Dr. Ranjit Prasad
Sunil Kumar Meena(489)
Dhanraj Meena(458)
Ashutosh kumar(449)
Geological work of sea
It is well known that about 71 % of the surface of the earth is covered by the oceans and seas. The
oceans and seas cover an area of about 361 million square kilometre out of 510 million square
kilometre of the surface of the entire globe.
About 1.4 billion cubic kilometres of water is concentrated in oceans and seas. The greatest known
depth in the ocean is 11022 metres at the Mariana Trench in the Pacific. Land is concentrated mainly
in the northern hemisphere and the water bodies in the southern hemisphere.
Nearly 61 per cent of the area in the former and 81 per cent in the latter are covered by water. The
four recognized oceans in the world are - the Pacific, the Atlantic, the Indian and the Arctic ocean.
The Pacific Ocean covers about 49% of the earth surface, the Atlantic Ocean-26%, Indian ocean-21%
and Arctic ocean - 4% of the world ocean.
The geological activity of seas and oceans, like other geological agents, comprises the processes of
erosion, transportation and deposition, which depend on a large number of factors such as :
(i) Relief of the floor.
(ii) Chemical composition of the sea water.
(iii) Temperature, pressure and density of sea water.
(iv) Gas regime of seas and oceans.
(v) Movement of sea water.
vi) Work of sea organisms etc.
(i) Relief of the floor
It has been established that the floor of the oceans exhibits an uneven topography with prominent
elevation and depressions.
On the basis of available bathymetric maps, the ocean is divided into definite regions as indicated
below:
(a) Continental shelf
The ocean floor gradually slopes down wards away from the shore. The shallow-water zone adjoining
the land, with average depth down to 200 metres constitutes the continental shelf.
It varies in width from a few kilometres to several hundred kilometres. The continental shelves cover
about 7.6 per cent of the total area of the oceans and 18 per cent of the land. About 20% of the world
production of oil and gas comes from them.
(b) Continental slope
From the edge of the continental shelf, the sea floor commonly descends to the ocean basin, with an
average gradient of 3.5° to 7.5° and is known as continental slope.
Its depth ranges between 200-2500 metres and covers about 15% of the total area of the ocean. It has
an average width of 16 to 32 Kilometres.
(c) Continental rise
It extends from the bottom of the continental slope to the floor of the ocean basins. The rise has a
slope of 1° to 6°. Its width varies from a few kilometres to a few hundred kilometres. The material of
the rise has been derived from the shelf and slope.
(d) Ocean floor
It begins at a depth of 2000 metres and goes down to 6000 metres. It covers 76 per cent of the total
area of the oceans and has a very gentle gradient, being measured in minutes.
It contains a number of distinctive topographic units such as abyssal plains, seamounts and guyots,
mid-ocean canyons, and hills and rises that project somewhat above the general level of the ocean
basins.
Apart from these features, the most significant feature of the ocean floor is the occurrence of midoceanic ridges and deep-oceanic trenches.
(ii) Chemical composition of sea water
The oceanic water contains a large number of dissolved salts and have almost a uniform
composition. These salts result in the property of salinity. The average salinity of sea water is 35
parts per thousand i.e. one litre of sea water contains 35 grams of various dissolved salt.
But the value is small where large rivers meet the sea and the value is higher within the zone of hot
and dry climate. In the Mediterranean Sea, for example, the sea level. is lowered by evaporation and
the salinity as well as the density of the water increases.
Sea water of normal salinity contains mostly chlorides which aggregate above 88% followed by
sulphates more than 10% and small amounts of carbonates and other compounds.
Sodium chloride constitute the bulk of the dissolved salts in sea water, followed by Magnesiumchloride, Magnesium sulphate, Calcium sulphate, Potassium-sulphate.
Apart from these salts, there are also elements like iodine, fluorine, zinc, lead, phosphorous etc. in
sea water.
Salinity determines features like compressibility, thermal expansion, temperature, density,
absorption of insolation, evaporation, humidity etc. It also affects the movements of the ocean
waters.
(iii) Temperature, pressure and density of sea water
The temperature of the oceanic waters plays a significant role in the movement of large masses of
oceanic waters and distribution of organisms at various depths. The temperature of the oceans is not
uniform.
The temperature of the water on the surface of oceans and seas is determined by the climatic
conditions. In tropical zones it is usually higher than in polar regions. Besides the temperature also
varies with the depth of the sea water.
There are two main processes of the heating of oceanic waters, viz. absorption of radiation from the
sun and convection; whereas cooling is caused by back-radiation of heat from the sea surface,
convection and evaporation. The interplay of heating and cooling results in the characteristics of
temperature.
The pressure in the oceans and seas varies vertically and increases with depth by 1 atmosphere for
each 10 metres of the water column. It is highest in the oceanic trenches (between 800-1000
atmosphere). At great pressure the dissolving capacity of sea water increases.
The density of sea water varies with narrow limits between 1.0275 and 1.0220, due mainly to
variation of temperature and salinity. It is highest in higher latitudes and lowest in the tropical areas.
(iv) The gas regime
The sea water contains mostly dissolved oxygen and carbon dioxide. Sea water derives oxygen from
the air and also through photosynthesis by marine plants. Similarly the content of the carbon dioxide
is mainly due to the atmosphere, river waters, the life activity of marine animals and volcanic
eruptions.
It has been seen experimentally that at a temperature of 0°C, the sea water can absorb about 50
cubic centimetre of carbon dioxide and 8 cubic centimetre of oxygen. The content of oxygen and
carbon dioxide are of much significance in the processes of marine sedimentation and dissolution of
chemical compounds.
