THE ROCK CYCLE

Sediment is produced either directly or indirectly by the weathering and erosion of pre-existing rocks.

Grains can originate as:A) Broken down particles from older, pre-existing rocks. B) Pyroclastic particles from explosive volcanism.C) Secondary Minerals (iron oxide, clay minerals) that

form at the site of weathering.

These grains are usually referred to as extrabasinal sediments because they originate outside the depositional basin and are transported into it. Main source of siliciclastic (quartz composition) rocks like shale and sandstone.

Grains can also originate in two other ways:A) Chemical and biochemical precipitates These form

what we call chemical sedimentary rocks. These grains are usually referred to as intrabasinal sediments because they are forming within the depositional basin. They are the source for limestones, dolostones, cherts, and evaporites

B) Organic residues that form from the decomposition of plant and animal remains. Enriched in carbon, these residues are the constituents of coal.

The fact that one generation of sedimentary rock can be the source material for another suggests that the processes responsible for sedimentary rock formation occur in cycles – a fact first recognized by James Hutton in 1788.

Thus, the part of the Geologic Cycle that we are most interested in is the Sedimentary Cycle:

1) Weathering + Erosion 2) Transportation 3) Deposition 4) Lithification

5) Diagenesis 6) Recycling, again

Let’s Begin with WEATHERING: What Happens To Rocks At

The Earth’s Surface:

DEFINITIONS:

1. Weathering – The destruction and decomposition of rocks at the Earth’s surface.

2. Erosion – The incorporation and transport of weathered material by wind, water, and ice.

3. Mass Movement – The downslope movement of weathered material under the force of gravity

WEATHERING

1. Mechanical – The physical breakdown of rocks into smaller and smaller pieces.

2. Chemical – The chemical reactions that alter or destroy minerals.

1. MECHANICAL WEATHERINGa) Frost Wedging – Water increases by 9% in

volume when it freezes.

1. MECHANICAL WEATHERINGb) Exfoliation or Sheeting – A pressure release

phenomenon.

Granite pluton

Downward pressure

SHEETING

1. MECHANICAL WEATHERINGc) Organic activity – tree roots (also chemical),

animal burrowing, lichens (chelation), bacteria.

d) Day/Night Temperature Variation

DesertPavement

1. MECHANICAL WEATHERINGe) Spheroidal Weathering:

CHEMICAL WEATHERING

1) Dissolution H2O + CO2 + CaCO3 Ca2+ + 2HCO3

-

2) Oxidation 4Fe + 3 O2 2Fe2O3 (Hematite or Rust)

3) Hydrolysis Carbonic Acid + Silicate Mineral Dissolved ions + Clay Minerals4) Hydration CaSO4 + 2H2O CaSO4* 2H2O (Annhydrite) (Gypsum)

1) Dissolution – Karst Topography

1) Dissolution – Florida Sinkhole

1) Dissolution – Cave Formation

Extreme Karst Topography:The Stone Forests of Yunnan Province, China

2) Oxidation:

2) Oxidation:Honeycomb Weathering at Hocking Hills

2) Oxidation:Artists Drive: Oxidized Volcanics in Death Valley

3) Hydrolysis

HYDROLYSIS

HYDROLYSIS

WHY DO ROCKS WEATHER ? Form Under Higher Pressure/Temperature Unstable at Surface Pressure & Temperature

BOWEN’S REACTION SERIES

Temperatures at which silicate minerals crystallize out of a cooling melt

1200o

600o

WHAT CONTROLS THE TYPE & RATE OF WEATHERING?

WHAT CONTROLS THE TYPE & RATE OF WEATHERING?

Obelisk was carved in granite 3500 years ago.

Pedestal was carved in marble (limestone) only 1600 years ago.

Chemical Weathering &Rock Type