Download A Brief Introduction to the Earth`s Surface

A Brief Introduction to the Earth's Surface
Elevation
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).
(1) What fraction of the Earth's area is above sea level?
(2) What is the median elevation of the area above sea level? What is the median depth of the
oceans? Remember that the median is the mid-point in a statistical distribution. So one-half of
the continental elevations lie above the median elevation, the other half below. Likewise for the
median depth.
Because the distribution of depths and elevations is not symmetric about the median, the average
depths and elevations are a little different to the values you estimated above. 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. This provides us with information about the average thickness and density of
the continental and oceanic crust.
(3) For example, we can estimate the average thickness of the continental crust. To do so,
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.
Now suppose that we know (from refraction of seismic waves) that the average thickness of
oceanic crust is 20 km, and 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. Finally, assume that the density of mantle
lithosphere underlying both continental and oceanic crust is 3.3 x 103 kg/m3.
Calculate the thickness of average continental crust. The diagram below may help:
(4) You can see from the hypsometric curve that very little of the Earth's surface lies at
elevations greater than a few kilometers above sea level. In fact, most of the really high ground
is found in a few very high plateaus, in places such as Tibet, the Altiplano in Peru and the
Colorado Plateau. Why do these areas of the continental crust stand so high?
For the next part of the 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.
Open up the program and browse around – find your house, or the trail you hiked over summer.
Make sure that you have the “Terrain” checked, so that you can view the landscape in relief. To
reduce clutter, turn off the cultural overlays suchs as roads, traffic, borders, and so forth. Notice
that you can vary the viewing angle from vertical to oblique, zoom in and out, and rotate the
image. Detail varies from region to region, depending on the satellite coverage. 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, sun angle, colours and shading can vary 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. Find rivers that disappear
into the deserts of Tunisia. Look for equatorial glaciers on Mt Kilamanjaro. Are they higher or
lower than the last tiny ice patches 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. Hours
of planetary exploration, and it's all free.
Explore a collisional mountain belt
Today we'll use Google Earth to look at the architecture of the Himalayas, and examine the
competing processes of mountain building and erosional down-wearing.
Go to northern India and locate the Himalayas, the greatest collisional mountain belt on Earth.
Tilt and rotate the image to take a look at the terrain. With a bit of exploration, you can get a
sense of the structure and physiography of the range.
(5) Zoom back and examine the geologic "grain" of the range. On the figure below, sketch
where you think you'd find the trace of the thrust fault (or faults) that have carried the Eurasian
plate over the Indian plate. Bear in mind that these are enormous structures – we are looking for
continent-scale faults that have accomodated nearly 2000 km of convergence between India and
Asia over the past ~ 40 Myr.
What other geophysical data could we use to locate these structures?
The Himalayas and Tibetan Plateau comprise the largest area of high topography on Earth.
Sketch a N-S topographic cross-section following longitude 87°, from latitude 26° north to
latitude 40° N. Move the cursor from point to point and read off latitude and elevation at the
bottom of the screen. Measure enough points to represent the topographic relief along the way.
Draw your section below, using latitude on the x-axis, and a healthy vertical exaggeration.
(6) What is the approximate width (in km) of the high topography? [Read from your section and
multiply by 111 km per degree of latitude, or use the "ruler" from the <tools> menu to measure
the width directly]. Give your answer in kilometers:
(7) India is moving north towards Eurasia at ~5 cm/yr. How long would it take to build the
Himalayas and Tibetan Plateau at this plate convergence rate?
(8) Your answer should come out at less than 40 Myr, the approximate time since collision
began. In other words, there is less thickened crust than expected. Why might this be so?
[Hint: Is all the rock ever pushed into the Himalayas by plate convergence still there? Or has
some been removed?]
(9) Take note of the surficial features related to climate and erosion along your traverse (e.g.
rivers and lakes, glaciers, vegetation, sand dunes, colour of the landscape, etc).
Now write brief notes describing your interpretation of the climate from south to north – from
the plains of India to central Asia.
(10) Based on your thoughts about climatic differences from north to south, and features you can
see on the ground, list the main processes of erosion at work (i) in the high Himalaya at the south
end of the transect, (ii) in the interior of Tibet, at the north end of the transect.
Topographic relief is defined as the difference in elevation between the highest and lowest points
within a given distance of one another. Local relief varies dramatically from one region to
another on the Earth's surface (compare Kansas to the Himalayan topography you've just been
looking at).
With the cursor or the ruler tool, make a few measurements of relief at the 10 km scale: Choose
a point. Now locate the highest and lowest points within 10 km. The difference gives you the
maximum local relief at the 10 km scale.
(11) Make a few quick measurements to characterise relief (i) in the region of the highest
mountain peaks, and (ii) on the central Tibetan Plateau. List your locations (lat/long) and the
local relief at each point below:
(12) What is the relationship between the erosional processes you described in Q (10) and local
relief? Write a couple of sentences on the conditions and processes 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? Can you think of some feedbacks
that tend to limit the development of topography?