ESS 211 - Google Earth Lab

ESS 211 – Google Earth Lab
                                      For this lab, we'll be working with Google Earth.
                                      Think of this as a digital field trip to the most
                                      remote and inaccessible corners of the planet… and
                                      an introduction to quantitative landscape analysis.

Learning goals:
   - Understand general concept of using remote sensing data for quantitative
       landscape analysis
   - Gain facility with the basic functions of Google Earth
   - Use Google Earth to investigate what topography can tell us about Earth
       structure and processes from the mega scale (isostasy), to macro scale (hillslope
       relief, river profiles), to meso scale (near-surface hydrology).

Deliverables (turn in both the complete lab and the summary answer sheet):
       1) Isostasy problem
       2) Local relief questions
       3) River profile
       4) Water table map

Open up the program. Take some time to get used to the interface and browse around –
find your house, or get a bird's eye view of the trail you hiked the last time it stopped
raining. Notice that you can vary the viewing angle from vertical to oblique, zoom in and
out, and rotate the image. You'll find that the level of detail varies from region to region.
In some patches you'll be able to see objects as small as a couple of meters. There are
some oddities – because the program drapes satellite images over a shaded representation
of the topography, clouds seem to stick to the ground. Also, because it draws on so many
different sets of satellite photos, the sun angles, colors and shading can vary wildly
between adjacent image tiles.

Take some time to explore – go to Chile and look at volcanoes in the Atacama Desert.
Which ones are active? See if you can find the youngest lava flow in Iceland. Check out
the surf beaches on the southern tip of Madagascar. Find rivers that disappear into the
sands of the Sahara Desert in Tunisia. Look for equatorial glaciers on Mt Kilamanjaro.
Are they higher or lower than the icecap remnants on Carstensz Pyramid in Irian Jaya.
Why? Wander through the fold-and-thrust belt of southern Iran, all from the comfort and
safety of your desktop. Use the “clock” icon to scroll through images taken at different
times. Hours of planetary exploration, and it's all free.

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Part 1. Mega-scale Elevation and Isostasy
    The shape of the Earth’s surface at all scales, from individual landforms up to
mountain ranges and ocean basins, is defined by the distribution of elevation. For
convenience, we measure elevation relative to mean sea level. Obviously this datum is
well defined around the coasts, but it has to be established by careful surveying and
calculation at locations far from the ocean. You can think of it as the height to which
water would rise in a canal cut across a continental interior. More technically, sea level
defines the geoid, a surface of equal gravitational potential over the Earth. The shape of
the geoid is influenced both by the Earth's rotation and the distribution of mass below the
surface.
    The global frequency distribution of elevation is shown in the plot below, called a
hypsometric curve. It shows the cumulative percentage of the Earth's surface area below
any elevation on the y-axis. Notice how the most common elevations in the histogram in
Fig. (a) correspond to the flatter portions of the curve in Fig. (b).

(1a) What fraction of the Earth's area is above sea level?

(1b) At what distance inland from the Atlantic coast of North America does the
topography rise to the average height of the continents? Report distance in km and
note the latitude at which you made the measurement.

As shown on the right-hand figure, the average height of the continents is 0.8 km, and the
average depth of the oceans is 3.7 km. We can use this information to estimate the
average thickness of the continental crust.

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Example: Estimate the thickness of average continental crust

To do this, we assume that the crust and mantle are isostatically compensated at a depth
of 100 km – in other words, that the lithostatic stress is the same beneath the continents
and oceans at this depth.

Suppose that we know (from refraction of seismic waves) that the average thickness of
oceanic crust is 20 km. We also know that the average density of rocks in the continental
and oceanic crusts is 2.85 x 103 kg/m3, and 3.0 x 103 kg/m3, respectively, and that the
density of mantle lithosphere underlying both continental and oceanic crust is 3.3 x 103
kg/m3. This information is depicted on the diagram below for “column 1.”

In the following, subscript numbers denote column number, and subscript letters denote
water (w), oceanic crust (o), mantle lithosphere (m), or continental crust (c).

We wish to find the thickness of average continental crust in column 2, hc2. We have two
unknowns, hc2 and the thickness of the mantle, hm2, so we will need two equations to
solve for them.

First, we know hc2 and hm2 must sum to the total thickness of column 2 (100.8 km):
        hc2 + hm2 = 100.08 km                        (equation 1)

Second, we know the lithostatic stress at the base of columns 1 and 2 must be equal:
      ρwhwg + ρohog + ρmhm1g = ρchc2g + ρmhm2g        (equation 2)

To solve this system of equations, rearrange equation 1 to solve for hm2:
       hm2 = 100.08 km – hc2                        (equation 3)

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Plug equation 3 into equation 2, simplify, and rearrange to solve for hm2. After doing
  some algebra and plugging in values, we find
                   ρ c (100.8km) − ρ shs − ρ o ho − ρ m hm1
          hm 2 =                                            = 63 km
                                 ρc − ρm
  Plugging this value of hm2 into equation 1, we find the thickness of average continental
  crust:
€        hc2 = 100.8 km – 63 km = 38 km

  (note: answers reported with two significant digits). We know from seismology that this
  is actually a very good estimate.

