Welcome Geology Students!

Using remote sensing data to make a geologic map #1. We will start drawing geologic contacts and completing maps interpreted from relatively high-resolution satellite imagery.

Navigating geologic maps #1

Intro to mapping geologic units remotely

Steps in making your map

More remotely sensed data

What to turn in and how

Background

Creating geologic maps is a complex task. A geologic map consists of

  1. A base map, which might be a georeferenced satellite image, an aerial photograph, a topographic map or a sketch that you make. Here, georeferenced means that the satellite image can be viewed as a map, and is not distorted by the track of the satellite.
  2. Geologic contacts. These are lines drawn on the base map that delineate rocks of a specific type. For example, a geologic contact might be drawn to surround a lava flow.
  3. Color-shaded regions, showing specific geologic units. These units are, or are thought to, consist of one rock type. Different colors represent different rock units on the map.
  4. Symbols are added to the map to clarify rock units. For example, the symbol Qlv might be added in the region color shaded as lava flow to represent a Quaternary-aged lava flow. If many Quaternary-age lava flows appear on a map they might have the same color shading, but be labeled Qlv1, Qlv2 and so on.
  5. Additional symbols showing faults, explosion craters, volcanic vents and so on.
  6. A map scale and north arrow. Sometimes the map scale is approximate.
In this exercise we will begin making geologic maps by sketching geologic contacts, shading units and adding map symbols directly on to satellite images.

It is not possible to make a complete and accurate geologic map only from interpretation of satellite images. Often geologic units can only be distinguished with certainty by going to the field and studying the mineralogy of rock units in hand sample. Sometimes it is only possible to distinguish geologic units by looking at thin sections of rocks with a petrographic microscope, or by analyzing the geochemistry of samples. Nevertheless, almost all mapping efforts start with the interpretation of satellite images and related information. In this exercise we will tentatively assign geologic units using the information that can be obtained directly from satellite images.

By the way, to our knowledge, no one has ever published a detailed geologic map of Mount Cleveland volcano. This is how such a mapping effort would start!

Steps in making a geologic map on a satellite image base map

Consider this satellite image of an area on the eastern flank of Mount Cleveland volcano, near where it connects across an isthmus to the rest of Chuginadak Island. Study this image and you will begin to pick out specific geologic features, mostly based on changes in color and texture.

first area

It takes practice to identify specific volcanic units on satellite images. In the following image, a lava flow is labeled. Notice the texture of the lava flow. This flow is elevated compared to surrounding topography. The surface is lobate (these are literally individual pulses of lava effused from a vent that have frozen in place). Long ridges (lava flow levees) occur parallel to the direction of the lava flow. These levees show how the flow channelized as it flowed downhill. Notice in this case the lava flow is not a dramatically different color on the satellite image than the surrounding terrain. Like the surrounding terrain the lava flow is sparsely vegetated. Sometimes relatively young lava flows stand out because of their dark color, especially when they are not vegetated.

The smooth circular feature NE of the lava flow is an explosive volcanic vent - a scoria cone. It is characterized on the satellite image by its dark gray to brown color, the presence of a crater, and its more or less circular form. Scoria cones are often oxidized to a red color (look at the south rim of this cone). Scoria cones often have very many small gullies because they are so easily eroded. This cone has many small gullies on its NE flank. Although we cannot visit this cone, we can infer from the textures we see on satellite image that this cone consists of relatively loose tephra particles. The crater of this particular scoria cone is very large compared to the volume of the cone. This is characteristic of a particularly violent eruption, perhaps resulting from interaction of magma and surface water during the eruption.

At the toe of the lava flow and the base of the scoria cone on its seaward side there is a flat area with reworked sediments. These are beach deposits.

first area
Once these units are identified geologic contacts can be drawn on the map. It is best to start with the most obvious geologic contacts. In this case, the edges of the lava flow and scoria cone can be mapped fairly well from the satellite image. In the following image a line is drawn marking the edge of the lava flow. This line is approximate, we cannot see the edge of the lava flow in the image everywhere, but we can infer the boundary. The line represents the geologic contact between the lava flow and whatever is around the lava flow. The lava flow is shaded and the symbol Qlv is added to indicate this is a Quaternary lava flow (all rocks on Mount Cleveland volcano are Quaternary in age) and lv stands for lava. There are some conventions for labeling geologic units. Usually start with the age (Q) and then use some intuitive method for labeling based on the inferred rock type.

