Lab

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Purpose of this lab

The Earth is one of four terrestrial (Earth-like) planets.  Our Moon would be considered a planet if it wasn't in orbit around the Earth.  All five of these worlds (four planets and one moon) are composed of rock and metal.  All five have been affected by impacts and volcanism.  The purpose of this lab is to explore some of the ways in which these objects are similar and different.


Part 1: A Comparison of Planetary Sizes

Background

A basic property of planets is their size. To compare sizes, we can compare the diameter (distance from one side to the other) of one planet to another, or we can compare the radius (half the diameter) of one planet to another. Size comparison is better shown graphically than with numbers. 

Graphing the Terrestrial Worlds

Table 1. The average diameters of the terrestrial worlds in kilometers (km)

MercuryVenusEarthMoonMars487912,10412,74234756779

Table 1 gives the average diameters for the terrestrial worlds in kilometers.  Use this data to plot circles representing the different planets to their correct sizes on the graph paper provided (.png version; .docx version; also shown below before the video). The video below shows how to do this.  Use a different color for each circle. Clearly identify which circle corresponds to which planet (labels or keys to colors).

graph paper for plotting planet sizes

Figure 1. Example of graph paper used for plotting planet sizes. Links to downloadable .png and .docx versions.

How to draw planet sizes explained

The video below attempts to show you how to draw the planets' sizes on the graph paper, but refuses to be centered in this box.

Please read the assignment instructions that follow Table 1, and watch the example of how to do this in the video.  Complete this assignment and upload it to the assignment box (link below).

UPLOAD TO ASSIGNMENT BOX FOR LAB 4 - Planet-Sizes

Upload your diagram to the Assignment Box—name your files: [Yourlastname]_Planet_Sizes

Lab 4: Question 1 

SHORT ESSAY: Spend a bit of time looking at the graph you've created. Describe the variation that you see for the sizes of these worlds. This should be at least a paragraph, not just a sentence or two.  This is worth 5 points (regular questions are worth 1 point).

Part 2: Global Topography

Background

A topographic map uses color coding to indicate the relative elevation (highs and lows) of landforms, such as plains, volcanoes, impact craters, etc.  Generally, violet and blue are used for low elevations, shades of green for average elevations, and yellow/brown/orange/red/white for high elevations.  Maps should include a key indicating which color corresponds to what elevation.  Unfortunately, not all maps include a key.

Be sure you have read Lecture 4.2, which covers the four geologic processes that shape the surfaces of the terrestrial worlds. While all four processes occur on all planets, moons, and smaller objects that have solid surfaces, one or two may dominate (be most important) for shaping a surface. This is most apparent when looking at a topographic map of the entire world.

Comparing Mercury and the Moon

One thing to keep in mind is that any map will show you a distorted view of a planet's surface, because it is difficult to project a spherical surface onto a flat one.topographic map of Mercury

Figure 2. Topographic map of Mercury with color coded elevation key on right. The topmost number on the scale is 4140 meters, and the bottommost number on the scale is -5020 meters.  Source: NASA/USGS. 

Lab 4: Question 2 

Look at the topographic map of Mercury (Figure 2). Which of the four geologic processes that shape the surface of a planet are the most obvious for Mercury at this scale (looking at a global topographic map)?

•  Impact Cratering
•  Volcanism
•  Tectonics
•  Erosion

Lab 4: Question 3 

TRUE or FALSE:  Based on Figure 2, the higher elevations are distributed evenly over the surface of Mercury.

Lab 4: Question 4 

Looking at the elevation scale on the right of Figure 2, what is the total change in elevation (in meters) on the surface of Mercury (from lowest to highest elevation)? Enter a single number only. [Hint: you should look at the elevation scale for this information - what you can actually see depends on how large or small you make the image.]

Now look at rotating model of Mercury below, which gives you a more representative view of the surface than does a flat map.  

