Lecture 9 surface processes chemical and physical weathering and sedimentary rocks

Lecture 9: Surface Processes: chemical and physical weathering and sedimentary rocks • Questions – What is the rock cycle?. Weathering: decomposition of rocks• There is a distinction be

Lecture 9: Surface Processes:

chemical and physical weathering and

sedimentary rocks

• Questions

– What is the rock cycle? How do rocks get destroyed

and recycled at the surface of the Earth?

– At the other end of the transport system, how do

weathered and eroded materials end up making the

various kinds of sedimentary rocks?

– What can observations of the sedimentary record reveal

about the tectonics, petrology, and climate of both

depositional environments and upstream source

environments?

• Reading

– Grotzinger and Jordan, Chapters 5, 16, 18, 19

Weathering and Sedimentation in the Rock Cycle

• Our geology so far has focused on internally-driven processes:

plate tectonics, magmatism, metamorphism, orogeny.

• The rest of geology is

• Weathering and erosion

are the processes that

form and transport form

sediment

• Sedimentation, burial and

lithification are the

processes that transform

weathering products into

Weathering and Sedimentation in the Rock Cycle

• A more detailed view of the surface-driven parts of the rock cycle

shows the various steps between source rock and sedimentary

product

Weathering: decomposition of rocks

• There is a distinction between

weathering and erosion:

– Weathering converts exposed

rock to soil in place

– Erosion transports dissolved

or fragmented material from

the source area where

– But the sediment layer is thin

in most places, with respect to

overall crustal thickness, so

sedimentary rock is a minor

volume fraction of the crust

(in part by definition: once

buried to the mid-crust,

sediments get cooked to

Weathering: chemical and physical

• The destruction of rocks at the Earth’s surface by weathering has two

fundamental modes of operation:

– Chemical weathering is dissolution or alteration of the original minerals, usually by reactions with aqueous solutions

• Chemical weathering puts ions from the source minerals into solution for subsequent erosion by transport in flowing water as dissolved load.

– Physical weathering is fragmentation into progressively smaller particles, from intact outcrop to boulders and on down to mineral fragments and sand grains.

• Physical weathering makes loose pieces of rock available for downslope movement by mass wasting or transport

in flowing water as suspended or bed load.

Chemical Weathering

• Chemical weathering is driven by

thermodynamic energy minimization, just like chemical reactions at high

temperature

– The system seeks the most stable

assemblage of phases.

– The differences are that (1) kinetics are

slow and metastability is common; (2) the stable minerals under wet, ambient

conditions are different from those at high

T and P; (3) solubility in water and its

dependence on water chemistry (notably pH) are major determinants in the stability

of minerals in weathering.

• A fresh rock made of olivine and

pyroxenes will end up as clays and iron oxides, with other elements in solution

• A fresh rock made of feldspars and quartz will end up as clays, hydroxides, and

quartz in most waters

Chemical Weathering

Chemical Weathering

• The most common alteration product of feldspars is kaolinite, Al2Si2O5(OH)4,

which serves as a model for the formation of clays by weathering generally

– The reactions of feldspars to kaolinite illustrate some of the basic trends:

• K, Na, Ca are highly soluble and readily leached by chemical weathering.

• Excess Si can be removed as silicic acid although quartz is relatively insoluble.

• Al is extremely insoluble, and is essentially conserved as source rock is converted to clays.

• Weathering is a hydration process, leaving H 2 O bound in the altered minerals.

– 2 KAlSi 3 O 8 + 9 H 2 O + 2 H + -> Al 2 Si 2 O 5 (OH) 4 + 2 K + + 4 H 4 SiO 4

• Note the H+ on the left-hand side…only acidic water can drive this reaction

• Natural waters are acidic due to equilibrium of carbonic acid with CO2 in the atmosphere

– CO2 (g) + H2O = H2CO3

– 2 KAlSi3O8 + 9 H2O + 2 H2CO3 ->

Al2Si2O5(OH)4 + 2 K + + 4 H4SiO4 + 2HCO3–

– Alteration of rock transforms acidic rainwater into neutral surface or ground water, with bicarbonate the dominant species (relative to CO2 and CO32– ).

– Mg and Fe 2+ are also readily leached, but Fe 3+ is very insoluble…the ultimate residue of alteration of mafic

Chemical Weathering

• Knowing the chemistry of reaction of minerals to kaolinite, it is possible to

reconstruct from the dissolved ions in stream water the amount of each source

mineral that reacted with the water

Chemical Weathering

• Some minerals are congruently soluble in acidic water, leaving no residue

– The most abundant is calcite: CaCO 3 + H 2 CO 3 = Ca 2+ + 2HCO 3– (the Tums reaction) – Effects of dissolution (and precipitation) of calcite can be dramatic, to say the least.

