Rocks & Weathering – Colo-i-Suva Forest Park
Tasks:
-
Describe the range of colors visible on the rock face and surrounding soil. Where are the red-brown hues most intense? Based on the geological background, why do you think these areas show stronger coloration?
-
Identify the primary weathering processes active at the waterfall. How do chemical and physical weathering interact to shape the landscape here?
-
How does the tropical climate of Colo-i-Suva contribute to the rate of weathering and the formation of iron oxide staining? Consider factors like rainfall, humidity, and temperature.
-
Please take a picture of yourself or an object on site to prove your visit and post it with your log.
Log this cache as “Found it” and send me the answers to the questions to “silka03.earthcache@gmail.com”, my profile e-mail or via Geocaching Message Center. Not in your log!
You may then log immediately. I will contact you if something is wrong.
If there are no answers after 14 days I have to delete your log.
Have fun!
The Colo-i-Suva Forest Park, located just outside Suva, Fiji, is renowned for its lush rainforest, clear swimming holes, and cascading waterfalls. The park lies within a tropical ecosystem that significantly influences the surrounding geology. The underlying geology of the area consists primarily of sedimentary rocks, formed over millions of years through the accumulation and compaction of mineral fragments, organic materials, and volcanic ash. Fiji’s history of volcanic activity contributed iron-rich sediments to these layers, making them particularly susceptible to weathering processes.
Over time, tectonic movements uplifted these sedimentary layers, exposing them to surface conditions. The region’s high rainfall and warm temperatures create an ideal environment for chemical and physical weathering, both of which are key to the distinctive geological features seen today.
At the Colo-i-Suva waterfall, visitors can observe striking red-brown discolorations on both the rock surfaces and the surrounding soil. This coloration results from chemical reactions occurring within the rock, primarily driven by the tropical climate and continuous water flow.
How Does the Red-Brown Color Form?
The dominant red-brown hues around the waterfall are caused by the oxidation of iron minerals present in the sedimentary rocks. As water seeps through cracks and flows over the rock surface, it carries oxygen, which reacts with iron within the rock, forming iron oxides—similar to how metal rusts. The most common compounds responsible for these colors are:
-
Hematite (Fe₂O₃) – gives deep red tones.
-
Goethite (FeO(OH)) – results in yellowish-brown hues.
-
Limonite (FeO(OH)·nH₂O) – produces earthy browns.
The constant presence of water from the waterfall accelerates these oxidation processes. In areas where water flow is strongest or where it pools temporarily, the coloration tends to be more intense due to prolonged exposure and mineral leaching.
Additionally, the surrounding soil often shares these red-brown hues as iron-rich sediments erode from the rocks and settle into the ground. This creates a cohesive visual link between the waterfall’s rock face and the forest floor.
The Role of Weathering
The rocks at Colo-i-Suva are shaped by a combination of chemical and physical weathering. While chemical weathering causes the iron oxidation responsible for the coloration, physical weathering — driven by the constant force of falling water — breaks down rock fragments and transports sediments downstream.
Physical weathering processes include:
-
Erosion from water flow, which smooths rock surfaces.
-
Freeze-thaw cycles (though limited in tropical climates), where temperature fluctuations cause minor expansions and contractions in rock.
-
Biological weathering, where roots and plant life penetrate cracks, widening them over time.
Together, these forces create a dynamic environment where rocks are continually shaped, colored, and reformed. The resulting landscape is a mosaic of smooth rock surfaces, sediment-laden pools, and iron-stained soils.
https://www.mineralienatlas.de/lexikon/index.php/Oxidation?lang=de
https://www.mineralienatlas.de/lexikon/index.php/Oxidationsverwitterung?lang=de
https://www.mineralienatlas.de/lexikon/index.php/Mineral?lang=de