Mercury Rotation Video

Credit: NASA/USGS

Lab 4: Question 5 

TRUE or FALSE:  Based on the rotating model above, the higher elevations of Mercury are distributed evenly over the surface of Mercury.

topographic maps of moon (flat vs globe perspectives) and image of nearside with labeled features

Figure 3.  Top: Topographic map of the Moon, centered on the far side of the Moon (so the near side is split on the left and right sides of the map). The scale on the right side of the image is in meters. The scale is difficult to read: the top of the scale (white color) says >8400, and the bottom of the scale (dark purple/black) say <-8200.  Image source: LRO/ASU.  Bottom left: An image of the near side of the Moon, with maria labeled using blue type and craters labeled using yellow type.  Image source: Wikipedia.  Bottom right: Topographic maps of the near side and the far side of the Moon.  The scale below the map is in kilometers (thousands of meters).  Image source: NRL.

The maria (Latin for "seas") on the Moon are darker colored regions, that are formed from large flat basaltic lava flows that filled in very large impact basins.  The lighter colored regions on the Moon are composed of a lighter colored rock, and are known as the lunar highlands.

Lab 4: Question 6 

Look at the topographic map of the Moon (Figure 3), and identify the regions which are maria (using the labeled photograph of the near side).  The maria occur at an elevation __________ the average (zero or 0) elevation on the Moon (given by a light gray color).

•  above
•  below
•  commonly both above and below
•  exactly at

Lab 4: Question 7 

Looking at the elevation scale for Figure 3, what is the total change in elevation (in meters) on the surface of the Moon (from lowest to highest elevation)? Enter a single number only.

Lab 4: Question 8 

Looking at the bottom right of Figure 3, where do the very lowest elevations on the Moon occur?

•  northern hemisphere on the near side of the Moon
•  northern hemisphere on the far side of the Moon
•  southern hemisphere on the near side of the Moon
•  southern hemisphere on the far side of the Moon

We covered impact craters and how they can be used to determine the age of a surface in Lecture 3.6 and Lab 3 part 1.

Lab 4: Question 9 

The pair of images in the bottom of Figure 3 are best for answering this question.  Looking at the region with the lowest elevation that you identified in Question 8, this region is

•  similar in age to the maria on the near side of the Moon
•  similar in age to the highlands on the far side of the Moon
•  much younger than the maria on the near side of the Moon
•  much older than the highlands on the far side of the Moon

Lab 4: Question 10 

Compare the total range of elevations for Mercury (Question 4) and the Moon (Question 7).  The total range from lowest to highest elevations on the Moon is __________ than the total range from lowest to highest elevations on Mercury.

•  almost the same (within 1000 meters)
•  significantly greater (closer to double)
•  significantly less (closer to half) 

Lab 4: Question 11 

Compare the flat topographic maps of Mercury and the Moon (Figure 2 and the top of Figure 3).  Is the distribution of high and low elevations spread around the Moon's surface similar (very close) to the distribution of high and low elevations spread around the surface of Mercury.

•  yes
•  no

Comparing Venus, Earth, and Mars

Venus and Earth

Venus and Earth are the largest terrestrial planets, which means they are the two most geologically-active terrestrial planets.  Similar in size, both have substantial atmospheres. 

topographic map of Venus

Figure 4.  Topographic map of Venus courtesy Calvin J. Hamilton.  Elevation scale is difficult to read.  Pink/violet (lowest elevation) says <-2 km (less than 2 km below average); pale red (highest elevation) says >11 km (greater than 11 km above average).

Lab 4: Question 12 

Looking at the elevation scale for Figure 4, what is the total change in elevation (in meters) on the surface of Venus (from lowest to highest elevation)? Enter a single number only.

Lab 4: Question 13 

Examine Figure 4.  Which of the following processes are visible in a global topographic map of Venus?  Choose all that apply.

•  volcanism (see small high spots that are individual volcanoes)
•  tectonics (see narrow rift valleys or trenches)
•  impacts (see numerous round depressions that are impact craters)

topographic map of Earth with elevation key

Figure 5.  Topographic map of Earth.  Source: Wikimedia. The highest elevation on Earth is Mt. Everest on the border of China and Nepal. "The altitude of 8,848 meters is officially recognized by China and Nepal. Both countries agreed to use the elevation of the mountain's snow cap, rather than a bedrock elevation of 8,844 meters." (Geology.com).  The lowest depth in the ocean is the Challenger Deep in the Marianas Trench, which is 11,034 meters below sea level (NOAA).

