The Volcanic Soils of Western Molokai Earthcache EarthCache

The Volcanic Soils of Western Molokai - An Exploration into Soil Composition

If you arrived to Molokai by plane, you inevitably observed one of the most striking features of the "Friendly Island;" the soil. Western Molokai in particular boasts soil so bright in hue that it appears to have been dyed an extraterrestrial color! As you make your observations here, please remember that this is an Earthcache; there is no container to find here nor log to sign. Instead you will need to make observations and answer the questions at the bottom of this page in order to claim your smiley. Please send all answers to me via the message center link (top of the cache page) and do not post them in your log. I hope you enjoy this cache!

A Volcanic Beginning

As with all the Hawaiian Islands, Molokai was formed by volcanoes. In fact the "Friendly Isle" can trace its origins to three separate volcanoes - The Western Molokai Volcano, the Eastern Molokai Volcano and the Kauhako Crater Volcano².  These volcanoes produced multiple eruptions and formed the island as we know it today.

An igneous rock is one which is formed during a volcanic eruption. In general these can be broken up into two broad categories: Felsic igneous and Mafic igneous³. Felsic igneous rocks, on a whole, tend to have light colors or shades: white, pink, light brown, light gray. Mafic igneous rocks, on the whole, tend to be dark colored, commonly black or dark gray. Most mafic magma originates by the melting of rocks in the mantle that are extremely rich in iron and magnesium. Felsic magma usually originates in the crust or by the shedding of mafic minerals as magma rises through the crust and are therefore relatively low in iron content³. As the Western Molokai Volcano erupted, it laid down a parent layer of igneous rock. After thousands and thousands of years of weathering that rock began to turn into soils. 

The Weathering Process

As soon as the island of Molokai had formed, it was subjected to erosion. Erosion can occur in three primary ways - mechanical, biological and chemical. Mechanical erosion is a process that most people are familiar with; it involves the eroding of rock by physical forces including wind and water. By extension, biological erosion occurs as living organisms break down rocks. Finally, chemical erosion, as the name suggests, relies instead on breaking down the rock via chemical processes. 

The chemical processes can be the result of man-made activity, or the chemical composition of soil or water as it moves across the rock's surface. Just as in other forms of erosion, such as wind, water, or tectonic activity, chemical erosion changes the surface structure of the rock over time⁴.

Chemical erosion causes an alteration to the actual composition of the rock, primarily in its surface minerals. This process seeks out the minerals that are already fairly unstable in the rock's surface. Water plays a very effective role in chemical erosion, as it introduces active agents that react with the minerals in the rock. This is especially true when combined with natural water erosion when the water seeps into existing fractures and causes the rock to break apart. It can also occur when water that contains key chemical agents dislodges thin pieces of shale material⁴. Below are a few common examples of natural chemical weathering processes:

• Carbonation - When carbon dioxide is present and carried by water, carbonation weathering can occur. The carbon dioxide reacts with the water, which forms a weak acid and eats away at the affected rock.

• Hydration - This type of weathering occurs when rocks absorb water, and the resulting hydrogen and hydrate ions form new bonds with minerals present within the rock. This type of chemical erosion can actually result in a change to a new form of rock, such as the process by which gypsum is formed.

• Hydrolysis - This erosion takes place when minerals in the rock form a new solution, usually due to the presence of water. An example of this would be the formation of salt water when natural minerals in the rock form a salt solution once water is introduced.

• Oxidation - Much like carbonation-which occurs in the presence of carbon dioxide-oxidation takes place when rocks react with the presence of oxygen. Oxidation also requires the presence of water, but this can even be in such small amounts as the presence of moisture in the air. Oxidation is more commonly known as rust.

Laterite Soil is Born

Laterite soil is one that is rich in iron oxide and may be derived from a wide variety of igneous rocks weathering under strongly oxidizing and leaching conditions. It forms in tropical and subtropical regions where the climate is humid. Typically laterite is porous and clay-like. It contains a variety of iron oxide minerals⁵. Laterite is not uniquely identified with any particular parent rock, geologic age, single method of formation, climate, or geographic location, rather it is a rock product that is a response to a set of physio-chemical conditions. These conditions must include iron-containing parent rock, a well-drained terrain, abundant moisture for hydrolysis during weathering, relatively high oxidation potential, and persistence of these conditions over thousands of years⁵. 

An essential feature for the formation of laterite is the repetition of wet and dry seasons. Rocks are leached by percolating rain water during the wet season; the resulting solution containing the leached ions is brought to the surface by capillary action during the dry season. These ions form soluble salt compounds which dry on the surface; these salts are washed away during the next wet season. Laterite formation is favored in low topographical reliefs of gentle crests and plateaus which prevents erosion of the surface cover⁶.

Laterite is typically found below the topsoil layer, however on Molokai this is not the case⁷. This is because in addition to an environment which promotes chemical weathering, this island is susceptible to mechanical weathering processes as well. These natural processes have stripped the topsoil layers away in many locations (including here at GZ) leaving the laterite soils exposed to the elements. 

Logging Your Earthcache

As mentioned at the beginning of this page, there is no container to find here at GZ. Instead you will be required to answer the below questions and send the answers to me via the message system in order to claim your "found it!" log. 

1. At the coordinates what color is the soil? 
2. Based on your answer to question 1, was the parent rock here mafic or felsic? Why do you suspect this?
3. Based on your current location and the answer to question 1, what chemical process do you think is most responsible for the coloration of the soil?
4. Based on your reading and observations, what element do you suspect is responsible for the soils coloration?
5. Describe the soil's texture. Is it fine? Coarse? Can this be attributed to chemical weathering or mechanical weathering?
6. (Optional) Take a photo of yourself at GZ and include it with your log!

Sources

1. https://www.spaceanswers.com/deep-space/whats-the-missing-link-between-stars-and-planets/
2. https://themolokaidispatch.com/naturally-speaking-6/
3. https://courses.lumenlearning.com/wmopen-geology/chapter/outcome-igneous-rocks/
4. http://www.softschools.com/examples/science/chemical_erosion_examples/319/
5. http://clarkscience8.weebly.com/weathering-erosion-deposition.html
6. https://www.britannica.com/science/laterite
7. http://gis.ctahr.hawaii.edu/SoilAtlas