  Although most continental regions reside at elevations of

(2a) What is the approximate width and height (in km) of the high topography?

To answer this question, use the "ruler" tool to trace a path along the 85º meridian from
northern India into Tibet. Start at latitude 26° up and across the highest part of the range,
then continue north to latitude 40° N. Record the elevation every 0.5° at the beginning of
your path. Once you get to ~30° N you can reduce the spacing to 1 degree. The ruler
tool will keep track of the distance from your starting point. Transfer your points to the
section below, using a healthy vertical exaggeration.

South                                                                                North

Height:

Width:

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What is the thickness of the continental crust beneath the Tibetan plateau?

Now that we know the thickness of average continental crust, answering this question is
straightforward if we consider how the change in elevation between the continental
average and the elevation of the Tibetan plateau is compensated by changes in the
thickness of the crust and mantle lithosphere (we assume the densities of the crust and
mantle lithosphere do not change).

Column 3 in the diagram on p. 3 is a cartoon of the lithosphere beneath the Tibetan
plateau. We wish to know the change in thickness of the crust between column 2 and
column 3, Δhc. We have two unknowns: Δhc and the change in thickness of the mantle
lithosphere between column 2 and column 3, Δhm. Thus we need two equations.
First, we reason that the change in elevation of the surface between columns 2 and 3, ΔE,
is equal to the sum of the changes in thickness of the crust and mantle layers:

       ΔE = Δhc + Δhm                                        (equation 4)

Second, it follows from equation 2 that the sum of the change in stress at the base of a
column at the depth of compensation is zero:

       Δ(ρcghc) + Δ(ρmghm) = 0                               (equation 5)

(2b) With this information, estimate the thickness of the continental crust beneath
the Tibetan plateau, hc3.

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(2c) Estimate the thickness of the Indian plate.

You have measured the width of the Tibetan plateau and estimated the thickness of the
crust beneath the plateau in the previous section. To answer this question, also assume:

1) approximately 2000 km of convergence has been accommodated between India and
Asia over the past ~ 40 Myr; and

2) the total volume of Indian plate material in equals the volume of material in the
Himalaya-Tibetan plateau system today.

The diagram below may be useful.

                                 Himalaya-Tibetan system
          India

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Part 2. Macro-scale Elevation and Hillslope and River Steepness

From the point of view of a geomorphologist interested in processes that erode and wear
down topography, most continental landscapes can be divided into hillslopes and rivers.

Hillslope erosion processes such as soil creep and landsliding are influenced by hillslope
steepness. Erosion rates generally increase with the steepness of hillslopes up to a
limiting (or threshold) slope, at which point increased frequency of landslides increases
erosion rates without further steepening hillslopes.

Erosion of landscapes by rivers also is influenced by river channel steepness. Erosion
rates generally increase with the capacity of rivers to transport sediment and incise
bedrock; the concept of “stream power” tells us that the capacity of rivers to transport
sediment and erode bedrock generally increases with channel steepness and the discharge
of water.

Geomorphologists use metrics of hillslope steepness and river steepness to make
quantitative predictions about spatial patterns of erosion rates and to quantify the relative
importance of different factors (e.g., the pattern of rock uplift from tectonics and faults,
rainfall, vegetation, rock type, etc.) in driving erosion.

Today, we will use measurements of mean local relief as a proxy for hillslope steepness
and map spatial variations in river steepness using longitudinal profiles.

2.1. Hillslope erosion and topographic relief

(3a) Re-examine your topographic cross-section from (2a) in Google Earth. List the
important surficial features along your traverse (e.g. rivers and lakes, glaciers,
vegetation, color of the landscape, etc.), contrasting the High Himalayas in Nepal with
the interior of the central Tibetan Plateau.

For a representative location in the High Himalaya, search for “Khumbu.” For the
interior of the Plateau, go to 30°20’ N, 86°40’ E, but also scroll around elsewhere on the
plateau to get a sense for the features.

Surficial features in High Himalaya:           Surficial features in interior of Plateau:

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(3b) Based the observations in 3a, concisely describe how climate (e.g., temperature,
aridity) varies from High Himalaya to the interior of the Plateau.

High Himalaya Climatic Interpretation:       Plateau interior Climatic Interpretation:

(3c) List possible erosion processes that you think should dominate in the High
Himalaya vs. the interior of the Tibetan Plateau:

Inferred erosion processes:                  Inferred erosion processes:

(3d) Quantitatively compare local topographic relief in the High Himalaya and in
the interior of the Tibetan Plateau.

Local relief is the difference in elevation between the highest and lowest points within a
given distance of one another.

