Lab 4.1 – How DO they measure water depth?

Fundamental concept: Explain how water depth data are calculated from water pressure data
Estimated time to complete: 20 minutes
Data skills preparation: Lab 1.2 – GeographyLab 2.1 – Time series, Lab 2.2 – Bathymetric charts
Materials needed: Calculator

The map below highlights the location of some of the seamounts in the Cobb-Eickelberg Seamount Chain.  At the southeast end of the chain is Axial Seamount, the feature that is monitored by the numerous devices illustrated in the previous page.  When hovering over the map, scrolling with your mouse will zoom in/out on the map.  Zoom in and out of the map to develop familiarity with Axial Seamount and other seamounts in the chain, as well as nearby features.  Note that Axial Seamount does not have a prominent cone shape that many people associate with volcanoes, which is characteristic for many shield volcanoes mentioned in the previous page.


Orientation Questions:

  1. Axial Seamount is located at the southeast end of the Cobb-Eickelberg Seamount Chain. Several of the seamounts in the chain are pinned in the above map.  Zoom in on the map and describe how Axial Seamount compares to other seamounts in the chain with regards to shape, size, width (using the line measure in upper left corner of map), ruggedness, etc.

Axial Seamount is the most active seamount in the northeast Pacific. It is a hotspot volcano that happens to sit on a mid-ocean ridge, the Juan de Fuca Ridge.  As you likely learned in your oceanography class, hot spot volcanoes and seamounts are thought to form as a lithospheric plate moves across a subsurface volcanic hot spot where more magma is produced and added to volcano magma chambers than in the lithosphere surrounding the hotspot.   Because Axial sits above the hot spot, it experiences a “robust magma supply” (Chadwick et al. 2010).  Older peaks in the chain become less active as they are moved further away from the hot spot.  The Cobb hot spot forming the Cobb-Eickelberg Seamount Chain has not always been located at a divergent plate boundary; in fact, it is thought that the divergent boundary has moved overtop the Cobb Hot Spot making the geologic processes in the region very complicated!

The top of Axial sits at a depth of approximately 1400 meters and its base sits at 2500 meters depth making the seamount about 1100 meters tall.  Scientists use metric measurements for several reasons.  Very importantly, to share results, using the same units across all studies enables ease in sharing and comparing results.  It is also an easy system to convert units since it is based on powers of ten; converting from kilometers to meters simply requires multiplying the km value by 1000, for example.   For this exercise, you will practice converting the metric measurements in the above paragraph to units many students are more familiar with in the U.S. (e.g. feet, miles) and then different scale metric units (e.g. meters to kilometers).  

  1. Now convert both the depth of the top of the seamount and the depth of the base of the seamount from meters to feet and subtract the difference (hint: it should be close to your answer to the first quick check question). Show your math.
  2. Let’s now review metric measurement conversions.  Distance is measured in various scales of meters.  You will look at meters converted to kilometers here.  1 km = 1000 m.  Convert the height of the seamount, 1100 m, to km.  Then convert it to miles using 1 mi = 1.61 km.

You will learn next specifically how these depths can be calculated from water pressure readings.  

Study the following Axial Seamount Caldera graph of water pressure and depth. Turn off and on the two variables to ensure you know which data set is which.  Use the sliders to change the time scale on the x-axis to look at shorter and longer time intervals.  (Note, sometimes the y-axis on this graph will flip when zooming out. To reset it, click the “All Data” button.)

Water pressure is plotted with water depth in this graph to show the “raw” data for pressure exerted by the overlying water column on the seafloor that was used to calculate water depth. Pressure can be used because the greater depth of the water column, the greater the pressure exerted downwards by the mass of that water column above any depth considered.  Thus the greater the water pressure, the deeper the water depth.  So one can graph the raw data collected for pressure at the seafloor and add to it the calculated depth determined from the pressure data.

  1. The volume of water at each depth in the water column exerts pressure downwards; the deeper the water column, the greater the pressure. Water pressure increases about 15 psi (pound-force per square inch) for every 10 meters (33 feet) of depth. From a data point in the graph, what calculation can you run that would enable you to convert pressure to depth? Select a point on the water pressure curve and calculate the depth from the information provided here.

Your answer will depend on the data point you select. For example, at 2 am on 4/24, the water pressure was 2253.54 psi. The depth can be calculated using the following equation:

2253.54 psi × (10 m/15 psi) = 22535.4/15 = 1502.4

This is close to the depth of 1509.81 m shown on the graph at this time (which was calculated with a more precise method). Make sure to choose a different point for your calculation.


Interpretation Questions:

Next, turn off depth (uncheck the box) and study only pressure in the graph.

  1. What do you think might cause the variations in pressure you notice in the plot? Record your best guess first, then click below to read an explanation.

If you suggested tidal fluctuations you are correct!  Tides are normally studied after geology topics in an oceanography class so you probably have not learned too much about them yet. They are natural daily fluctuations in the height of the water surface resulting from complex interactions of the earth, moon and sun.  The site of this study experiences two high tide levels and two low tide levels every 24 hours as a result of these interactions.  After you learn about tides, revisit the graph in this exercise and see if you can convince yourself that this study site experiences “mixed semi-diurnal tides”.

  1. What challenges might the phenomenon described above present for oceanographers when they want to study average water depth at this site?

When multiple processes occur at the same time, their impact on a particular feature (water pressure in this case) can cause the data to be complicated and “messy”.  Scientists need to process messy data somehow when possible. If scientists want to study average water depth, filtering out the tidal fluctuations can be done since they are well documented.  Note that the approximately 3 psi range in the water pressure data fluctuations due to tides converts to a fluctuation of about 2 m which, for a depth of about 2254 m, is about 0.1% of the total water depth.

Because scientists studying the Axial seamount want to know average depth, the water pressure data are filtered to remove fluctuations from processes that are not relevant for the phenomena they are trying to study, in this case seafloor depth changes. The gold line on the graph shows seafloor depth calculated from water pressure in the Axial Seamount caldera from March through May 2015.  Deselect the water pressure data and view just the calculated depth data to answer Questions 7-10. 

  1. Describe the patterns in the depth data. Be sure you zoom in and out of the data to notice small and large changes.  (Note, sometimes the y-axis on this graph will flip when zooming out. To reset it, click the “All Data” button.)

    Think about these prompts to help you answer #7:

    • What is the range in depths using actual measurements?
    • How long do various depths occur?
    • When does depth change and how quickly does it change?
  2. What do you think may have caused the changes you described? Formulate some preliminary ideas in this step to discuss with classmates in class or in a discussion board; you will revisit this question in the next activity.
  3. Adjust the widget to zoom in to the day before and after the major change in depth. How much depth change occurred and how long did it take for the change to take place?
  4. a. Describe the pattern(s) you see in the depth plot; be as thorough as you can.
    1. If you are continuing to Lab 4.2, before you proceed, consider what the graphed line might be revealing about changes to the seafloor over time and record your ideas. Remember, this is a volcano; what do you think might be happening beneath the surface that could cause the depth of the seafloor to change?