(v) Movement of sea water
The sea is a mobile mass of saline water and the movements of sea water are of great geological
importance as they determine the intensity of destruction caused by the oceans and seas on the shore
and the floor and also the distribution and differentiation of the sedimentary materials that enter the
seas and oceans.
The waters of oceans and seas are subjected to the action of wind, the attraction of the sun and the
moon, and to changes of temperature, salinity, density etc. All these factors give rise to three main
types of movements. Such as Waves, Currents and Tides.
Waves
Waves are generated mainly by the wind blowing over the surface of the ocean and sea water. The
friction of wind moving over the water surface causes the water particles to move along circular or
near- circular orbits in a vertical plane parallel to the direction of wind.
There is almost no forward motion. Thus energy is transferred from the atmosphere to the water
surface by a rather complex mechanism involving both the friction of the moving air and the direct
wind pressure.
The ocean waves are oscillatory waves (or transverse waves) as they cause an oscillatory wave
motion. The waves consist of alternating crests and troughs. Wave length is horizontal distance from
crest to crest or trough to trough.
Wave height is the vertical distance between trough and crest. Wave period is the time taken by two
consecutive crests to pass any reference point.
Wave velocity is the ratio between the Wave length and the Wave period.
The waves of oscillation are characteristic of deep water. As waves move into shallow water, they are
slowed by the friction with the sea-floor and thus the wavelengths become shorter while the wave
height increases, the paths become elliptical and the wave steepens.
Since the front of the wave is in shallower water than its rear part, there is an increase of the
steepness of its frontal slope and the wave becomes highly unstable. At this stage, the wave is transformed into a breaker, which then collapses forward in a curling, frothing zone of surf.
As the wave breaks, its water suddenly becomes turbulent. The turbulent water mass then moves up
the beach as the swash or uprush. Thus, both water and wave energy move forward against the shore
and the wave is called a Wave of translation.
This energy causes erosion and transports material along the shore. The return flow which sweeps
the sand and gravel sea ward is called backwash.
Currents
In the currents, there is an actual movement of the water over great distances, which may be caused
by various factors, such as-the differences in temperature, salinity, action of steady and periodic
winds etc.
Tides are periodic movements of the ocean waters due to the gravitational attraction of the sun and
moon on the earth. Twice a day, about every 12 hours 26 minutes, the sea level rises and it also falls
twice a day.
When the sea rises to its highest level, it is known as 'high tide' and similarly when it falls to the
lowest level, it is called 'low tide'.
There are two tides of a special occurrence viz. (a) the spring tide and (b) the neaptide. The spring
tides occur twice every month at new moon and full moon, whereas in the first and third quarter the
attraction of the sun and moom tends to balance each other and small tides, which are termed 'neap
tides', occur.
The currents caused by high tides in the littoral zone are quite strong and can carry quite big
fragments of rocks to the shore or along it, eroding the bottom.
(vi) Work of sea organisms Seas and oceans are inhabited by a large variety of animals and plants.
Their development and distribution depends on the depth of the sea, its temperature, salinity,
pressure, penetration of light and the dynamics of the sea water etc. Marine organisms are divided
into three major groups such as benthos, plankton and nekton.
The benthos group includes organisms both mobile and sessile which inhabit the bottom of the sea.
The plankton group includes organisms which are passively floating by the waves and currents.
Unicellular organisms (animals) like foraminifers and radiolarians, and, diatoms (plants) belong to
this group. The nekton group includes all actively swimming animals which comprises all the sea
vertebrates and invertebrate molluscs.
These marine animals are important in producing biogenic sediments.
The above factors together play significant roles in the erosion, transportation and deposition by the
oceans and seas.
GEOLOGICAL WORK OF WATER
I. Introduction
A. The hydrologic cycle is the unending circulation of Earth’s
water, driven largely by energy from the Sun. The cycle
describes the storage and movement of water near Earth’s surface
that is it describes the reservoirs in which water resides and
the processes that move it around. The ocean is by far the
largest reservoir, followed by glaciers and then by groundwater.
Although these volumes are relatively constant, significant
amounts of water move continuously in and out of these
reservoirs.
Reservoir
Percentage of Earth’s Water
Ocean
Glaciers
Groundwater
Freshwater Lakes
Saline lakes/Inland Seas
Soil Moisture
Atmosphere
97.2
2.15
0.62
0.009
0.008
0.005
0.001
B. This circulation of water from one reservoir to another
is
called the hydrologic cycle.
1. Water moves out of the ocean via evaporation and is
carried by atmospheric currents of air over the land masses
where precipitation occurs in the form of rain and snow.
2. Water is carried by run-off along the surface of the
ground and eventually runs into rivers or seeps down into
the ground to become part of the subsurface water
reservoir.
3. Water flows in rivers and in underground conduits from
higher to lower elevations, towards the sea.
4. Water also evaporates directly from the land surface or
is returned to the atmosphere by plants in a process
collectively called evapotranspiration.
C. Average annual USA water budget:
Precipitation
76 cm
Evapotranspiration
53 cm
Run-off
23 cm
Infiltration
0.025 cm
D. Rivers contain much less than 1% of the total water on
Earth. Their importance stems from their major role in the
transport of water, dissolved solids and suspended matter. In
this light, they are much more important than the other
transport agents - the atmosphere and glaciers.
E. Rivers are the major routes by which continental rain and
the products of continental erosion reach the oceans.
II. Stream flow - key concepts
A. The complex interactions between flowing water, channel
shape, climate, channel material, etc. cannot be adequately
expressed mathematically. The multitude of variables that define
the system are constantly adjusting and readjusting to minor
variations in flow. Scientists studying river flow have often
had trouble determining what was causing observed changes. It is
also often difficult to determine, which are the dependent and
which are the independent variables.
