Lecture 10 Transcript
Lecture 10
Natural Resources Conservation
NRPI 110
Humboldt State University
Fall 1997
Susan Bicknell

Slide 1

Our discussion of energy in the biosphere, and how it influences concentrations of gasses in the atmosphere leads naturally into a consideration of long time scales. Long time scales are the domain of geology. Geologic evidence recorded in the rocks of the crust of the earth tells us many things about the history of life on earth, and about the changing conditions of the earth environment over its 4.5 billion year history. We often speak of environmental issues assuming that nature, undisturbed by human activity, is constant. Geology tells us that we are quite mistaken in that perception.

Understanding processes that operate over geologic time scales is essential to understanding the processes of change in the biosphere and the role that humans play. Geologic processes are important from very long term perspectives; but they are also relevant to human activities on shorter time frames.

Slide 2

The purpose of this lecture is to provide an overview of the geologic time scale over which whole earth processes occur.

I will look at the present day composition of the earth and briefly describe how it came to be that way.

We will review the basic theory of plate tectonics, and briefly look at how the movement of the earth's continents has changed the environment for living organisms.

We will return to the basic idea of the system model to look at the phenomenon of rock cycles, and then focus on the role that surficial geologic processes play, both in rock cycles and in human activities.

Slide 3

First, let's try to place some of the processes we have been discussing with regard to energy and the atmosphere in a geologic time scale.

The origin of the earth is believed to have occurred some 4.5 billion years ago. The abbreviations used for geologic age are shown in the box. Ga is short for giga-annum and means one billion years; Ma is short for mega-annum and means million years; ka is short for kilo-annum and means thousand years. BP means "before present" but "present" is defined as 1950, the year radiocarbon dating was initially used.

At the origin of the earth, there were no living organisms. Before life began, the earth's atmosphere was composed mostly of N2, CO2, and H2O vapor. There was no free oxygen. And thermodynamic processes assured that oxygen remained in a bound state. The chemical bonds of water are strong, requiring energy to break them. The earth was initially very hot, and water existed mostly as vapor in the atmosphere.

The first traces of life are found in rocks 3.8 billion years old. By this time, the earth had cooled to below 100oC, and water had precipitated to form the earth's oceans. Synthesis of simple organic molecules was taking place in the ocean in the absence of life. The earliest life forms used these organic molecules as their energy source. In the process of extracting energy from the abiotically formed organic compounds, the earliest single celled creatures released CO2, CH4 and H2O.

Geologic evidence in the form of deposits of oxidized iron (Fe2O3) tells us that photosynthesis was occurring as long ago as 3.5 billion years. These iron beds indicate that photosynthesis was evolving free oxygen that oxidized the iron dissolved in the ocean and it precipitated as Fe2O3.

Reduced substrates were abundant both on the barren land, and dissolved in the oceans, and so O2 released by photosynthesis was consumed until the readily available supplies of reduced substances were depleted.

Oxygen began to accumulate about 2 billion year ago when the rate of O2 production by photosynthesis exceeded its rate of consumption by the oxidation of reduced substances. The concentration seems not to have varied much since the Silurian, 430 million years ago, and has remained about 20% since then.

Slide 4

As oxygen accumulated in the atmosphere, light from the sun began the photochemical process of ozone formation in the stratosphere. The ozone served, and still serves, to protect living organisms from the damaging effects of ultraviolet radiation. As the ozone shield developed, it enabled organisms to colonize land.

While multicellular organisms proliferated in the oceans as early as 680 million years ago, the first life on land did not appear until 400 million years ago. But once land was colonized, evolution rapidly radiated life forms and abundance of life increased.

The origins of today's fossil fuels were between 290 and 360 million years ago.

Although it is a commonly held belief that dinosaurs preceded mammals in the history of the earth by hundreds of millions of years, the fossil record tells us that early mammals and early dinosaurs arose about the same time, roughly 240 million years ago. Angiosperms later rose to dominate the vegetation of the earth starting about 100 million years ago.

