WEATHERING SEDIMENTARY ROCKS WEATHERING EROSION REMEMBER THE ROCK

WEATHERING & SEDIMENTARY ROCKS

WEATHERING & EROSION

REMEMBER THE ROCK CYCLE…

What is Weathering? Physical breakdown and/or chemical alteration of rocks and minerals at or near the earth’s surface. What is Erosion? Removal of weathered material from its place of origin. -eroded material is called sediment. Are they the same thing? ? NO!

Weathering is the break. Weathering up of rock due to exposure to the atmosphere.

Weathering involves two Weathering processes that often work together at the same time to decompose or break down rocks.

CHEMICAL AND MECHANICAL WEATHERING!

Mechanical (Physical) weathering, (Physical) weathering or disintegration, involves physically breaking rocks into fragments without changing the chemical make-up of the minerals within them.

INCREASING SURFACE AREA LEADS TO INCREASE OF CHEMICAL WEATHERING

There are four main sources of power for mechanical weathering. Gravity, Water, Wind, & Waves Of these, water appears to be the leader in changing the surface.

PHYSICAL PROCESSES OF MECHANICAL WEATHERING Frost Wedging � Unloading � Biological Activities �

FROST WEDGING When water freezes it expands by 9% Frost Wedging is the Wedging freezing and thawing of water in cracks, breaking rocks into pieces Talus: sections of rock that are wedged loose and tumble into large piles; found at the base of steep, rocky cliffs

MECHANICAL UNLOADING AND EXFOLIATION Mechanical Unfolding: when pressure on igneous rocks is reduced after large masses of igneous rocks are exposed through uplift and erosion of overlying rocks Mechanical Exfoliation is the peeling off of sheets of rock as they expand

BIOLOGICAL ACTIVITIES � Occurs because of the activities of organisms including plants, burrowing animals, and humans

ROOT PENETRATION Plant roots have to be strong to survive the elements. They grow into cracks, wedging the rock apart as they grow

MORE BIOLOGICAL ACTIVITIES Animals: � Burrowing animals move rocks to surface � Decaying Animals produce acids that cause chemical weathering Humans: � Accelerate mechanical weathering through deforestation and blasting

Chemical weathering, or Chemical weathering decomposition, takes place when at least some of the rock’s minerals are changed into different substances.

Chemical processes include rain, acid etching by plants, oxidizing, and demineralizing by water.

CHEMICAL WEATHERING � Water � � Promotes chemical weathering by absorbing gases from the atmosphere and ground Oxygen dissolved in water reacts to form oxides (rust) Water absorbs carbon dioxide when rain falls to form carbonic acid Also aborbs sulfur oxides and nitrogen oxides to cause Acid Rain

CHEMICAL WEATHERING OF GRANITE � � � Consists of mainly feldspar and quartz When exposed to carbonic acid, feldspar converted to mostly clay material Quartz remains unchanged and are released from the granite OF SILICATE MATERIALS � Make up most of the Earth’s crust � When these minerals undergo c. w. the sodium, calcium, potassium, and magnesium are dissolved and carried away by groundwater � Iron reacts with oxygen to form iron oxide (rust) � Aluminum, silicon, and oxygen combine with water to produce clay

SPHEROIDAL WEATHERING � Causes the corners and edges of rock to be more rounded

RATE OF WEATHERING Mechanical weathering increases the rate of chemical weathering � Two other factors affecting the rate of weathering are rock characteristics and climate. 1. Rock Characteristics: � Physical characteristics like cracks � Chemical Composition � Crystallization Patterns (if it crystallizes first it weathers more rapidly) �

RATE OF WEATHERING 2. Climate: � Temperature � � Higher temperature increases rate Moisture � Higher amounts of moisture increases rate

DIFFERENTIAL WEATHERING When different parts of a rock mass weathers at different rates � Why? � § § Mineral composition differences Number and Spacing of Cracks

If a particle is loosened by weathering, but stays put, its just weathering. If it starts moving, its Erosion.

SEDIMENTARY ROCKS FORMED FROM THE WEATHERED PRODUCTS OF PREEXISTING ROCKS THAT HAVE BEEN TRANSPORTED, DEPOSITED, COMPACTED, AND CEMENTED.

SEDIMENTARY ROCKS Weathering, Erosion, and Deposition • Weathering any process that breaks rocks into sediments • Chemical Weathering when minerals in rocks change into new substances • Erosion involves the weathering and the removal of rock. • Deposition occurs when an agent of erosion—water, wind, ice, or gravity— loses energy and drops sediments.

SEDIMENTARY ROCKS Compaction and Cementation • Compaction is a process that squeezes, or compacts, sediments. o Caused by weight of sediments, most of the water in sediments is driven out • Cementation takes place when dissolved minerals are deposited in the tiny spaces among the sediments.