Next draw the geologic contact for the scoria cone. As with the lava flow, the contact is not always clear. In this case, it appears (!) that the crater floor is light colored and may show the underlying older rock. We are not exactly sure, but we interpret the image that way. Another interpretation that is reasonable from the satellite image is that the floor of the crater is covered with the same scoria found on the cone. Likewise, the beach deposit is outlined. The contact between the beach deposit and surrounding rocks is not always clear - especially in the cloudy area of the image - but is inferred from the satellite image as best we can.

Notice the NE corner of the map. This area is a black dark plain. One interpretation is that this flat dark plain is reworked tephra deposits eroded off of Mount Cleveland volcano and partially building an isthmus between the volcano and the rest of the island. It is labeled Qtr for "reworked tephra".

first area with some geology

Now we complete the map of this particular image. The entire land surface area of the image is colored and each geologic unit is labeled. The new unit (Qopl, green color) included older rocks upon which the younger lavas, scoria cone and beach deposits have accumulated. These are undifferentiated, older pyroclastic rock (tephra, pyroclastic density current deposits) and older lavas which are mapped as a single unit. Notice that the deposit is vegetated and sometimes buff, tan, or yellow colored.

Triangles are added to show the locations of volcanic vents. One triangle is in the center of the scoria cone crater. Another is located on the lava flow. Study the satellite image and you will see a dome-like feature at the head of the lava flow. This is interpreted to be the location of the volcanic vent that erupted the lava.

first area with geology

Different parts of the volcano have different volcanic features and different geologic units. The following image is of an area on the NE flank of Mount Cleveland volcano. Note the dark gray to brown colors in the SW corner of this image. The smooth areas in this zone are marked by many gullies, just like the scoria cone in the previous example. This is easily eroded pyroclastic deposits and tephra fallout from explosive volcanic eruptions. This unit makes an irregular contact with the older deposits lower on the flank of the volcano.

Some channels low on the flank of the volcano are filled with dark brown and gray material. These are pyroclastic deposits that have been re-mobilized in lahars (debris flows) and transported further down the volcano.

Also note that this image has a change in color trending roughly N-S across the image. This color change is not real, but is a change in the time at which the satellite data were collected. Be careful to evaluate color changes that might be associated with the change in satellite data quality or time of acquisition.

second area

Try to differentiate lava flows and pyroclastic deposits on this image from the N flank of Mount Cleveland volcano.

third area

Remotely-sensed data for Mount Cleveland volcano

It helps to try to look at deposits from different perspectives, especially when interpreting the location of geologic units and geologic contacts from remotely-sensed data. The following window shows ESRI satellite data for the region. Zoom in and look around the island!

You can also use Google Earth Pro to investigate the mapping area.

Do the following:

  1. This satellite image covers a slightly larger area than the example given above, including all of the isthmus. Make a geologic map of this area using the same geologic units and symbols provided in the example.
    third area
    1. Download and print the map (you may be provided with a printed copy of the image).
    2. Outline the geologic contacts and map units
    3. Write descriptions of the units you identify on the map. Include a legend and symbol key
    4. Make an approximate scale for the map (e.g., following previous examples or using Goggle Earth Pro).
  2. Using the digital map in the remotely sensed data section, pan and zoom to observe the geology of all four islands. Use your hand-drawn base map of the four islands to put geologic contacts on your base map. It is not possible to map in the same detail you used in question #1. Instead, show geologic contacts on your base map that show areas of predominantly young pyroclastic deposits (Qp) mostly high on the slopes of the volcanoes), areas that are predominantly lava flows (Qlv), and areas that are predominantly older (Qopl). Add additional units as needed. Color the base map and include a map key (legend).

Turn in your answers to the questions using a word document. Scan your geologic maps and paste into your word document. Or, take a picture of the geologic maps and insert in your word document.