But our earliest hominid ancestors arose only about 1 million years ago.

And the origin of our own species, Homo sapiens, has been dated at sometime before 40,000 years ago and perhaps as early as 70,000 years ago.

Slide 5

Scientists have estimated that only 4% of the oxygen ever liberated by photosynthesis remains in the atmosphere today. The rest is bound in various oxidized sediments like banded iron formations and Red Beds. Current rates of weathering and oxidation of soil minerals suggests that if photosynthesis stopped, it would take about 1 million years for all of the free oxygen in the atmosphere to be consumed. Most of the oxygen would be bound in iron oxides.

Remember that photosynthesis removes an amount of CO2 from the atmosphere equal to the amount of O2 liberated. The total net amount of free oxygen ever released is balanced stoichiometrically with the storage of reduced carbon-containing organic sediments including coal, oil, and other so-called fossil fuels.

Carbon dioxide is also depleted from the atmosphere by the formation of carbonate sediments which form from the precipitation of dissolved carbonates in sea water formed from the skeletons of marine zooplankton. The total sedimentary storage of carbon is estimated at 1022 g. Converted to Petagrams, a unit you should be more used to by now, that is 10 million Pg.

Slide 6

Astrophysicists theorize that our solar system formed from the remains of a supernova that ended its existence as a cloud of hot gases in space.

As the gases coalesced, the sun, planets and moons began to form. Their gravitational masses contributed to the process of condensation. The sun apparently achieved pressures sufficient to reinitiate the fusion of hydrogen to form helium. Elements became assorted to the planets relative to the positions of the masses in relation to the sun, and the rates at which the masses grew. The unique assortment of elements that became earth served to form the environment for life.

As the gaseous planet condensed, elements and minerals assorted based on relative atomic and molecular weights and densities. Radioactive decay of elements in the interior of the mass maintained temperatures high enough to melt iron, nickel and other metals. Their fluid nature allowed them to settle to the interior, forming the core. Lighter elements and minerals rose to form the mantle, and lighter materials still formed the crust.

Earth's original atmosphere was apparently much smaller than today, and consisted of the lightest gaseous materials remaining from the stellar supernova. Additional atmospheric contributions came from emissions of volatiles from the planet's interior through volcanism, and from planetary accretion, which is the addition of mass to the earth from meteors and planetesimals which continued to arrive in some abundance until 3.8 billion years ago. Hydrogen gas is not present in our atmosphere because earth's gravity is not sufficient to hold it in.

All of these processes resulted in the composition of the atmosphere, crust, mantle, and core of the earth each becoming distinct.

Recall that the composition of the atmosphere is dominated by N2 and O2. The core is mostly iron and nickel, while the mantle is iron, silica, magnesium and their oxides. The composition of the crust is dominated by lighter aluminosilicate rocks with abundant parts of iron, calcium, sodium, potassium and magnesium.

Slide 7

The composition of the earth's crust breaks down as follows.

Oxygen comprises the largest fraction of the crust at 46.6%. It is present in the form of oxides of silicon, aluminum, and iron.

Silicon contributes 27.7%, aluminum 8.1% and iron 5% of the crust.

The more rare elements of the crust include calcium at 3.6%, sodium at 2.8%, potassium at 2.6%, and magnesium at 2.1%.

The remaining 1.5% of the crust is made up of many other elements including sulfur, manganese, phosphorus, chlorine, zinc, copper, molybdenum, selenium, iodine, and cobalt.

Many of the elements present in very small amounts are essential to life.

Slide 8

Plate tectonics is a relatively new theory, as theories go, considering its importance to understanding the biosphere.

This theory was only first articulated for the general public in a book by Nigel Calder called "The Restless Earth," published in 1973. The theory of plate tectonics replaced prior theories of how the continents came to assume their current positions and shapes. Previous theories were called "continental drift" and the theory of the shrinking planet. If you are over 25 years old, you have lived through one of the most important paradigm shifts in science.

This theory says that the earth's lithosphere (that's the crust plus the top of the mantle) is made up of plates sliding on the asthenosphere (the lower mantle and core).