B. Turbulent flow of water in streams is by far the most
important type of fluid flow observed there. When the velocity
of the stream exceeds some critical value the flow changes from
laminar to turbulent.
1. In turbulent flow, as the velocity of the water
increases, the individual water particles follow a swirling
chaotic path with eddies superimposed on the main forward
flow.
C. The velocity of the stream water is equal to the distance the
water travels in a given amount of time:
Velocity = Distance / Time
1. The typical range of stream velocities is
15 - 750 cm/sec
0.32 - 16 mph
D. Factors determining stream velocity
1. It's important to understand the factors which determine
stream velocity because erosive capability of a stream is
proportional to velocity.
a. The faster the stream flows, the more it can erode.
2. Many factors affect the velocity of a stream, but a
generalized formula can be used to describe most of them:
G
V =
√ ------R
V
G
A
R
P
the
=
=
=
=
=
A
P
velocity
Gradient
cross-sectional area of the channel
roughness of the channel floor
wetted perimeter (total length of that part of
channel sides and bottom that are underwater)
a. A change in one of these factors can cause changes
in the other factors, thus contributing to the
complexity of stream flow.
3. Gradient or slope - the drop in vertical elevation of
the stream's surface for a specific horizontal distance the
stream flows.
a. Geologists used to think that this was the
predominant factor determining velocity, but depth may
be more important.
b. Usually measure in feet of elevation drop / miles
of horizontal distance traveled.
c. It's measured just like you would measure the
gradient of a hill.
d. Longitudinal profile- cross section of stream along
its course from headwaters to mouth.
e. Generally a stream has a steeper gradient near its
headwaters at its highest elevation giving it a
concave upward profile.
f. Low gradient = 5 cm/km = 3"/mile = parts of
Mississippi River
g. High gradient = 6600cm/km = 350'/mile
4. The force of gravity that drags the water downhill from
the higher elevations must compete w/ frictional forces
generated between the water and the stream bed. The
magnitude of these frictional forces varies with the size,
shape and roughness of the channel.
a. Consider the following different channel shapes,
all with the same cross-sectional area (10 units2)
1) Wide, shallow channel (Perim. = 12 un)
2) Deep, narrow channel (Perim. = 12 un)
3) Semicircular channel (Perim. = 8 un)
5. Summary of how the above factors affect velocity
a. Steeper gradient (G)
= higher
velocity
b. Larger cross-sectional area (A) =
"
"
c. LOWER PERIMENTER = LESS FRICTION =
"
"
d. Less channel roughness (R)=smoother = "
"
6. The competition between gravity and friction results in
velocity differences within a single stream. The velocity
is higher for water that travels in the stream where the
friction is the least.
a. For example, in a straight channel, its is fastest
in the center, near the surface and slowest along the
bottom.
b. Also, in a curved channel the velocity is highest
around the outside of the bend.
c. The maximum turbulence occurs where these two
opposing forces go head to head such as along the
walls and bed of the stream. And as it turns out, the
more turbulent the flow the higher the eroding
capability of the stream
E. When you put all this information together you come up with
the concept that the parts of the stream where the velocity is
the highest and the interaction with the stream bed is the most
intense- the most erosion occurs. That is, the most intense
erosion occurs on the outside of the bends of a curved stream
channel or along the bottom and the edges of a straight stream
channel.
F. Another important factor that affects stream velocity
is DISCHARGE, which is the volume of water flowing past a fixed
point (gauging station) in a specified amount of time.
1. Discharge varies from stream to stream, and from time to
time AND place to place in a single stream.
a. It is usually higher in the spring due to melting
of winter snow.
b. Usually higher farther downstream as
more tributaries (smaller streams that flow into
larger stream) merge into the big stream.
c. Usually higher during the rainy season in a region
that has distinctly wetter and drier seasons of the
year.
G. A diagram called a HYDROGRAPH is used to display the
variations of stream discharge with time, at a specific
location.
1. Hydrographs can show yearly, monthly, daily or
instantaneous discharges and periods of high and low
discharge can be determined from them.
2. Hydrographs compiled over long periods of time are
useful for designing irrigation and power systems, and for
predicting water-supply patterns and for forecasting
floods.
H. Flooding
1. When the amount of water in a stream exceeds the
capacity of the stream bed the water rises up over the
banks and floods the adjacent lowland called floodplain
a. Bankful stage=maximum capacity of stream bed that
just fills stream channel. Observations have shown
that a stream rises to its bankful stage about once
every 1.5 - 2 years. The size of the stream channel is
adjusted to handle the volume of flow that occurs once
every 1.5-2 years.
2. Floods are a natural, recurring phenomenon. The time
elapsed between occurrences of a particular size flood is
predictable if historical data is available. In other
words, floodplains flood periodically, and we must keep
this in mind when we contemplate building on a floodplain.
3. Perhaps the greatest immediate practical use of stream
monitoring and hydrographs is to provide knowledge of the
magnitude and probable frequency of floods.
a. A simple statistical method enables hydrologists to
construct a diagram such as this that allows them to
predict flood recurrences.
4. Flood-control dams help to modify the shape of the
hydrograph and to lessen the impact of flooding on
downstream regions by controlling the rate at which flood
water is allowed to go downstream.
5. Urbanization and agriculture usually have an opposite
negative effect on stream flow.
a. Effects of urbanization
1) Decreases the amount of water sinking into the
ground because it is sealed off by concrete
buildings, streets and parking lots.
2) Increases the amount of run-off.