NATURE OF SEDIMENTARY ROCKS � Sedimentary � Fragments rocks are composed of: of other rocks � Chemical precipitates � Organic matter

� Sedimentary rocks are common at the Earth’s surface �Cover ~75% of the continents �Cover nearly all of the ocean floor �occur in distinct layers (strata) Stratified sedimentary Rocks; Badlands Nat’l Park

John Day Fossil Beds, Oregon

� Layers are easily identified �Majors layers-formations: easily recognized over large distances �layers separated by bedding planes �Gradation in grain size, composition or physical features Schematic: Sediment layers filling a basin

Layered sedimentary rocks exposed in the Grand Canyon, AZ

� Sedimentary rocks contain evidence of their environment of formation �Animal and plant fossils �Sedimentary structures indicate environment & mode of transport of sediment

SEDIMENTARY ROCKS-CLASSIFICATION � Two main groups: � Clastic Sedimentary � composed of weathered bits of rocks and minerals. � Classified by clast size � Clast=fragments � Chemical/Biochemical Sedimentary � form when dissolved substances precipitate, or separate, from water.

SEDIMENTARY ROCKS: CLASTIC � Clastic Rocks are classified by particle shape and particle size � Particle size is controlled by: � Source rock � Carrying capacity of transport process � Weathering and erosion during transport

CLASTIC SEDIMENTARY ROCKS � Common clastic sedimentary rocks �Conglomerate �Sandstone �Mudrock or Shale �Siltstone �Claystone

Gravel: Grain size greater than 2 mm; Coarse Rock Type: -rounded clasts = conglomerate -angular clasts = breccia

CONGLOMERATE

Sand: Grain size 1/16 to 2 mm; Medium Rock Type: -dominated Sandstone by quartz grains = quartz sandstone by feldspar grains = arkose by rock fragments = graywacke)

Silt: Grain size 1/16 to 1/256 mm; Fine Rock Type: Siltstone Clay: Grain size less than 1/256 mm; Very Fine Rock Type: Shale (Most Abundant!!)

� How do you distinguish siltstone from shale? � Siltstone is thin and platy like shale, but it tends to be a little harder than shale. Siltstone has the texture of a very fine-grained sheet of sandpaper � Geologists working on an outcrop will often do a field test to distinguish siltstone from shale by dragging a sample across their front teeth. Shale is smooth whereas siltstone is more abrasive. � Shales split along planes of weakness into thin

CHEMICAL/BIOCHEMICAL SEDIMENTARY ROCKS � Processes that take ions from solution to form a solid �Chemical Sediments �Precipitates process �Biochemical �Formed from water by an inorganic Sediments during the growth of some organism

� Subdivided by composition and mode of formation � Limestone Biochemical formation by algae, coral, etc. � Direct chemical precipitate from warm sea water - oolites � Chemical precipitate from springs and in caves � MOST COMMON CHEMICAL SEDIMENTARY ROCK �

[fossileferous] limestone [micritic] limestone

Capitan Reef in Guadeloupe Nat’l Park -example of a Permian reef limestone

Coquina -fossil hash cemented together -may resemble granola

Chalk -microscopic planktonic organisms (coccolithophores) (fizzes in acid)

� Chemical/Biochemical �Chert rocks: - microcrystalline quartz �Various modes of formation �Evaporites �Rock salt - halite �Gypsum

FOSSILIFEROUS LIMESTONE

CLASSIFICATION OF SEDIMENTARY ROCKS

CLASSIFICATION OF SEDIMENTARY ROCKS

SEDIMENTARY ROCKS AND CLUES OF FORMATION u Features of sedimentary rocks are clues to how and where the rocks are formed u Some Surface Impressions o Ripple marks: beach or stream bed o Mud lines: dry environment Oldest Strata. Layers on bottom, Newest layers on top

SEDIMENTARY STRUCTURES � Strata or beds �Distinct layers having variations in texture, color or physical properties � Formations �Large scale stratification �Contain numerous individual strata

�Fossils �Fossil remains or trace fossils �Indicate paleo-environment �Detailed study may provide �Depth of water �Temperature and salinity �Relative age of rock

SHALE WITH PLANT FOSSILS

SEDIMENTARY SYSTEMS � Systems include: �Weathering �Transportation �Deposition �Lithification � Sedimentary rock records processes that produced it

Idealized diagram of major sedimentary systems.

� Advance and retreat of shallow seas �Common example of a stratigraphic sequence �Cycle consists of �Sandstone-shale-limestone-shale- sandstone

Transgression and regression of shallow sea

Transgressive Sequence in Outcrop Shale [shallow water] Limestone [deepwater]

SEQUENCE STRATIGRAPHY � Sedimentary rock formations classified by rock type � Sequences of formations may be grouped together � Sequences separated by unconformity � Relate global scale events � e. g. , Change in sea level

Changes in sea level throughout geologic history

� Plate tectonics plays a major role �Sediment deposition & geographic distribution �Sediment source areas �Formation of sequences