This theory explained how it is possible that the continents of the earth may have been in the positions shown on the globe to the left 225 million years ago, and moved to their positions today, shown on the globe to the right

Slide 9

This theory says that the earth's surface is made up of plates. Where the plates meet, three kinds of interactions between the plates can occur. The types of interactions determine the classification of the plate boundary.

Divergent boundaries are locations from which the surface of the earth is spreading. The plates are moving apart, and magma from the mantle and core is rising to fill the void left by the separating plates.

Convergent boundaries occur where plates are moving towards one another. Convergent boundaries generally form oceanic trenches as plates of heavier oceanic crust plunge beneath plates of lighter continental crust. Where the plates rub together, friction melts the crust, which may then be recycled to the surface through volcanic activity.

Transform faults occur where plates are moving in opposite directions along a common boundary. The San Andreas Fault with which you may have some experience, is a transform fault.

This figure is from your textbook, as is the previous one. Other figures included in pages 225-240 of your textbook do a very good job of explaining the operations of the theory of plate tectonics. Page 239 provides a graphical summary of the efforts to date to map all of the plate boundaries worldwide.

Slide 10

Plate tectonics operate in long term geologic time scales, but influence human activity in immediate time.

Earthquakes and volcanism, as anyone who lives on the Pacific Rim can tell you, create risk of exposure to hazards that are in many ways uncontrollable.

Increasing knowledge about the principles underlying geologic catastrophes can help us avoid some exposure to risk.

Plate tectonics can also explain the movement and concentration of useful ore materials. The past locations of the continents explains what environments were present in what locations, and what geologic processes would have been dominant. For example, knowing the locations of continents during the Carboniferous, the coal forming ages about 300 million years ago, tells us which continents were the most likely to have had the tropical environment necessary to promote the productivity required for the organic matter deposition which was the precursor for coal formation.

Slide 11

The deposition of organic matter in coal formation leads us naturally to think of the rock cycle.

This diagram illustrates the general pattern of the rock cycle. Let's begin by considering rock at the earth's surface. The earth's surface is constantly acted upon by the ravages of wind and weather.

Water and wind act to erode and transport materials from their points of origin and to deposit them in distant locations at lower topographic positions.

We refer to the deposit as sediment. As time goes on, our sediment may become buried by more sediment being deposited on top of it. Shallow burial creates modest pressure. The movement of water through buried sediments may cause slight chemical changes and rearrangements of particles which when combined with the pressure are sufficient to cement the particles of the sediment together. The resulting product is called sedimentary rock.

Sedimentary rock may either be uplifted and return to the surface by having its overburden removed by erosion, or it may be buried more deeply by the deposition of more sediment. Deep burial with its accompanying high pressure, the potential for high temperatures to develop, and the possible presence of water cause the rock to metamorphose. The water may remove some parts of the sediment, and precipitate others. Pressure and heat may cause both chemical and physical changes in the rock. The result is called metamorphic rock.

Metamorphic rock can either be uplifted, or may undergo further pressure and heat causing it to melt. Melting and the solidification resulting from cooling and uplift result in igneous rocks. The process of melting may involve the rock becoming part of the magma. And magma from deeper may be uplifted or rise to the surface to cool and solidify as igneous rock.

Slide 12

Diagram is from Schlesinger, W. H. 1991. Biogeoghemistry: An Analysis of Global Change. Academic Press. San Diego. page 4.

The rock cycle connects the geosphere to the biosphere. This diagram, from a book by Bill Schlesinger called "Biogeochemistry: An Analysis of Global Change," relates organic matter accumulation and the deposition of minerals. This particular example, shows the effects of an increase in organic matter in the biosphere. The repercussions of the fixation of carbon are connected to dissolution and sedimentation of magnesium, calcium, sulfides, carbonates, silicates, sulfates, and iron oxides.

This is a very complex system model. Its complexity stems from the nature of chemical reactions to lead from one to another like a domino effect. Each reaction is in stoichiometric balance with the previous one -- all reactions in the chain, or more appropriately here, the web, fueled by the energy of photosynthesis that caused the increase in biomass.