3) Drops the level of groundwater.
b. Effect on stream flow
1) The time lag between precipitation and flood
peak is decreased because there is no
infiltration and less vegetation to slow the
water down.
2) The flood peak is higher because the stream
must carry more water in a shorter time.
c. Urbanization often creates flashy streams. That is
streams with a low normal flow and a high, short flood
peak.
I. Base Level
1. Another key concept in the study of stream flow
is base level, which is the lowest point to which a stream
can erode its channel.
a. The ocean is the ultimate base level and as we have
seen, since sea level fluctuates, this ultimate base
level does not have a permanent elevation.
2. Alternate definition-the elevation or level at which the
mouth of the stream enters a large standing body of water
(lake or ocean), and disappears as a river.
3. Graded stream
a. Nature looks for the easiest way to get her work
done, which includes getting water from high
elevations down to the ocean. Therefore, over the
course of time a stream deposits and erodes its
channel to achieve the most efficient profile for
carrying its sediment load. A stream that has achieved
this most efficient profile is called “graded”.
4. A stream can have a number of local base levels (e.g.,
lakes or waterfalls) that affect streamflow directly
upstream of them. Along the course of the river upstream
from these local base levels the river can not erode below
the elevation of the lake or waterfall.
a. These local base levels are temporary and disappear
when the lake or waterfall or dam disappears.
5. Artificially raising or lowering the base level of a
stream affects the stream flow, gradient and channel
characteristics.
a. If the base level is raised such as by building a
dam, the upstream gradient becomes less steep, this
leads to decreased stream velocity, decreased energy
available to transport sediment and results in
deposition of material. This is one factor that
contributes to the "filling-in" behind dams.
b. Downstream from the dam, the gradient of the stream
is increased which has the opposite effect and leads
to erosion of material below the dam.
III. The work of running water
A. Introduction, The work or funning water includes:
1. Transportation of sediment
2. Erosion of river channel
3. Deposition of sediment
B. Transportation
1. Terminology of stream carrying characteristics
a. Load-amount of material a stream carries at any
time.
b. Capacity-the maximum load of particles of any size
that a stream can transport.
c. Competence- a measure of the largest grain a stream
can transport. It is a measure of the ability of a
stream to carry particles of diff. sizes.
1) Higher competence is usually associated with a
higher stream velocity.
2. Ways in which streams carry particles
a. Solution - the dissolved load, carried as ions
1) Commonest compounds carried in streams: Ca,
Mg, HCO3, Cl, SO4, NO3, Si
2) Amount of dissolved material varies with:
a) Climate
b) Season
c) Geologic terrain being eroded
b. Suspension - Turbulent currents lift particles up
into the flowing water. Particles carried in this way
are said to be in suspension.
1) Turbulence increases when velocity increases
so the greatest amount of material of the largest
size can be carried during floods.
2) Mainly mud and silt are carried in suspension.
c. Bed load - particles that roll or slide along the
stream bottom make-up the bed load.
1) Mainly sand and gravel are carried in bed load.
d. Clastic load = suspended + bed load
1) Greatly increases during periods of increased
discharge.
C. Erosion
1. Streams erode material by a number of different means:
a. Streams dislodge material from their beds and carry
it away.
b. Abrasion - Solid particles carried by the stream
act as erosive agents by wearing down the bedrock.
1) Also the impact of large particles knocks
fragments off the bed.
c. Dissolves channel debris and bedrock.
D. Deposition
1. When the velocity of the stream decreases below that
needed to keep material in suspension, deposition begins.
a. Alluvium - any stream-deposited material.
2. Channel deposits - any alluvium deposited within the
bounds of the channel.
a. Bar - any such deposit
b. Point Bar - bar on inside of bend
3. Braided stream results when sediment load is large with
respect to discharge, gradient, and channel depth.
a. The shallow channels tend to become filled &
streams break through their walls & form new channels.
What forms is a complex network of converging &
diverging channels that thread their way among bars
1) Many of the subsidiary channels will rejoin
further downstream giving the stream a familiar
pattern that gets the name braided.
b. For example, if a steeper, more turbulently flowing
tributary enters a main stream, its rocky bed load may
be deposited at the junction. Excessive load may also
be provided when debris from barren slopes is flushed
into a channel during a heavy downpour, or at the end
of a glacier
c. Braided streams also form where there is an abrupt
decrease in gradient of discharge.
E. Floodplain deposits
1. Natural levees - elevated land forms made of alluvium
and deposited immediately on either side of the stream
channel
a. Lower Mississippi R. = 20 ft.
b. They parallel streams and confine waters near the
river creating marshy conditions (back swamp)
c. Natural levees can also prevent tributaries from
making their way into the main channel, so that they
are forced to flow through the back swamp zone
parallel to the main river for many kilometers = Yazoo
tributary
F. Delta deposits - accumulation of sediment formed where a
stream enters a standing body of water (e.g., lake or ocean).
1. Relatively flat, barely above the water surface.
2. The sedimentary structures observed in a delta reflect
the different particle sizes being deposited and the
differing energy levels available to the stream for
depositing them.
3. As the delta grows outward, the stream's gradient
continually lessens, which decreases its ability to carry
sediment. Therefore, the channel becomes choked with
sediment and the river seeks a shorter route to base
level.
4. Main channel also divides into smaller ones
called distributaries, which do the opposite job to that of
tributaries. Instead of feeding into the main stream, they
carry water away from the main channel.
5. The delta of the Mississippi River began forming
millions of years ago near the town of Cairo, Illinois.
