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. 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
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 + H2CO3 + H2O Al2Si2O5(OH) + Na2CO3 + CaCO3 + SiO2
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 Fe2+ Fe3+
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)
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
Individual spheroidal boulder ~0.5
m across, showing concentric
weathering
Spheroidal weathering of originally
angular boulder, Kolekole Pass,
O‘ahu
Soil Classification and respective abundances in Hawai‘i
Soil Type % 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
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
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
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)
Stream Processes
Hawaiian Stream Valleys
Amphitheater-headed valleys
Landscape evolution by coalescence of amphitheater-headed
stream valleys
Geomorphic development varies with age and location
Waterfalls
With time, waterfalls migrate up-stream