I am showing you this system model to make the point that the rock cycle is connected to the biosphere. The system models that I will expect you to use and remember as part of this class are much simpler because they will not attempt to consider so many factors at once.

Slide 13

The rock cycle is driven both by surficial processes - processes that happen at the surface of the earth, and by processes happening deep in the mantle and core of the earth.

Deep in the core and mantle, heat of radioactive decay of elements creates currents of mass movement of materials.

Remember the contribution solar radiation and friction as explained by the Theory of Isostacy. The Theory of Isostacy explains the manner in which solar radiation contributes to mass imbalance of the plates perpetuating their motion.

With the exception of mass wasting, surficial processes including weathering, erosion, transportation and deposition are powered by solar radiation .

Slide 14

Weathering is defined as the chemical and mechanical alteration of rock materials during exposure to air, moisture, and organic matter.

Weathering transforms solid rock and other forms of parent material into smaller particles. The particles are altered physically (or mechanically) from the parent material. They are different sizes and shapes from the parent material. They may also be altered chemically. Water may remove some elements or minerals, and deposit others. Some elements or minerals may be oxidized by exposure to atmospheric oxygen. The presence of organic matter may chemically alter the composition of the material, and may contribute acids that will increase the effectiveness of water to dissolve parts of the parent material.

Biological organisms can contribute to the weathering of parent material. Roots may dislodge particles from rock through their expansion. Burrowing organisms, from tiny worms to large mammals, can provide channels for water to reach previously unexposed substrate below the surface. The presence of the organisms adds organic matter that contributes to chemical alteration.

Slide 15

Weathering processes include the impaction of rain drops that can physically remove particles from the surface of a substrate.

Because water expands when it freezes, the action of water freezing and thawing can wedge small and large pieces of rock apart when ice forms in rock crevices.

Temperature fluctuations alone even in the absence of water can cause the substrate itself to expand and contract. Expansion and contraction create forces that can break apart pieces of rock. They will heat differentially more at the surface than deeper, so expansion will be greater in some parts of the rock than in others.

Some minerals, and organic matter in particular, shrink and swell with hydration state. Hydration state involves the amount of water that is bound by contact with surfaces of the substance. The shrinking and swelling create mechanical forces that may break the substrate apart. Chemical weathering processes include oxidation, as in the formation of aluminosilicates and iron oxides that we talked about earlier.

The presence of water will dissolve some elements and minerals, while depositing others. Often, water will dissolve minerals in the top layers of substrate, and deposit them in lower levels as temperature and chemical properties of the substrate change with depth. This causes some substances in the substrate to migrate. Additions of organic matter, dissolution and substitution can change the pH of the substrate. pH is a measure of the acidity, or concentration of hydrogen ion in a substrate.

Slide 16

The process of weathering is essential to soil formation.

While we may often think of weathering, erosion and transportation as bad processes, and ones we want to avoid in our human activities, weathering is a natural process.

Without weathering we would not have the soils we do today.

Without weathering, soils would not be renewed as they are eroded.

Without weathering, nutrients would not be freed from rock substrates for uptake by plants.

Slide 17

Weathering plays a fundamental role in the basic principles of soil and soil formation. We will talk about this in greater depth later.

Soil is defined by this statement:

Soil develops as the result of the interaction between substrate, climate and organisms.

Another way of stating it is that soil is the product of weathering just as we have described it, and of the deposition of organic matter.

Our description of weathering has emphasized the chemical and physical alteration of rock. But rock is not the only substrate that undergoes weathering.

Slide 18

Substrate is the general term we use for anything upon which a soil may develop.

Rock, of course, of all kinds, is substrate.

Colluvium is also substrate. Colluvium is what results when a hillside fails, and a jumble of material cascades down the hill, perhaps to the valley floor.

Water deposited sediments like sand, silt or clay, for example on the flood plain of a river, may serve as substrate for the development of soil.