Since then it has advanced nearly 1000 miles south.
a. The Mississippi River delta is actually a series of
7 coalescing subdeltas.
b. For many years the Mississippi has been trying to
cut through a narrow neck of land and shift its course
into that of the Atchafalaya River. If this happens
the river will abandon 500 kilometers of its path in
favor of a much shorter route. Only the construction
of massive dams has prevented this from happening.
G. Alluvial fan deposits - fan-shaped deposit formed where a
stream's gradient is abruptly reduced. These commonly form where
a high-gradient stream leaves a narrow mountain valley and
passes out onto a broad, flat plain or valley floor.
1. The sudden drop in velocity causes the stream to drop
its load in a fan- or cone-shaped deposit.
2. The dryland equivalent of a delta, but can be steeply
sloping
IV. Features of stream valleys
A. If not for mass wasting all stream valleys would have
straight, vertical walls. However, mass wasting generates a
characteristic V-shaped cross-section.
B. The features developed in stream valleys depend on:
1. Geologic Age of region
2. Tectonic activity
3. Bedrock composition
4. Climate - affects mass wasting & weathering
5. Because of these factors, stream valleys vary, but there
are two main types.
C. Narrow, V-shaped valleys
1. Valley walls - rise on either side of floodplain to the
crests of the flanking hills or mountains which are
called:
a. Divides - separations between valleys.
2. Rapids form where there is a sudden gradient increase
3. Waterfalls form where there is an extremely large &
sudden increase in gradient.
a. Not all falls form in the same way.
1) Niagara Falls - resistant layer overlying less
resistant layer
2) Hanging Valley Falls in glaciated regions form
where a smaller tributary glacier entered a
larger glacial valley
b. These "knickpoints" in the stream's profile are
actually the cause of their own disappearance, since
the steep slope results in shooting flow, with
increased erosion from impact,
eddies, and cavitation.
D. Wide valleys with flat floors
1. Meanders - form by a combination of erosion and
deposition
a. Obstruction forces current against bank
b. Outside of bank erodes due to increased turbulence
c. Material carried downstream & deposited in center
of stream & on inside of next bend
d. They migrate across and down valley
2. Cut bank - outside of meander
3. Cut-offs - new, stream channel formed when the
narrow
neck of a meander is cut through and the
stream abandons
the old bend.
4. Ox-bow lakes = Water-filled abandoned meanders
5. Meander scars = Dry abandoned meanders
V. Drainage Networks
A. Drainage basin - entire region from which a stream and
its
tributaries receive their water.
1. Every stream, no matter how small, has one.
B. The type of drainage network that develops depends on
composition and structure of the underlying rock.
1. Dendritic = tree-like = develops in areas where surface
materials all erode at the same rate (i.e. where the
underlying strata are horizontal and the same material is
exposed for large distances.) This situation is found in
regions underlain by flat-lying sedimentary rocks &
massive, homogenous igneous and metamorphic rocks.
2. Radial = streams radiate out in all directions from a
central high = domal uplift or volcano.
3. Rectangular = differential weathering of fractures or
joints in bedrock localizes flow into an ordered geometric
pattern.
4. Trellis = forms when bands of rocks resistant to
weathering alternate with bands that erode more
rapidly. This is typical where rocks have been deformed
into a series of parallel folds and are, therefore, very
common in the Appalachian Mountains.
C. To understand fully the features of a drainage network it is
often necessary to study the history of the region.
1. Water gap - Steep-walled notch followed by a river
through a raised structure in the landscape
2. Why does the river go through the mountain instead of
around it?
a. Antecedent stream - existed before the structure,
which was raised later while the stream continued
to downcut. Sequence of events:
1) Stream forms
2) Tectonic Uplift generates ridge
3) Downcutting of stream keeps pace with uplift
b. Superposed stream - stream let down upon structure,
which already exists in the rocks that underlie the
unit the stream is cutting into.
3. Headward erosion and stream piracy
a. Streams can lengthen their course
by headward erosion extending into
previously undissected land.
b. Can lead to stream piracy and wind gaps.
VI. Stages of Valley Development
A. Until recently geologists believed that river valleys evolved
through predictable stages of development as time passed.
Recently, however, this concept has fallen out of favor with
fluvial geomorphologists. It is now believed that the features
developed in a stream valley result from a much more complex
interaction of factors and don’t just follow one another
predictably as time passes. In nature there are no clearly
definable differences in the features found in streams of
different ages. Instead, the features developed in a stream
valley are much more dependent on tectonics, climate, and
bedrock, than on the number of years since they originated. For
example, it may take 106 years to develop rapids and waterfalls
along a stream cutting into granite, whereas, in a region of
more easily erodable mudstone a valley may evolve to this point
in a much shorter period of time.
B. What is observed is that a stream that is
actively downcutting rather than eroding laterally, is typified
by the presence of a narrow stream channel that almost
completely fills the stream valley (i.e., not much of a
floodplain), rapids, waterfalls, few meanders, and a steep
gradient.
C. A stream that more closely approaches base
level increases the amount of lateral erosion and decreases the
amount of downcuttingit is doing. This increases the valley
width so that it may greatly exceed the channel width. Also,
meanders, cut-offs, oxbows, a lower gradient, and a smoother
profile develop.
D. A stream that is mainly reworking previously-deposited
alluvium, which is easier to do than downcutting, tends to
meander extensively over an exceptionally wide floodplain. For
example, the lower Mississippi River meanders migrate at a rate
of ~20m/yr. Such streams are seldom near their valley walls so
the valley doesn't widen anymore. That is, there is virtually no
more lateral erosion of the valley walls and the
only downcutting that occurs is into previously-deposited
alluvium. The stream gradient is low and oxbow lakes, meanders,
cut-offs, natural levees, yazoo tributaries, and back swamps are
common.
