Weathering The effects of the physical and chemical environment on the decomposition of rocks

Size: px

Start display at page:

Download "Weathering The effects of the physical and chemical environment on the decomposition of rocks"

Rafe Virgil Jackson
5 years ago
Views:

Weathering The effects of the physical and chemical environment on the decomposition of rocks - Igneous rocks form at high temperatures and the constituent minerals reflect the conditions of

1 Weathering The effects of the physical and chemical environment on the decomposition of rocks - Igneous rocks form at high temperatures and the constituent minerals reflect the conditions of formation. The minerals are not necessarily stable at surface conditions, long after they cooled. - Weathering can be thought of as an attempt by rocks to attain physical and chemical equilibrium with the surface environment. - Most igneous minerals are not stable on Earth s surface. Given time most will decompose. Addition of extra energy (heat or mechanical energy) reactions will proceed sooner, rather than later

Chemical and Physical Weathering Factors that affect the efficiency of physical weathering 1. Topography and relief e.g., falling down hill; pounding by waves 2.

2 Chemical and Physical Weathering Factors that affect the efficiency of physical weathering 1. Topography and relief e.g., falling down hill; pounding by waves 2. Climate Especially temperature, and diurnal variations 3. Original structure Porosity and permeability are especially important Hot and wet is hardest on rocks Lahaina Cliffs Trail, W. Maui

3 Factors involved in Chemical Weathering 1. Original Composition - Some substances are more resistant to alteration than others. - In Hawai i, glass and olivine are the least stable substances 2. Physical Environment (see above) 3. Chemical and Biological Environment - Microbial activity is important in all weathering processes - Most rain water and ground water in Hawai i is acidic The decomposition of feldspar Plagioclase feldspar + H 2 CO3 + H 2 O Al 2 Si 2 O 5 (OH) + Na 2 CO 3 + CaCO 3 + SiO 2 carbonic acid kaolinite (clay) soluble carbonates quartz carbonic acid derived from breakdown of organic matter soluble cations (K, Ca, Na) are leached from rocks end products are hydrated (water-rich) clay minerals and quartz In addition, there is oxidation (rust), with Fe 2+ Fe 3+

The End-Product of Weathering is Soil Soils can be generally divided into two main types: 1. Residual soils form by weathering in place by the breakdown of rocks beneath them 2.

4 The End-Product of Weathering is Soil Soils can be generally divided into two main types: 1. Residual soils form by weathering in place by the breakdown of rocks beneath them 2. Transported soils have been brought in by wind or streams, and may be unrelated to rocks beneath them. In Hawai i: Most soils are residual. Transported soils are restricted to outwash from streams (alluvial fans)

$begins along permeable fractures in$ Progressive weathering leaves

5 Residual soil formation by spheroidal weathering Weathering begins along permeable fractures in the outcrop. Progressive weathering leaves rounded cores of less-weathered rock Mo omomi, Moloka i Makamakaole, Maui

6 Spheroidal weathering of originally angular boulder, Kolekole Pass, O ahu Individual spheroidal boulder ~0.5 m across, showing concentric weathering

7 Soil Type Soil Classification and respective abundances in Hawai i % land area Characteristics Location Inceptosols 24.4 Poorly developed, sub-surface horizons Steep slopes, recent deposits Histosols 14.3 Rich in organic matter Wet, poorly drained areas Oxysols 5.1 Highly weathered soils, low in silicate Smooth, gentle slopes Ultisols 2.5 Clay-rich soil Valley edges Mollisols 2.3 Soft, organic soils Grassy, forested areas Aridisols 1.2 Dry most of the year Deserts Entisols 1.1 Without subsurface development Coastal beaches, recent ash deposits Vertisols 0.8 Soils subject to expansion and shrinkage Low, level areas Spodolsols 0.6 Severely leached; acidic Wet mountain slopes Alfisols 0.2 Clay-rich, similar to ultisols Stable slopes, steeper than ultisols Soil types (orders - from U. S. Dept. Agriculture) depend on: 1. Parent material, 2. Climate (temperature and rainfall), 3. Topography and drainage, 4. Organisms in the soil, and 5. Length of time exposed to weathering

9 Inceptisols (24 % of state land area) the most abundant soil type in Hawai i, a fairly immature soil type. Histosols (14 % of state land area) Rich in organics (= good stuff). Rainforest floors, almost entirely restricted to Big Island

10 Oxisols (5 % of state land area) These are the soils that used to be called laterites. Depleted in nutrients (leached out from prolonged weathering) Potential commercial source of bauxite (an ore of aluminum) Major sugar cane and pineapple growing areas, mainly because of gentle slopes. (require massive use of fertilizers) Oxisols dominate de-vegetated areas

11 Ultisols (2.5 % of state land area) Clay-rich soils, mainly windward areas Molisols (2.3 % of state land area) Moderately organic-rich soils, mainly in grassland areas Good stuff for growing (Kula, Kahuku, Hale iwa, Kekaha, Waimea)

24 Stream Processes

25 Hawaiian Stream Valleys

26 Amphitheater-headed valleys

27 Landscape evolution by coalescence of amphitheater-headed stream valleys Geomorphic development varies with age and location

28 Waterfalls

29 With time, waterfalls migrate up-stream

Physical Geology, 15/e

Lecture Outlines Physical Geology, 15/e Plummer, Carlson & Hammersley Weathering and Soil Physical Geology 15/e, Chapter 5 Weathering, Erosion and Transportation Rocks exposed at Earth s surface are constantly