Lab 4: Question 14 

Looking at the elevation scale for Figure 5, what is the total change in elevation (in meters) on the surface of the Earth (from lowest to highest elevation)? Enter a single number only.

Lab 4: Question 15 

Examine Figure 5.  Which one of the following processes is most visible in a global topographic map of Earth?  

•  volcanism (see small high spots that are individual volcanoes)
•  tectonics (see narrow rift valleys or trenches, see very long thin ridges)
•  impacts (see numerous round depressions that are impact craters)

Lab 4: Question 16 

Compare the total range of elevations for Venus (Question 12) and Earth (Question 14).  The total range from lowest to highest elevations on Venus is __________ than the total range from lowest to highest elevations on Earth.

•  almost the same (within 1000 meters)
•  significantly greater (closer to double)
•  significantly less (closer to half) 

Lab 4: Question 17 

The highest elevations on Earth are marked in red (Figure 5).  Some of these regions are mountain ranges (Himalayas, Andes, etc.).  There are two regions (Greenland and Antarctica) that are not mountain ranges.  Why are these regions so high?  You will need to do some research online to answer this.  Write a short paragraph in the box provided and include the URL that was the source for your information.  You will not get credit for your answer without a citation/URL. [This question is worth 5 points]. 

Hypsometric curve

"A hypsometric curve is essentially a graph that shows the proportion of land area that exists at various elevations by plotting relative area against relative height." Quote from Britannica. There are two related graphs that scientists use for this information.  One is a histogram that shows the amount of area for on specific elevation; the other is a cumulative hypsometric (also called hypsographic) curve that totals to 100% for the entire surface of a planet.  Some graphs put elevation on the x-axis and area on the y-axis, while other graphs switch the x-y axis (as in the example Figure 6 below).

examples of hypsometric curve in two formats (histogram and cumulative)

Figure 6.  Original caption: "Global histogram and hypsographic curve of Earth's surface." Source: NOAA.

Figure 7 (below) shows hypsometric data in histogram form for Earth and Venus.  Note that the x- and y-axes on the graphs below are opposite those in Figure 6.

graph comparing the % of surface at a given elevation on Earth and Venus

Figure 7. Histograms showing hypsometric data for Earth and Venus.  "Unloaded" refers to the fact that the curves plot the solid surfaces on Earth, rather than the liquid oceanic surface. Source: Kostana.

The Earth shows two distinct peaks in Figure 7.  The peak on the left represents the oceanic crust on Earth, while the peak on the right represents the continental crust on the Earth.  Continental crust on Earth formed as a result of plate tectonics.

Lab 4: Question 18 

Based solely on the hypsometric curves of Venus and Earth (Figure 7), we can conclude

•  there is evidence for continental and oceanic crust, as well as plate tectonics on both Earth and Venus
•  there is evidence for continental and oceanic crust on both planets, but only Earth has plate tectonics
•  there is evidence for continental and oceanic crust, as well as plate tectonics only on Earth

Mars

The diameter of Mars is approximately half that of Venus and Earth. Figure 8 below shows topographic maps of Mars from different viewpoints.  

topographic maps of Mars

Figure 8.  Topographic maps of Mars. Notice that the flat map at the top does not include polar regions (which would be 90 N and 90 S), going only to latitudes of about 75 N and 75 S.  The polar regions (latitudes above 60 N and 60 S) are shown in the lower left. The lower right shows the topographic information on round hemisphere views centered on Mars' equator. The scales in both images say -8 km on one end and 12 km on the other. Sources: Flat map and polar views from Wikipedia, global views from JPL/NASA.

Lab 4: Question 19 

Examine Figure 8.  Which of the following processes are visible in a global topographic map of Mars?  Choose all that apply.

•  volcanism (see small high spots that are individual volcanoes)
•  tectonics (see grabens or rift valleys)
•  impacts (see numerous round depressions that are impact craters)

Lab 4: Question 20 

Look at the map on the top of Figure 8.  Which of the following ranges of elevations represent the oldest Martian surfaces?

•  highest elevations (white and red)
•  high, but not highest elevations (yellow and orange)
•  low, but not lowest elevations (green)
•  lowest elevations (blue)

Lab 4: Question 21 

What geologic feature is represented by the highest elevations (in white)?