E. A stream may be rejuvenated and begin
vertical downcutting
again because of a change in base level
caused by processes such as tectonic uplift or decreasing sea
level, both of which increase gradient. In this situation
entrenched (or incised) meanders and stream terraces can result.
The Grand Canyon of the Colorado is an example of a rejuvenated
stream.
Geological work of atmosphere
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through
contact with the Earth's atmosphere, biota and waters. Weathering occurs in situ, or "with no movement",
and thus should not be confused with erosion, which involves the movement of rocks and minerals by
agents such as water, ice, snow, wind, waves and gravity.
Two important classifications of weathering processes exist – physical and chemical weathering; each
sometimes involves a biological component. Mechanical or physical weathering involves the breakdown
of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and
pressure. The second classification, chemical weathering, involves the direct effect of atmospheric
chemicals or biologically produced chemicals (also known as biological weathering) in the breakdown of
rocks, soils and minerals,
The materials left over after the rock breaks down combined with organic material creates soil. The
mineral content of the soil is determined by the parent material, thus a soil derived from a single rock type
can often be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock
types (as in glacial, aeolian or alluvial sediments) often makes more fertile soil. In addition many of
Earth's landforms and landscapes are the result of weathering processes combined with erosion and redeposition
Physical weathering
Physical weathering is the class of processes that causes the disintegration of rocks without chemical
change. The primary process in physical weathering is abrasion (the process by which clasts and other
particles are reduced in size). However, chemical and physical weathering often go hand in hand.
Physical weathering can occur due to temperature, pressure, frost etc. For example, cracks exploited by
physical weathering will increase the surface area exposed to chemical action. Furthermore, the chemical
action of minerals in cracks can aid the disintegration process. Physical weathering is also called
mechanical weathering, disaggregation
Thermal stress
Thermal stress weathering (sometimes called insolation weathering) results from expansion or contraction
of rock, caused by temperature changes. Thermal stress weathering comprises two main types, thermal
shock and thermal fatigue. Thermal stress weathering is an important mechanism in deserts, where there
is a large diurnal temperature range, hot in the day and cold at night. The repeated heating and cooling
exerts stress on the outer layers of rocks, which can cause their outer layers to peel off in thin sheets.
The process of peeling off is also called exfoliation. Although temperature changes are the principal
driver, moisture can enhance thermal expansion in rock. Forest fires and range fires are also known to
cause significant weathering of rocks and boulders exposed along the ground surface. Intense localized
heat can rapidly expand a boulder.
Frost weathering
A rock in Abisko, Sweden fractured along existing jointspossibly by frost weathering or thermal stress
Main article: Frost weathering
Frost weathering, frost wedging, ice wedging or cryofracturing is the collective name for several
processes where ice is present. These processes include frost shattering, frost-wedging and freeze-thaw
weathering. Severe frost shattering produces huge piles of rock fragments called scree which may be
located at the foot of mountain areas or along slopes. Frost weathering is common in mountain areas
where the temperature is around the freezing point of water. Certain frost-susceptible soils expand
or heave upon freezing as a result of water migrating via capillary action to grow ice lenses near the
freezing front. This same phenomenon occurs within pore spaces of rocks. The ice accumulations grow
larger as they attract liquid water from the surrounding pores. The ice crystal growth weakens the rocks
which, in time, break up. It is caused by the approximately 10% (9.87) expansion
of ice when water freezes, which can place considerable stress on anything containing the water as it
freezes.
Freeze induced weathering action occurs mainly in environments where there is a lot of moisture, and
temperatures frequently fluctuate above and below freezing point, especially
in alpine and periglacial areas. An example of rocks susceptible to frost action is chalk, which has many
pore spaces for the growth of ice crystals. This process can be seen in Dartmoor where it results in the
formation of tors. When water that has entered the joints freezes, the ice formed strains the walls of the
joints and causes the joints to deepen and widen. When the ice thaws, water can flow further into the
rock. Repeated freeze-thaw cycles weaken the rocks which, over time, break up along the joints into
angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree
slope). The splitting of rocks along the joints into blocks is called block disintegration. The blocks of rocks
that are detached are of various shapes depending on rock structure.
Hydraulic action
Hydraulic action occurs when water (generally from powerful waves) rushes rapidly into cracks in the rock
face, thus trapping a layer of air at the bottom of the crack, compressing it and weakening the rock. When
the wave retreats, the trapped air is suddenly released with explosive force
Salt-crystal growth
Tafoni at Salt Point State Park, Sonoma County, California.
Salt crystallization, otherwise known as haloclasty, causes disintegration of rocks when saline solutions
seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals
expand as they are heated up, exerting pressure on the confining rock.
Salt crystallization may also take place when solutions decompose rocks (for
example, limestone and chalk) to form salt solutions of sodium sulfate orsodium carbonate, of which the
moisture evaporates to form their respective salt crystals.
The salts which have proved most effective in disintegrating rocks are sodium sulfate, magnesium sulfate,
and calcium chloride. Some of these salts can expand up to three times or even more.
It is normally associated with arid climates where strong heating causes strong evaporation and therefore
salt crystallization. It is also common along coasts. An example of salt weathering can be seen in
the honeycombed stones in sea wall. Honeycomb is a type of tafoni, a class of cavernous rock
weathering structures, which likely develop in large part by chemical and physical salt weathering
processes.
Biological effects on mechanical weathering
Living organisms may contribute to mechanical weathering (as well as chemical weathering, see
'biological' weathering below). Lichens and mossesgrow on essentially bare rock surfaces and create a
more humid chemical microenvironment. The attachment of these organisms to the rock surface
enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger
scale, seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a
pathway for water and chemical infiltration.
