrock. In some places, no sediment exists and, therefore, the surficial map may show bedrock. But in others, for example in northeastern and northwestern Pennsylvania (where multiple glaciers dumped their sediment load over the past several hundred thousand years), surficial geologic maps differentiate between the various materials moved into the region by glaciation and may even differentiate between deposits of different ice ages.

Lesson Plan

Unit 3.1 - Day 1 (75 minutes total)

Pre-class Individual Reading (this combined with Group Reading, see below, is a major component of this unit so be prepared to devote 4-6 hours to this review).

Pre-class Reading Groups

  • The class will be divided into 4 different reading groups
  • Each group will be assigned an article(s) to read.
  • Each group will make a brief (<10 minute) report to the class summarizing their reading assignment.
    • Articles should be reviewed prior to class to capture the main themes and topics discussed and how these relate to this class, module and entire course. As the course proceeds, students will begin to tie concepts learned in earlier lessons to the ongoing lessons and in the process will make the links that are at the heart of this very interdisciplinary science.
    • Research articles should also be reviewed prior to class using the following generic scientific analysis framework and any specific questions provided.
  • Each group will pre-read and meet outside of class to prepare their brief presentation on one of the 4 scientific papers listed below.
    • focus on concepts of general and broad importance, as well as topical questions specific to each article.

Pre-class reading articles:

  1. The Critical Zone can be thought of as a "feed-through reactor" in which physical denudation and erosion are closely tied to chemical weathering.
    • Anderson, S., von Blanckenburg, F., and White, A., 2007, Physical and chemical controls on the Critical Zone, Elements, v. 3, n. 5, p. 315-319. (Google citation for un-official web version)
    • Consider the following questions as your read the article:
      • Do you think all soil parent materials were subject to erosion and deposition?
      • Are some soils the result of weathering of bedrock in place, that is not subjected to erosion and deposition?
      • If so, how do soils developed directly from bedrock differ from soils developed on unconsolidated material, if at all?
  2. Critical Zone (and soil) formation can be greatly affected by landscape position, particularly in actively uplifting systems.
    • Goodfellow, B., Hilley, G., Webb, S., Sklar, L., Moon, S., and Olson, C., 2016, The chemical, mechanical, and hydrological evolution of weathering granitoid, J. Geophys. Res. Earth Surf., 121, 1410-1435, doi:10.1002/2016JF003822.
    • Consider the following questions as your read the article:
      • What is the progression of chemical weathering in an uplifting system?
      • What links exist between rock chemistry and physical properties as a function of weathering?
      • How do changes in rock physical properties feedback on chemical weathering?
  3. Biota exert a fundamental role on landscape evolution and development of Critical Zone architecture.
    • Dietrich, W., and Perron, T., 2006, The search for a topographic signature of life, Nature, 439/26, 411-418, doi:10.1038/nature04452.
    • Consider the following questions as your read the article:
      • How does life impact CZ architecture on long and short time scales?
      • What biotic mechanisms can be linked to various processes of erosion?
      • Do slope-dependent versus water-flow processes produce different landscapes?
  4. Bedrock disintegration into erodable soil declines with increasing soil mantle thickness.
    • Heimsath, A., Dietrich, W., Nishiizumi, K., and Finkel, R., 1997, The soil production function and landscape equilibrium, Nature, 388, 358-361.
    • Consider the following questions as your read the article:
      • What is the relationship between soil depth and hillslope curvature?
      • What is a cosmogenic nuclide and how might one be used to study soil production rates?
      • Why might the thinnest soils and highest soil production rates be found on ridge tops?

Activity 1 - Class Discussion (30-60 minutes with the remainder of the class time available for review of the website material, or general catch up from previous lessons). Consider the aforementioned questions and,

  • If all state factors of soil formation except parent material (e.g., rock or sediment type) were held constant in a select locale, do you think similar or different soils would be developed on the different parent material types? Why?

Unit 3.1 - Day 2 (75 minutes total)

Activity 2 - National Geologic Map Database (NGMD) Activity

  • Introduction (5 min) to USGS NGMD:
  • Class demonstration (20 min) Activity-BedrockMap-Intro (PowerPoint 2007 (.pptx) 18.2MB Mar31 17). View the activity worksheet below for important relevant information, and for an example of how to move through the classroom demonstration.
  • Each student should select a region of interest and produce a list of available resources and if possible, an example of a bedrock and surficial geologic map. (50 min)

Teaching Notes and Tips

The goal of this lesson is two-fold:

  1. To begin to introduce students to the complex geological processes that control the development, architecture and many processes in the CZ.
  2. To demonstrate to students the wealth of geological information that is available for the United States---including for very site specific studies---this information can guide fundamental understanding of the CZ at a given locale.

To accomplish the first goal, begin by assigning readings available at URLs listed below under references and resources: General geology, plate tectonics and the rock cycle; Geologic time; and Weathering, erosion and deposition. The information contained in these web resources is meant to convey a very introductory level understanding of geology to those without a geology background - students can expect to spend four to six hours reviewing this information. In addition, the scientific articles introduce students to concepts of the CZ as a "feed through reactor"and can be greatly affected by landscape position; that bedrock disintegration into soil varies with soil mantle thickness; and, that biota exert a fundamental role on landscape evolution and development of Critical Zone architecture. .

The combination of the introductory geology readings and these articles should form the basis of an at-least thirty-minute, in-class discussion that revolves around the questions given above.

The second half of the module (Day 2) will involve an activity that introduces students to the National Geologic Map Database developed and managed by the US Geological Survey, and the wide array of geological information that can be available for a locale. Work the students through an example led by the instructor (~15 minutes) and then guide the students to perform their own search for a study locale, assigning them to further search for specific resources identified in the search if time allows. This may require a brief primer on reading geological maps though the details of interpreting geologic maps are not necessary for this exercise. Students should be encouraged to view this information as useful/usable in their semester project - as baseline information on site characteristics of a potential study site or proposed CZO.


Students should knowledgeably and enthusiastically engage in an in-class discussion of the relationship between various geologic materials and soil that demonstrates their ability to organize and evaluate information gathered from their readings.

Students will also translate information gathered from an online resource to evaluate the underlying geology of a study site cleared with the instructor, and present this knowledge in a short report.

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These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
Explore the Collection »