Adam C. Simon Ph.D., University of Maryland, 2003 Research Associate Department of Geology

1. Adam C. Simon Ph.D., University of Maryland, 2003 Research Associate Department of Geology University of Maryland College Park, MD 20742 p: 301 405 0235 f: 301 314 9661 e-mail: asimon@geol.umd.edu

2. Weathering and Erosion Rocks exposed at the Earth’s surface are constantly being eliminated: physically and chemically destroyed and transported elsewhere. These are the processes of weathering and erosion. The Great Falls of the Potomac River

3. Terminology • weathering -- the chemical and physical breakdown of rock exposed to air, moisture, and organic matter • erosion -- the loosening and moving downhill of rock and soil • regolith -- the loose layer of broken rock and mineral fragments on the surface

4. The Rock Cycle Revisited The rock cycle traces the potential changes to rocks resulting from Earth’s internal and surficial processes.

5. Physical weathering is when rocks break down as the result of physical phenomena. This can be due to abrasion or processes related to expansion and contraction. Simple heating and cooling causes enough physical change to promote physical weathering. Physical Weathering heat spalling Much weathering exploits pre-existing fractures in rock -- joints:fractures along which no movement has occurred.

6. Exfoliation Exfoliation is the natural formation of fractures in rocks (mainly igneous plutons) due to the release of pressure upon reaching the surface.

7. Peeling Away Granite The classic Sierra Nevada (CA) granite mountains photographed by Ansel Adams are examples of exfoliation domes.

8. Weathering in Sheets Exfoliation can operate on very large scales. It is most important in areas of exposed igneous rocks, even though these rocks generally lack a layering or similar planar features.

9. The volume change as liquid water transforms to ice is an important physical weathering force in temperate climates. Freeze-Thaw (Frost Wedging)

10. Root Wedging The action of growing plants also promotes physical weathering.

11. A birch tree exploited a crack in this boulder in Maine and, together with freeze-thaw, has split it in half. Frost + Root Wedging

12. Chemical Weathering Chemical weathering is the decomposition of rocks and minerals as a result of chemical and biochemical reactions. As in all reactions, unstable minerals react to form a set of products more stable at near-surface conditions. So, broadly speaking, chemical weathering is the process whereby unstable high temperature/ pressure minerals are transformed into their Earth-surface-stable relatives.

13. Types of Chemical Weathering hydrolysis: chemical reaction in which water is involved Si also lost (~20%)

14. H2O + CO2 = H2CO3 (carbonic acid) Hydrolysis involves pure water, but most natural waters, from reaction with atmospheric CO2, are acidic (rain pH ~ 5.5). These naturally acidic waters are even more efficient at doing chemical work. electron microscope view of a weathered feldspar crystal

15. Oxidation is the breakdown of minerals exposed to a oxygen-rich environment. This is most obvious with minerals rich in elements like iron. Remember, iron in rocks is mainly Fe2+. When rocks weather, the iron is oxidized to Fe3+, producing red staining from minerals like hematite (Fe2O3). Oxidation

16. Oxidation of Iron-Rich Silicate Minerals

17. Iron in Hematite How can you tell what the oxidation state of iron is in a mineral like hematite, just from the formula? Oxygen in nature has a - 2 charge. If hematite is Fe2O3, then it carries (- 2) x 3 = - 6 charge from oxygen. This must be balanced by the iron, so both Fe atoms in the structure of hematite must be +3 charged. A trickier problem: what is the oxidation state of iron in the mineral magnetite: Fe3O4

18. The dissolution of calcite (CaCO3) is chemically easy, since calcite is much more soluble than silicate minerals. Dissolution In limestone caves we see the work of calcite dissolution, as well as the results of calcite precipitation (collectively called speleothems).

19. Chemical Weathering in Nature solution of calcite by mildly acidic rainfall (hydrolysis) over the past century oxidation of iron in Hawaiian soils: Fe2+ in rocks converted to Fe3+ in sediment

20. Consequences of Chemical Weathering One thing to consider during these chemical reactions is the fate of the ions that are released. In weathering feldspars, for instance, a lot of Si, K, and Ca are shed. These start out as ions in aqueous (water-rich) solutions (like groundwater), but where do they end up?

21. Weathering and Bowen’s Reaction Series The order in Bowen’s reaction series of minerals to crystallize from cooling magma is the same sequence when considering mineral stability at Earth surface conditions: calcite, halite least stable at low temperatures/pressures olivine Ca-rich plagioclase feldspar pyroxene amphibole Na-rich plagioclase feldspar biotite (mica) K-feldspar muscovite (mica) quartz most stable at low temperatures/pressures clay minerals, Fe-oxides

22. Weathering and Bowen’s Reaction Series Rocks chemically weather according to the stability of their minerals, the severity of their environment and the time available. For example, a granite (quartz, feldspars and mica) typically chemically weathers to a mixture of quartz and clay minerals.

23. When weathering begins along a perpendicular network of fractures, it will frequently weather out resistant rock in spherical shapes. Spheroidal Weathering (see Fig 7.10 in your text) the Devil’s Marbles in central Australia

24. Breaking rocks up (physical weathering) increases the ratio of surface area to volume. Surface Area / Volume

25. If there is more exposed surface, then more chemical weathering can take place, since it is on surfaces that chemical weather concentrates. Positive Feedback: Surface Area/Volume We will see many positive feedback processes like this during the semester. These are mechanisms in which the product works to advance the process.

26. Climate and Weathering • Climate (combination of temperature and rainfall) strongly influences weathering processes: • physical weatheringdominates in low rainfall, cold climate; • chemical weatheringdominates in high rainfall, warm climate.

27. Organisms are important agents of weathering, physical and chemical. Imagine how weathering on Earth would be different without life. Organisms and Weathering

28. Soil The fine-grained portion of the regolith is called soil. Soils characterize areas by type of bedrock and climate, and tend to have distinct layered profiles. weathered, disaggregated bedrock intact bedrock

29. Soil in Humid Regions Water from above percolates through the soil column and leaches the upper surface of soluble components. These components tend to precipitate lower in the column. Fig 7.11

30. Soil in Humid Regions Insoluble materials remain in the A horizon: Fe-oxides, Al-rich minerals, and organic compounds. Fig 7.12

31. In arid climates, groundwater flow may be principally from the bottom up, as water is drawn to the surface and evaporates. Soils in Dry Regions This reverses the profile we see in humid climates, such that hard, calcite-cemented material dominates the upper portion of the profile.

32. Arid Soils The hard calcite pavements associated with arid soils are called caliche or calcrete.

33. Soil Formation and Climate Equator North Pole tropical polar low-latitude desert temperate latitude Why is weathering enhanced in the temperate regions, even though temperature is lower than the tropics?

34. Factors Controlling Weathering Rates

35. Why does soil cover aid chemical weathering? Bury a nail for 10 years and compare it to a nail in a 19th century house. Chances are, the one in the soil is in far worse shape. Soils have high contents of moisture, organic materials and CO2, all of which enhance chemical weathering. Positive Feedback: Soil Formation

36. Credits Some images in this presentation come from: Hamblin and Christiansen, Earth’s Dynamic Systems, 8/e AJ Kaufman (Univ. Maryland) GS Solar (SUNY College Buffalo) Brodo, Sharnoff and Sharnoff, Lichens of North America Plummer, McGeary and Carlson, Physical Geology (8th ed.) Carlsbad Cavern National Park Press and Siever, Understanding Earth (3rd ed.) Marshak, Earth: Portrait of a Planet (1st ed.)