Rates of Chemical

Weathering

• Many factors affect the rate

at which a rock will

weather, as summarized

here.

• Some of these variables are

local (e.g., source rock),

some are global These

include temperature and

pCO2, leading to the CO2

-weathering feedback cycle

Physical Weathering

• Anything that promotes disaggregration of a rock so that pieces can form soil or be eroded away by wind, water, or gravity transport is physical weathering

– The distinction between physical weathering and erosion is subtle, but think of physical

weathering as fragmenting the rock and erosion as carrying the fragments away; at times these may be the same event, of course.

• Rocks that are jointed or faulted or have pre-existing weak zones are most easily

weathered

– Few of the stresses associated with physical weathering are significant compared to the

tensile strength of intact rocks; something, has to start the process, either initial cracks and weaknesses or chemical attack on mineral cohesion.

• Organisms, especially plants (think tree roots), are fond of breaking up rocks

• Freeze-thaw, frost wedging, frost heave…the volume change between ice and water

is effective in widening cracks in rock in suitable climates

• Physical abrasion by flowing air or water, or more often by rock particles already

mobilized by water or wind (think Fossil Falls)

• Tectonics…rocks caught in a fault zone are definitely undergoing physical

weathering

• Etc

Weathering feedbacks: chemical and physical

• Physical weathering and

chemical weathering

generally proceed in

parallel in most

environments.

• Physical and chemical

weathering promote one

another:

– Formation of cracks by

physical weathering

increases reactive surface

area, promoting chemical

weathering

– Chemical weathering

replaces intact

interlocking minerals with

weak clays or void space,

making the rock easier to

physically disaggregate,

promoting physical

weathering

Weathering feedbacks:

more generally

• Weathering of both kinds plays key roles in several feedbacks

• Tectonics affects weathering

through slopes and elevations, climate affects weathering

through temperatures (via

chemical kinetics and

freeze-thaw), rainfall, pCO2, etc

• Conversely, weathering and

erosion affect tectonics and

– Weathering controls water

chemistry, courses of streams and groundwater, removes CO 2

from the atmosphere, etc.

Soil formation

• A weathered surface develops a stratified structure, with intact rock at the bottom (or inside) and maximum weathering at the top

• Leachable ions are transported downwards by groundwater flow, possibly redeposited as water

chemistry adjusts towards equilibrium

• Chemically and physically weathered rock that is not

eroded or transported but remains in place becomes soil.

Soil formation

• The mineralogy and thickness of soil layers depends on source rock,

climate (temperature and rainfall), and age

• Which of these soil types would you rather farm?

Erosion and Transport

• Between weathering and sedimentation, matter must be

transported from source to destination This is erosion.

– We dealt with the landforms generated by erosion in the

geomorphology lecture; here our concern is with the effects of

transport on sedimentary rocks

• Modes of transport:

– Gravity (short distances and steep slopes)

– Wind (small particles only)

– Glaciers

– Water

• Surface runoff carries dissolved, suspended, and bed loads

• Groundwater flow only carries dissolved load

– All these mechanisms carry products of physical

weathering and insoluble residues of chemical

weathering.

– Only water transport carries away leached soluble

products of chemical weathering.

Erosion and Transport

• Certain modes of transport physically modify and

physically and chemically sort particles en route.

• Size sorting by surface water runoff flow:

Current of a given

velocity can generally

carry all noncohesive

particles smaller than a

critical size; since

current velocity drops

with decreasing slopes

from mountains to

lowlands, it follows that

sediments evolve from

poorly sorted and

coarse-grained near

source to well-sorted

and finer grained with

increasing transport

Erosion and Transport

• Chemical sorting with

increasing transport

distance is like a

continuation of

chemical weathering:

most stable minerals are

transported the farthest.

• Textures of particles are

modified by abrasion

during wind or water

transport Close to

source particles are

angular; far from

source particles are

rounded.

• Eventually transported particles and dissolved ions reach a place where they can

be permanently deposited and accumulated This is sedimentation.

• The sedimentary rocks that result from this accumulation are controlled by and

record the sedimentary environment where they were deposited.