Chemical weathering
Comparison of unweathered (left) and weathered (right) limestone.
Chemical weathering changes the composition of rocks, often transforming them when water interacts
with minerals to create various chemical reactions. Chemical weathering is a gradual and ongoing
process as the mineralogy of the rock adjusts to the near surface environment. New orsecondary
minerals develop from the original minerals of the rock. In this the processes
of oxidation and hydrolysis are most important.
The process of mountain block uplift is important in exposing new rock strata to the atmosphere and
moisture, enabling important chemical weathering to occur; significant release occurs of Ca++ and other
minerals into surface waters.
Dissolution and carbonation
A pyrite cube has dissolved away from host rock, leaving gold behind
Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic
acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as
sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to
produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide, SO2, comes from volcanic
eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution
weathering to the rocks on which it falls.
Some minerals, due to their natural solubility (e.g. evaporites), oxidation potential (iron-rich minerals, such
as pyrite), or instability relative to surficial conditions (see Goldich dissolution series) will weather
through dissolution naturally, even without acidic water.
One of the most well-known solution weathering processes is carbonation, the process in which
atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which
contain calcium carbonate, such as limestone and chalk. This takes place when rain combines
withcarbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate
(the limestone) and forms calcium bicarbonate. This process speeds up with a decrease in temperature,
not because low temperatures generally drive reactions faster, but because colder water holds more
dissolved carbon dioxide gas. Carbonation is therefore a large feature of glacial weathering.
The reactions as follows:
CO2 + H2O => H2CO3
carbon dioxide + water => carbonic acid
H2CO3 + CaCO3 => Ca(HCO3)2
carbonic acid + calcium carbonate => calcium bicarbonate
Carbonation on the surface of well-jointed limestone produces a dissected limestone
pavement. This process is most effective along the joints, widening and deepening
them.
Oxidation
Oxidized pyrite cubes
Within the weathering environment chemical oxidation of a variety of metals occurs. The most commonly
observed is the oxidation of Fe2+ (iron) and combination with oxygen and water to form Fe3+ hydroxides
and oxides such as goethite, limonite, and hematite. This gives the affected rocks a reddish-brown
coloration on the surface which crumbles easily and weakens the rock. This process is better known as
'rusting', though it is distinct from the rusting of metallic iron. Many other metallic ores and minerals
oxidize and hydrate to produce colored deposits, such as chalcopyrites or CuFeS2oxidizing to copper
hydroxide and iron oxides.
Hydration
Olivine weathering to iddingsite within amantle xenolith
Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions
to the atoms and molecules of a mineral.
When rock minerals take up water, the increased volume creates physical stresses within the rock. For
example iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum.
A freshly broken rock shows differential chemical weathering (probably mostly oxidation) progressing inward. This piece
of sandstone was found in glacial drift near Angelica, New York
Hydrolysis on silicates and carbonates
Hydrolysis is a chemical weathering process affecting silicate and carbonate minerals. In such reactions,
pure water ionizes slightly and reacts with silicate minerals. An example reaction:
Mg2SiO4 + 4H+ + 4OH- ⇌ 2Mg2+ + 4OH- + H4SiO4
olivine (forsterite) + four ionized water molecules ⇌ ions in solution + silicic acid in solution
This reaction theoretically results in complete dissolution of the original mineral, if enough water
is available to drive the reaction. In reality, pure water rarely acts as a H + donor. Carbon dioxide,
though, dissolves readily in water forming a weak acid and H+ donor.
Mg2SiO4 + 4CO2 + 4H2O ⇌ 2Mg2+ + 4HCO3- + H4SiO4
olivine (forsterite) + carbon dioxide + water ⇌ Magnesium and bicarbonate ions in solution +
silicic acid in solution
This hydrolysis reaction is much more common. Carbonic acid is consumed
by silicate weathering, resulting in more alkaline solutions because of thebicarbonate.
This is an important reaction in controlling the amount of CO 2 in the atmosphere and can
affect climate.
Aluminosilicates when subjected to the hydrolysis reaction produce a secondary mineral
rather than simply releasing cations.
2KAlSi3O8 + 2H2CO3 + 9H2O ⇌ Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ + 2HCO3Orthoclase (aluminosilicate feldspar) + carbonic acid + water ⇌ Kaolinite (a clay mineral) + silicic
acid in solution + potassium and bicarbonate ions in solution
Biological weathering
A number of plants and animals may create chemical weathering through release of acidic
compounds, i.e. moss on roofs is classed as weathering. Mineral weathering can also be initiated
and/or accelerated by soil microorganisms. Lichens on rocks are thought to increase chemical
weathering rates. For example, an experimental study on hornblende granite in New Jersey,
USA, demonstrated a 3x - 4x increase in weathering rate under lichen covered surfaces
compared to recently exposed bare rock surfaces.
Biological weathering of lava by lichen,La Palma.
The most common forms of biological weathering are the release of chelating compounds (i.e.
organic acids, siderophores) and of acidifying molecules (i.e. protons, organic acids) by plants so
as to break down aluminium and iron containing compounds in the soils beneath
them. Decaying remains of dead plants in soil may form organic acids which, when dissolved in
water, cause chemical weathering.[citation needed] Extreme release of chelating compounds can
easily affect surrounding rocks and soils, and may lead to podsolisation of soils.
