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