•  impact crater
•  continent
•  shield volcano

Lab 4: Question 22 

What geologic feature is represented by the large violet-colored oval in the bottom right of the map (which is the lowest elevation surface on Mars)?

•  impact crater
•  continent
•  shield volcano

Part 3: Hot Spots and Plate Tectonics

As shown in the figure below, a hot spot volcano forms when a stationary mantle plume (a column of hot rock rising from the deep mantle) gets near the surface and begins to melt. Magma works its way through cracks and creates a volcano. Because the Earth has plate tectonics, the movement of the plate pulls the volcano off of the mantle plume, causing the volcano to go extinct. Magma from the hot spot continues to erupt, creating a newer, younger volcano over the mantle plume.  Eventually, there is a long chain of progressively older extinct volcanoes spread out away from the hot spot. The islands and seamounts of the Hawaii-Emperor chain are an example of a hot spot island chain, and they allow us to calculate the speed and direction of the past motion of the Pacific plate.

cartoon showing how a chain of islands forms over a hotspot

Figure 9. Original caption: "Diagram showing evolution of volcanic islands formed by a hotspot. Seamounts are underwater volcanoes that have not yet breached the water surface yet or are now dormant volcanoes."  The top image shows the start of the hot spot some time in the past.  The bottom image shows the hot spot as it is today.  So this is a time sequence from past to present. Credit: Science Over Everything 

Hawaii: A scientifically valuable hot spot

The Hawaiian islands formed as the Pacific plate moved over a stationary hot spot. The location of the hot spot is currently beneath the eastern side of the Big Island of Hawaii and an undersea volcano off the southeast coast of the Big Island known as Loihi. The Big Island of Hawaii has formed over a hot spot. The island is made of five large shield volcanoes (Figure 10): Mauna Loa, Mauna Kea, Kilauea, Hualalai, and Kohala. According to the USGS, Kilauea is the youngest and most active of the five volcanoes, and has erupted almost continuously from 1983 to 2018.

map of big island of hawaii showing volcanoes and recent lava flows

Figure 10. Map of the Big Island of Hawaii showing the locations of the five large shield volcanoes. Lava flows that have erupted since 1800 are shown in red; major communities and roads in blue. Image from HVO.

The volcanoes on the Big Island are scientifically important to Geology, Climatology, Ecology, and Astronomy (among other sciences). In addition, several Hawaiian volcanoes (most notably Mauna Kea) are sacred sites for native Hawaiians and have been used for native science and ceremony since ancient times. Pele, the Hawaiian goddess of volcanoes and fire is believed to reside at the summit of Kīlauea, within Halema‘uma‘u crater at Hawaii Volcanoes National Park on the Big Island. Pele is perceived as both creator and destroyer of land.

Determining the motion of the Pacific plate

We want to know if the rate (speed) and direction of motion of the Pacific plate has remained constant over time or if it has changed over time (and if so, how).

map of hawaiian emperor chain

Figure 11.  Map of the Hawaiian-Emperor chain of volcanic islands and seamounts.  A section containing the Hawaiian Islands has been enlarged as an "inset map" (upper right of image), which contains a scale bar.  Numbers (and names) adjacent to lines point towards volcanic islands and volcanoes are the formation ages of the volcanoes in millions of years (when most of the lava erupted).  Source: Base map found at several locations on web and altered by M. Hutson (PCC).

Figure 11 shows the Hawaiian Islands (in the inset, with a scale bar). The numbers are the ages, in millions of years, of the volcanoes (when they formed). For example, there are two volcanic vents marked on Maui. One (Wailuku) formed 1.3 million years ago, the other (Haleakala) formed 0.8 million years ago. The Hawaiian Islands are part of a longer chain (the Hawaiian-Emperor chain) that includes islands such as Midway and undersea volcanic mountains (seamounts). These are shown in the larger map, again with dates in millions of years. On the larger map, the area that corresponds to the inset map has been colored a dark blue, and a large curving blue arrow connect this to the enlarged inset map.