You can make some simple measurements of local relief using the ruler tool in Google
Earth. Start from a high point (note the elevation) and draw a line roughly 10 km in
length. Now click on the end point and walk it over the map. As you shift the cursor,
you'll be able to read off the distance from your point of origin, and the corresponding
elevation at the cursor position. Locate the lowest point within 10 km of your origin,
which will give you an estimate of the maximum local relief.

Make 4 of these measurements in the region of the highest mountain peaks, and
another 4 measurements on the central Tibetan Plateau, and report average and
standard deviation of your measurements (in meters).

Local relief in High Himalaya:               Local relief on central Plateau:
1)                                           1)
2)                                           2)
3)                                           3)
4)                                           4)
average:                                     average:
std. dev.:                                   std. dev.:

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(3e) What is the relationship between the erosional processes you described in
question (3d) and local relief? Answer as concisely as possible, addressing what
conditions and processes are needed to create high local relief. You will need to
distinguish between high elevation and high relief, and consider both rock uplift and
erosion. What role might climate play in this?

2.2. Where does all the eroded rock go?

Rivers are agents of both erosion and sediment transport. Himalayan rivers are among
the most sediment-laden on Earth, carrying away almost all of the rock eroded from the
range. Locate the Brahmaputra River, which emerges from the range near its eastern end
and discharges into the Bay of Bengal. (The Indus River follows a similar course at the
western end of the range and plays a similar role in carrying away sediment, but carries
much less water.)

(4a) Draw the longitudinal profile of the Brahmaputra River from the Bay of Bengal
upstream through the mountains and into Tibet. (Use the map below for guidance
where the river takes a horseshoe turn around Namche Barwa, carving one of the deepest
gorges in the world. The Brahmaputra goes by the name of the Siang in far northeast
India, and Tsang Po in Tibet. Start near Moju Chowdhury Hat, Bangladesh. Follow the
river N towards the town of Dhubri in Assam, India. Continue NE to where the
Brahmaputra enters the mountains near Pasighat, India. Follow the Siang northward past
Ninging, and to Geling near the Chinese border. Cross the border and head NE near the
town of Medog. The river continues north to 29°54’02” N, 95°08’39” E, where it makes
a hairpin turn to the south, then flows west between the peaks of Namche Barwa and
Gyala Peri. Now zoom out, and you should be able to follow the river W to Langxian.

As before, use the ruler to mark occasional points, which will allow you to keep a record
of the distance upstream and elevation of the river channel. Don't try to record every tiny
meander, or you'll be here well into next week. Continue into Tibet, but don't feel
obliged to trace the river to its remotest headwaters. A few hundred kilometers will be
enough to define the channel gradient in the upper valley (Note also! – there are some
serious artifacts and errors in the elevation data along the channels which you will need
to filter out). Draw the river gradient on the graph paper below, using a healthy vertical
exaggeration.

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South                                                                           North

(4b) What is the average river gradient over its last 1000 km to the sea (“segment
1”)? Concisely describe how the channel looks in this region.

(4c) What is the average river gradient along the extended E-W upper channel in
Tibet (“segment 2”)? Use data over a length of at least 500 km to get a good average.
Concisely describe the form of the channel (and adjacent valleys) here. What can you
infer about the river's flow speed and its ability to carry sediment?

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(4d) What is the average river gradient in the narrow gorge where the river turns
the corner north of Namche Barwa (“segment 3”)? Compare this to the gradient you
measured in (4c).

(4e) Based on the measured river gradients and the character of the channel
observed for the three segments (1, 2 and 3), compare the relative rates of erosion
and deposition along the Brahmaputra. Answer as concisely as possible.

Part 3. Meso-scale topography and hydrology

Groundwater is the water occupying void space in soil, sediments and rocks below the
Earth’s surface. The upper surface of the saturated zone in an aquifer (permeable rocks
and sediments that store groundwater) is called the water table. Water emerges at the
surface where the water table intersects topography (e.g., river banks, springs).

Go to the following coordinates in Florida and zoom out a little so you can see the view
pictured below:
                               29.56349 N, 81.957792 W

The series of lakes in this region result from the development of karst topography, which
is characterized by dissolution of soluble bedrock (commonly carbonate rock such as
limestone or dolomite).

(5a) Map out the elevation of the water surface in the area surrounding your
location on the map below. Record 25 to 30 water surface elevations on the map
(overlay a piece of tracing paper to map the elevations). Note that the image you see may
not exactly match the image below. Click on the little clock in the tool bar at the top of
the window in Google Earth, and scroll back through the available images that have been
taken of the area from 1994 to the present. The image from 5/24/2005 looks nice and
clear.

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(5b) Contour the water surface elevations on the map.

(5c) To the left of the map, label the direction the water table is sloping in this area
with an arrow. It turns out that the surface elevation of the water in these lakes
represents an unconfined aquifer with 100% permeability (we will discuss these terms in
class). This means that the slope of the water surface defines the hydraulic gradient.

(5d) Given that groundwater flow through the permeable rocks between the lakes
must follow the hydraulic gradient, what is the azimuth of the flow direction?

(5e) How does this azimuth relate to the contour lines you drew in (5b)?

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