The symbiotic mycorrhizal fungi associated with tree root systems can release inorganic
nutrients from minerals such as apatite or biotite and transfer these nutrients to the trees, thus
contributing to tree nutrition.It was also recently evidenced that bacterial communities can
impact mineral stability leading to the release of inorganic nutrients. To date a large range of
bacterial strains or communities from diverse genera have been reported to be able to colonize
mineral surfaces and/or to weather minerals, and for some of them a plant growth promoting
effect was demonstrated.The demonstrated or hypothesised mechanisms used by bacteria to
weather minerals include several oxidoreduction and dissolution reactions as well as the
production of weathering agents, such as protons, organic acids and chelating molecules.
Geologic work of glaciers
Glacial erosion
Glaciers themselves do relatively little significant erosion because ice is so soft. Under the
weight of an ice sheet thousands of feet thick continental glaciers detach material from the
surface by crushing the underlying bedrock. Once the material is loosened from the
surface, ice can quarry (also known asplucking) the rock by freezing around and into
fractures, then lifting it from the surface. The rock embedded in the ice gouges and
smoothes bedrock surfaces by abrasion. Striations are fine scratches left in bedrock by
abrasion. At a larger scale, linear grooves are ground into the bedrock in the direction of
ice movement. Episodic movement leaves crescent-shaped marks called chatter
marks gouged into the bedrock. The constant abrasion of exposed rock also
creates polished bedrock.
Figure 19.7
Grooved bedrock, Quebec
Figure 19.8
Chatter marks
Glacier Transport and Deposition
Figure 19.10 Glacial till on Mt. Rainier.
Glacial drift is the general term applied to materials
eroded from the surface and deposited by glaciers.
Glaciers transport the embedded material towards the
front of the glacier as if they were on a conveyor belt,
or is deposit directly beneath the ice. Most material is
embedded in the lowest few meters of the glacier and
along its sides. Little drift material is lodged in the
interior as flow through most of the glacier is laminar,
except at the nose where thrust faulting of the ice
occurs. When the ice becomes so burdened by its load
of soil and rock fragments, it deposits the mixture of
fine and coarse textured material in place as glacial
till. Till is distinguished by its lack of sorting.
Figure 19.9
Striations
Figure 19.11 Outwash along a road
cut
At the margin of the glacier, massive
amounts of rock debris are deposited as
the ice stagnates and melts in
place. The meltwater flushes through
the accumulated debris, spreading drift
ahead of the decaying glacier
as stratified drift.
Landforms of Continental Glaciation
Examine the diagrams of a region during glaciation and
the same region after glaciation while reading the material
below.
Figure 19. 12 Aerial photo of a portion of the Northern
Unit of Kettle Moraine, WI.
A moraine is a glacially formed accumulation of unconsolidated debris. Moraines often take
the form of a belt of low hills composed of till. Where the leading edge of the glacier was
located a terminal or end moraine
can be found. The terminal moraine marks the
furthest advance of the ice sheet. Behind the terminal moraine is found a recessional
moraine deposited when the ice sheet receded and stopped for a period of time. Often,
uplands will cause an ice sheet to
separate into lobes.
Figure 19.13 Wisconsin-age
moraine in northern Illinois
Interlobate moraines form between
lobes of the ice sheet. Ground
moraine is till that was lodged beneath
the glacier and generally found behind
the terminal moraine. Ground moraine.
Wetland areas are often created in
ground moraine which is a convenient
way of identifying them from a
topographic map.
Figure 19.14 Outwash plain, Copper River
region, Alaska
An outwash plain forms ahead of the terminal
moraine as melt water from the snout of a
glacier deposits stratified drift. The outwash
plain is a relatively flat surface that may be
pock marked with depressions called kettles. If
numerous kettles are present the surface is
called a pitted outwash plain.
Figure 19.15 Sinuous form of an esker is
seen in this aerial photograph
Eskers are sinuous ridges of glacio-fluvial material that form in tunnels in an ice sheet . The
sides of the tunnel act as part of the channel for a melt water stream. As the glacier
recedes, the support for the stream is removed and the stream deposits its load into a long
ridge-like form. Eskers are good sources for sand and gravel, and many of them have been
destroyed by mining for these materials.
Figure 19.16 Streamline profile of a drumlin in
Alberta, Canada
direction of ice flow.
Drumlins are stream-lined hills that appear separately or
in "swarms" . Their formation is not well known but form
by the deposition of till. As the ice rides over the till it is
smoothed into an inverted spoon-shaped feature. The
steep side faces the direction the ice sheet came from
while the more gentle slope of the tail points toward the
Figure 19.17 Kame in northern unit
of Kettle Moraine State Park, WI
Kames are steep mounds or conical
hills built by the deposition of stratified
drift in or around ice. Some kames form
in holes in the ice where sediment
accumulates. A mound of glacial drift is
left behind once the ice melts.
Figure 19.18 Kettle lake in moraine.
Kettles are pits in the surface that may or may not be occupied by water. They form when
an isolated block of ice is surrounded by till or stratified drift. After a period of time the ice
block melts away leaving behind a hole in the surface. Kettles are often found on outwash
plains or embedded in moraines (hence the name for Kettle Moraine State Forest in
Wisconsin). Go here to view kames and moraine topography in the Northern Unit of Kettle
Moraine, Wisconsin
Figure 19.19 Lateral and end
moraine.
Glacial till is deposited along the
valley sides as lateral
moraine. Till is transported and
deposited at the nose of the glacier
as an end moraine. The end or
terminal moraine marks the
furthest advance of the glacier.
Behind the terminal moraine are
found recessional
moraines indicating positions of
the glacier front during times of retreat. When lateral moraines merge upon entering a main
glacial trough, medial moraines are formed and run the length of the glacier.