Determine the scale factors for the two maps

The first thing you need to do is determine the scales for both the larger map and the inset map.  A scale will tell you something like "1 inch on the map is equal to 60 miles in real life" or "1 cm on the map is equal to 40 km in real life".  If you aren't familiar with this, please review Working with Units in the Math Resource Module.  Please note that the scale that you determine will depend on how you view or print out Figure 11.  The figure below shows Figure 11 as viewed in two different browsers.  I put the same centimeter ruler in front of each screen shot.  The scales are not the same, because the field of view for the screen was not the same in each browser.

example of measuring scaleanother example of scale

STEP 1: 

Start by determining the scale on the inset map, as that map has a scale bar. Use a metric ruler to measure (in centimeters) the length of the scale bar (from 0 to 150 km).  Divide 150 km by the length in cm that you measured.  For example, if I have 4 cm = 150 km (the right hand example above), then I would have a scale of 1 cm = 37.5 km.

You won't be entering this part into the lab quiz.

  • The scale for the inset map is 1 cm = ________ km. (Keep this information to use below for step 3 and in the table).
STEP 2:

Now you need to determine the scale of the larger map (that shows the entire Hawaiian-Emperor chain).  To do this, you need to figure out how much the inset map (and its scale) has been enlarged.  So you need to compare the same length in both maps.  To do this, measure the length along the TOP edge of the inset map in centimeters.  Do the calculations described below.  You don't need to enter them into the lab quiz.

  • Part A:  The top edge of the inset map (in cm) = _____________ cm.  

Now measure the same TOP side of the same area (colored dark blue) on the bigger map (the area corresponding to the inset map is shaded dark blue on the bigger map; the edge you want is touching the curved arrow that connects the two maps).

  • Part B:  The top edge of the dark blue area in the bigger map (in cm) = ___________ cm.
STEP 3:

To determine the scale of the larger map, you need to take your answer for Step 2 Part A, multiply it by the scale you determined in Step 1, and then divide this number by your answer to Step 2 Part B.

EQUATION 1:  Scale on the large map=edge length on inset map×scale of inset mapedge length on large map{"version":"1.1","math":"<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>S</mi><mi>c</mi><mi>a</mi><mi>l</mi><mi>e</mi><mo>&#xA0;</mo><mi>o</mi><mi>n</mi><mo>&#xA0;</mo><mi>t</mi><mi>h</mi><mi>e</mi><mo>&#xA0;</mo><mi>l</mi><mi>a</mi><mi>r</mi><mi>g</mi><mi>e</mi><mo>&#xA0;</mo><mi>m</mi><mi>a</mi><mi>p</mi><mo>=</mo><mfrac><mrow><mi>e</mi><mi>d</mi><mi>g</mi><mi>e</mi><mo>&#xA0;</mo><mi>l</mi><mi>e</mi><mi>n</mi><mi>g</mi><mi>t</mi><mi>h</mi><mo>&#xA0;</mo><mi>o</mi><mi>n</mi><mo>&#xA0;</mo><mi>i</mi><mi>n</mi><mi>s</mi><mi>e</mi><mi>t</mi><mo>&#xA0;</mo><mi>m</mi><mi>a</mi><mi>p</mi><mo>&#xD7;</mo><mi>s</mi><mi>c</mi><mi>a</mi><mi>l</mi><mi>e</mi><mo>&#xA0;</mo><mi>o</mi><mi>f</mi><mo>&#xA0;</mo><mi>i</mi><mi>n</mi><mi>s</mi><mi>e</mi><mi>t</mi><mo>&#xA0;</mo><mi>m</mi><mi>a</mi><mi>p</mi></mrow><mrow><mi>e</mi><mi>d</mi><mi>g</mi><mi>e</mi><mo>&#xA0;</mo><mi>l</mi><mi>e</mi><mi>n</mi><mi>g</mi><mi>t</mi><mi>h</mi><mo>&#xA0;</mo><mi>o</mi><mi>n</mi><mo>&#xA0;</mo><mi>l</mi><mi>a</mi><mi>r</mi><mi>g</mi><mi>e</mi><mo>&#xA0;</mo><mi>m</mi><mi>a</mi><mi>p</mi></mrow></mfrac></math>"}

  • The scale for the large map is 1 cm = _________ km. (Keep this information to use below in the table).

