Lab 4.1 – How DO they measure water depth?

Estimated time to complete: 20 minutes
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.  Zoom in and out of the map to develop familiarity with Axial Seamount and other seamounts in the chain, as well as nearby features.  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. 

  1. What are the three measurements noted above (top, base and height of Axial Seamount) converted to feet?  What are each of these three measurements in miles?  What equation can you set up to convert any measurement from km to miles and the reverse?


  1. 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.  Axial experiences a “robust magma supply” (Chadwick et al. 2010) from the Cobb Hot Spot. This hot spot has not always been located at a divergent margin; in fact, it is thought that the divergent margin has moved overtop the Cobb Hot Spot. The top of Axial sits at approximately 1400 meters (how many feet is this?) and its base sits at 2500 meters depth making the seamount about 1100 meters tall.  You will learn next specifically how these depths are determined from pressure readings.

Study the following Axial Seamount Caldera graph of pressure and depth. 

Turn off and on the 2 variables to ensure you know which data set is which.  Change the time scale on the x-axis to look at shorter and longer time intervals.   

  1. What are the y and x axes, units for the axes and what does the graph show?
  2. Explain why you think pressure and depth can both be plotted in the y-axis.

Pressure is what is normally measured and depth is then calculated from it.  The greater depth of the water column, the greater the pressure exerted downwards by the mass of that water column on the seafloor.  Thus the greater the pressure, the deeper the water depth.

  1. From a data point in the graph, what calculation can you run that would enable you to convert pressure to depth?

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?
  2. What challenges might they present for oceanographers when they want to study average water depth at this site?

Because scientists studying the Axial seamount want to know average depth, the pressure data are filtered to remove tidal fluctuations. The gold plot shows seafloor depth calculated from pressure in the Axial Seamount caldera from March through May 2015.  Deselect the pressure data and view just the depth data.

  1. Describe the patterns in the depth data. Be sure you zoom in and out of the data to notice small and large changes. 
  2. What do you think may have caused the changes you described?

Assessment questions

  1. 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?
  2. Describe the pattern(s) you see in the depth plot; be as thorough as you can and consider what the graphed line reveals occurred over time.