Likewise, wind-blown silt and sand, and volcanic ash can all serve as substrates upon which soil can develop.

All of these substrates can and do undergo chemical and mechanical alteration to form media appropriate for plant growth and habitation by soil organisms.

Slide 19

The next step in surficial processes of the rock cycle is erosion.

In everyday language, we usually use the word "erosion" to include the entire three step process of weathering, erosion and transportation.

But erosion more specifically refers to the weathering to the point of loosening particles from their parent material. It is specifically defined as the physical and chemical breakdown of rock, loosening and removal from point of origin.

It is the readying of the products of weathering for the transportation process.

Slide 20

Transportation then is defined as the movement of the products of weathering and erosion. Transportation is facilitated by water, in arid regions or in dry seasons by wind.and in cold wet climates by the movement of ice in the form of glaciers.

We are all familiar with the immense potential for flood waters to move great masses of eroded materials great distances down stream.

The largest portion of erosion and transportation occur during extreme events, like winter storms, hundred year storms, and hurricanes.

Some transportation is accomplished without water, wind or ice.

Slide 21

Mass-wasting is the movement of regolith down-slope by means of gravity without the aid of water, wind, or ice.

Regolith is weathered and eroded substrate. Mass-wasting produces the colluvium mentioned earlier.

Mass-wasting is a common phenomenon in the steep coast ranges of California.

Mass wasting can be facilitated by the presence of water saturated layers in the soil that provide lubrication. Water may also add extra weight to the hill side, contributing to its tendency to fail.

Slide 22

All of this material that has been weathered from substrate, eroded, and transported must eventually be deposited. The eventual destination may be the ocean floor. But many interim destinations are possible.

Materials may be deposited as lake sediments. The picture of this dam is to remind us that deposition of sediments in reservoirs is one of the things limiting their useful life span for electrical generation, irrigation or municipal water supply, flood control, and recreation. Colluvium may be deposited on the valley floor.

Wind deposits result in sand dunes and loess deposits on plains. Glaciers deposit a wide array of formations that come under the inclusive heading of moraines.

All of these deposits may serve as substrate for the development of soils if they are not submerged. And they can all participate further in the rock cycle, either by being further transported and deposited, or by being buried more deeply, and becoming sedimentary rocks, metamorphic and igneous rocks, yet to be uplifted again.

Slide 23

There are may reasons to be interested in surficial processes.

Weathering provides for soil renewal, and the liberating of plant nutrients from parent material -- but it also initiates erosion.

Weathering affects not only naturally occurring rocks, but also human erected structures like buildings and bridges. Civilization is working constantly to repair the damage of weathering Erosion causes the loss of productive soil. If erosion exceeds weathering rates, soils will eventually be depleted.

Transportation results in polluted water. Water carrying particulate matter is not drinkable, is difficult to purify, deposits sediments in watercourses and in pipes.

Mass wasting is a potential hazard wherever human settlements occur in hilly terrain.

Sedimentation reduces navigability of waterways, shortens the life span of dams, creates nuisance and destroys human settlements. Wind deposited sand dunes and loess blow across roads and highways, and bury productive agricultural soils.

We are living with and coping with surficial geologic processes all the time. Planning of human activities to reduce risk of harm from exposure to the hazards of surficial geologic processes requires knowledge of the principles governing them.

Slide 24

To reiterate -- We have looked at the geologic time scale to place in perspective some of the issues of energy and its interaction with the atmosphere and biosphere. The earth's atmosphere is the result of billions of years of biologically mediated interactions between the atmosphere and the geosphere.

We compared the composition of the earth's crust, mantle, and core, with that of the atmosphere, and found that they are quite different.The distribution of elements and compounds is not unlike what one would predict based on their relative atomic weights.

We were reminded of the Theory of Isostacy as we reviewed the surficial and deep forces driving plate tectonics. And we reviewed the cycle of rocks in the geosphere.

Finally, we related the surficial geologic processes of the rock cycle to the identification of human environmental problems, asserting the importance of geologic knowledge and planning for risk reduction.