Calculate and graph the distances and lengths of time between pairs of volcanoes

Now use the two maps and their corresponding scales (from Step 1 and Step 3 above) to determine the distances between pairs of volcanoes. Be careful to use the correct scale for the map (inset or larger map) you are measuring. Also determine the difference in ages between that pair of volcanoes. Put your measurements in Table 2 (shown below). Notice that the last column is in “millions of years”, not years.  Table 2 and the graph for your data (also shown below) are both found in one document (.docx version; .pdf version).

Table 2. Distances and formation ages between pairs of volcanoes

From volcano to volcano

Distance on map in cm

Real distance in km (multiply by appropriate scale factor) – distance between volcanoes

Ages of younger volcano in millions of years

Age of older volcano in millions of years

Length of time (in millions of years) between formation of two volcanoes (subtract age of younger from age of  older) – difference in ages

Hualalei on the Big Island (at 0.15 Myr) to Lanai

Use inset map and scale factor

Lanai to Kauai

Use inset map and scale factor

Kauai to Midway

Use larger map and scale factor

Midway to Daikakuji seamount

Use larger map and scale factor

Daikakuji seamount to Suiko seamount

Use larger map and scale factor

Table 2 has 6 columns.  You will be plotting the data from the third column (real distance in km) and the sixth column (length of time in millions of years. 

  • Plot this data (columns 3 and 6 from Table 2) on the graph (shown below) which is found in the same file as Table 2.
  • Label each point (HL = Haulalei to Lanei; LK = Lanai to Kauai; KM = Kauai to Midway; MD = Midway to Daikakuji seamount; DS = Daikakuji seamount to Suiko seamount).
  • Draw a best fit line through the data. Reminder: A best fit line is a straight line that represents the trend of the data. You do not connect the points. There should be approximately the same number of points on either side of a best fit line. Some data points may lie on the line, but it is possible to have a best fit line with none of the points actually on the line.  If you need more information please check out the SERC page on best fit lines.

graph paper for hot spot data

Figure 12.  Picture of the graph for plotting data from Table 2. The actual graph paper and the table are found in the hotspot-table-graph file (.docx version; .pdf version).

When you have completed both Table 2 and the accompanying graph, upload your completed file to the assignment box (link below).

UPLOAD TO ASSIGNMENT BOX FOR LAB 4 - hotspot-table-graph

Upload your diagram to the Assignment Box—name your files: [Yourlastname]_hotspot_table_graph

Analyze your data

Examine the graph that you have just completed and answer the following questions.

Lab 4: Question 23 

Should your best fit line go through the point (0,0) on your graph?

•  yes
•  no

Lab 4: Question 24 

Explain the reason for your answer to Question 23, typing into the box provided.  This question is worth 5 points.

Lab 4: Question 25

Has the Pacific plate moved at a constant rate (speed) over the last 60 million years?

•  yes
•  no

Lab 4: Question 26 

Explain the reason for your answer to Question 25, typing into the box provided.  This question is worth 5 points.

Lab 4: Question 27 

A line drawn from the origin (0,0) through just the HL data point will have a steeper slope than the best fit line. This tells us that the current speed of the Pacific plate is ______________ as its average speed over the last 60 million years.

•  almost twice as fast
•  about the same speed
•  almost twice as slow

Lab 4: Question 28 

Looking back at Figure 11, which direction has the Pacific plate been moving for the last 40 million years (last 40 million year, not 60 million years)?

•  to the northeast
•  to the southeast
•  to the southwest
•  to the northwest

Lab 4: Question 29 

Between 50 and 60 million years ago, which direction was the Pacific plate moving?

•  to the north
•  to the east
•  to the south
•  to the west

Part 4: Greenhouse Effect on Earth and Keeling Curve

You will need to refer back to Lecture 4.6.  The main greenhouse gas in the atmosphere of Earth is carbon dioxide (CO2).

Lab 4: Question 30 

Which of the following are common greenhouse gases on Earth?  Choose all that apply.

•  Nitrogen
•  Water vapor
•  Methane
•  Oxygen
•  Nitrous Oxide
•  Hydrofluorocarbons (HFCs) - fluorinated gas

Lab 4: Question 31 

Which of the following terrestrial planets has the greatest greenhouse effect?

•  Earth
•  Mars
•  Mercury
•  Venus

Lab 4: Question 32 

Which of the following terrestrial planets has the smallest greenhouse effect?

