Weathering and Soils

Welcome to Poorna Pal's 'Earth Revealed' telecourse (Geol — 101: Physical Geology; section #7695) at the Glendale Community College

  • Weathering and Soils

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  • Module 2

    Plate Tectonics

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    Measuring Geologic Time

  • Module 3

    Evolution through Time

    Atoms, Elements and Minerals

    Volcanism and the Extrusive Rocks

    Intrusive Activity and the Igneous Rocks

    Weathering and Soil

    Mass Wasting

  • Module 4

    Sediments and Sedimentary Rocks

    Metamorphism and the Metamorphic Rocks

    Streams and Landscapes

    Underground Water

    Deserts and Wind Action

  • Module 5


    Waves, Beaches and the Coasts

    Earth Resources

    Environmental Matters



Weathering and Soil




  • Poorna's Outlines

Weathering and Soil

Click on the title to access and download this MS word document



Weathering heralds the sedimentation process and is therefore a crucial component of the "Rock Cycle" ― a theoretical concept that relates tectonism, erosion, and the various rock forming processes to the common rock types.

Try the following links for interesting "Rock Cycle" presentations:



Read about  chemical weathering of rocks at this USGS website and visit the site

to learn of the difference between weathering and erosion.



  • Weathering implies physical disintegration and/or chemical decomposition of surface and near-surface rock material.

EROSION is the physical removal of weathered material.
TRANSPORTATION is the moving of eroded material by wind, streams, waves, glaciers etc.

  • MECHANICAL WEATHERING (i.e., physical disintegration)

    • results from cosmic radiation, tides, frost action, abrasion and/or pressure release, and

    • causes jointing (block, sheet, exfoliation, columnar etc.)

  • CHEMICAL WEATHERING (i.e., chemical decomposition)

    • results from the chemical action of water solvents, and

    • causes chemical and mineralogical changes, e.g., iron/aluminum oxides, caverns, stalag-mites and stalactites, clay minerals etc.


  • Silica (or the mineral quartz) is the most resistant of all minerals to weathering and therefore forms the beach sands.

Two examples of mechanical weathering in granite. Bottom picture shows spheroidal weathering and the one on the right shows how the weathering along  isometric joint planes has left huge boulders resting over one another in the Joshua Tree National Park.

Indeed, looking at Bowen's Reaction Series shown on the right, note that minerals that are the last to form also resist weathering the most. This is because minerals are most stable at the pressure and temperature conditions at which they form. For instance, olivine and pyroxenes form at high temperatures. At the atmospheric pressure and temperature conditions, therefore, they also succumb the fastest to weathering. Quartz, on the other hand, is the last mineral to form in Bowen's Reaction Series. Little surprise, then, that it also withstands weathering the most.


  • Soil is the weathered, unconsolidated, top part of rock, often rich in organic matter and therefore suitable for plant growth (lunar soil is called regolith).

Click on this map or go to and browse an interactive map of U.S. soil surveys

  • Soil forms through loosening of particles, leaching by down-ward percolating water, and accumulation of clay minerals, iron oxides, calcite at the bottom of the weathered zone, while the top of bedrock defines the depth to which weathering has progressed.


  • Soil can be

    • pedalfer, pedocal, lateritic, and/or bauxitic, depending on temperature and humidity (e.g., compare the calcium rich pedocal soil of the rather dry western U.S. with the aluminum and iron rich pedalfer soil of relatively humid temperate latitudes in the eastern U.S. and Pacific northwest)

    • arnacious (sandy), argillaceous (clayey) and/or calcarious (loam), depending on the mineral and/or composition.


Note: Rotating globe at the top left corner of this page is from