– We interpret ancient sedimentary rocks by comparison to modern

environments where we can observe ongoing sedimentary processes and

relate them to the composition, texture, and structure of the resulting rocks

• Sediments and the environments

in which they form are

fundamentally divided into

clastic and chemical:

– Clastic sediments are made of physically transported and

deposited particles (they may later gain chemically grown cement during diagenesis)

– Chemical sediments are grown from solution, organically or inorganically; biochemical

sediment more specifically

refers to minerals grown from solution by organisms

• In some cases the relationship between the environment and the character of the sediment is absolute and obvious (carbonate

in reefs, boulder-strewn till in periglacial deposit, etc.); other

• The process of modification of

newly deposited sediments into

sedimentary rocks is diagenesis or

lithification.

– Processes include:

• physical compaction by the pressure of overburden, accompanied by expulsion of pore waters

• Growth of new diagenetic minerals and continued growth of chemical sediments from pore waters

• Dissolution of soluble elements of clastic rocks

• Recrystallization and remineralization as water chemistry, pressure, and

temperature evolve

• At the high-T and P end, diagenesis

merges smoothly into the low-T and P

end of metamorphism The distinction is

Sedimentary Rocks

• The preserved end-result of weathering, erosion, transport,

sedimentation, and diagenesis is sedimentary rocks.

– Like sediments and sedimentary environments, the resulting rocks are divided into clastic (or siliciclastic or volcaniclastic, etc.) and chemical (or

biochemical).

• Clastic rocks are classified by particle size (and sorting) and

composition.

Sedimentary Rocks

• Chemical sediments are primarily classified, of course, by

mineralogical composition.

Sedimentary rocks and environmental information

• How do sedimentary rocks preserve information about their

depositional environments?

– By composition, mineralogy and grain size, obviously, but also

through sedimentary structure

• Elements of sedimentary structure:

– Bedding

• Bed thickness, from finely laminated to massive

fine

30 cm

30 m

Sedimentary structure

• Cross-bedding indicates high and unidirectional current velocity, often winds in terrestrial settings, forming sand dune lee-slopes

• Character of bedding, from simple horizontal laminae to cross-bedding,

ripples, soft-sediment deformation, or bioturbated.

• Ripple marks record back-and-forth action by waves in shallow water

Sedimentary Structure

• Mud cracks demonstrate

drying-out of a thin layer of

sediment fine enough to

have significant cohesion

Definite proof of terrestrial

setting or very shallow water

marginal marine

• What about this structure?

(Hint: it is not the surface of

the Moon)

Sedimentary Structure

• Bioturbation is the vertical

mixing of sedimentary layers

by burrowing organisms

Evidence of such activity

can be preserved on bedding

surfaces as trace fossils

Indicative of water depth,

• Soft-sediment deformation indicates slumping or compression of layers

before complete lithification

Sedimentary Structure

– Alluvial settings, with wandering channels that fill up and become overbank deposits

– Continental slopes with turbidity currents

• Graded Bedding: sorting of particle sizes within beds

indicates time dependence and hence process of deposition

– An environment in which a episodes of high-energy transport give

way to periods of low-energy transport gives normal graded beds:

Carbonate Rocks

• Most carbonate rocks are entirely

biochemical sediment, made up of the body

parts of calcite or aragonite-precipitating

organisms

– Deep-sea carbonate ooze is made of foram shells

– Reef carbonates are made of coral reefs (usually)

– Stromatolites are formed by carbonate

precipitation by microorganisms

Tour of sedimentary environments

Let us go through each of the major categories of sedimentary

environment, keeping in mind the relationship between observable

processes in modern settings and the preserved features in ancient

examples, and the ways in which observation of a sedimentary rock

Sedimentary environments: Terrestrial

I Fluvial (rivers and streams of all kinds and sizes)

a Alluvial Fans

We saw alluvial fans on the field trip They form where drainages exit

mountain fronts onto surrounding lowlands

Individual fans may merge to form a piedmont slope (like Pasadena)

In arid regions like California,

sediment transport on

alluvial fans is dominated

by debris flows like

mudslides and landslides,

and by periodic stream

flows that divide the fan

into channel and overbank

deposits

Sorting is poor, but increases

downstream; grain size

decreases downstream;

sediments are often

Sedimentary environments: Terrestrial

I Fluvial

b River systems

Rivers are classified into meandering or

braided, most often

Braiding is favored by high sediment

load, steep gradients, variable stream

flow, and unstable poorly vegetated

banks

Meandering is favored by the opposite

Sedimentary environments:

Terrestrial

I Fluvial

b River systems

Meandering rivers develop in a fairly

regular pattern by channel migration,

leaving a predictable sequence of

cyclic, fining-upward sedimentary

deposits Braided river deposits are

more chaotic leave somewhat random

deposits, since channels wander

randomly across the floodplain