•  Earth
•  Mars
•  Mercury
•  Venus

We have been directly measuring the abundance of carbon dioxide (CO2) in the Earth's atmosphere at an observatory near the summit of the Mauna Load volcano on the Big Island of Hawaii since 1958. "This is the longest continuous record of direct measurements of CO2 and it shows a steadily increasing trend from year to year; combined with a saw-tooth effect that is caused by changes in the rate of plant growth through the seasons. This curve is commonly known as the Keeling Curve, named after Charles Keeling, the American scientist who started the project. Why Mauna Loa? Early attempts to measure CO2 in the USA and Scandinavia found that the readings varied a lot due to the influence of growing plants and the exhaust from motors. Mauna Loa is ideal because it is so remote from big population centers. Also, on tropical islands at night, the prevailing winds blow from the land out to sea, which effect brings clean, well-mixed Central Pacific air from high in the atmosphere to the observatory. This removes any interference coming from the vegetation lower down on the island.” Quote from Skeptical Science.

map of mauna loa showing offshore winds

Figure 13.  Image is titled "Ideal Atmospheric Sampling at Mauna Loa", and shows prevailing wind coming off the ocean, with off shore winds drawing well mixed air downward over the summit of Mauna Loa.  Text at the bottom says "The Mauna Loa volcano juts out of the middle of the Pacific Ocean, creating climate patterns that make it easy to know when CO2 measurements represent the well-mixed atmosphere and not local sources."  Image from Skeptical Science.

We are able to get reliable, albeit less precise, measurements of atmospheric carbon dioxide farther back in time by looking at trapped bubbles of atmosphere in ice cores thousands of years old.

Supplemental Video, not required

Credit: NSF Ice Core Facility

A graph that plots the abundance of atmospheric carbon dioxide (on the y axis) as a function of time (on the x axis) is call a Keeling  curve

Keeling curve covering a two year period

Figure 14. Keeling curve for a two year period ending October 4, 2020, showing carbon dioxide concentrations measured at Mauna Load Observatory. Source: Scripps.

Lab 4: Question 33 

Examine Figure 14. The concentration of carbon dioxide is cycling up and down.  What might be the cause of the variation?

•  increased fossil fuel emissions during the summer, as more people go on vacation
•  repeated eruptions of the nearby Kilauea volcano emitting carbon dioxide into the atmosphere
•  cold air in winter is dryer, holding less carbon dioxide
•  seasonal increases and decreases in plant growth (and therefore photosynthesis)

Keeling curve since 1700

Figure 15.  Keeling curve from 1700 to October 2020.  This figure combines carbon dioxide data from Mauna Loa (the thick line starting after 1950, which zigzags annually) with measurements from ice cores. Source: Scripps.

Lab 4: Question 34 

Examine Figure 15. How would you describe the concentration of carbon dioxide between the years 1700 and 1800?

•  relatively constant
•  increasing slightly
•  increasing steeply

Lab 4: Question 35 

Examine Figure 15. How would you describe the concentration of carbon dioxide between the years 1850 and 1950?

•  relatively constant
•  increasing slightly
•  increasing steeply

Lab 4: Question 36 

Examine Figure 15. How would you describe the concentration of carbon dioxide between the years 1960 and today?

•  relatively constant
•  increasing slightly
•  increasing steeply

Keeling curve for last 800,000 years

Figure 16.  Keeling curve for the last 800,000 years. Source: Scripps.

Figure 16 (above) shows the abundance of carbon dioxide in Earth's atmosphere for the past 800,000 years.  Low values of carbon dioxide occurred during global ice ages, while high levels of carbon dioxide occurred during warmer interglacial periods. For the past 10,000 years (since the end of the last ice age) up until about 1800, the abundance of carbon dioxide in our atmosphere was below 280 ppm (parts per million).  The abundance of carbon dioxide in Earth's atmosphere on October 4, 2020 was 411 ppm.  

Lab 4: Question 37 

Examine Figure 16. Based on this graph and what you've learned so far in this module, explain why we would expect the Earth's average temperature to increase as a result of this increase in carbon dioxide.  Be sure to include the balance between incoming and outgoing energy in your explanation.  Please type your answer in the box provided.  This question is worth 5 points.

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