~ An Earthcache is a “special place that people can visit to learn about a unique geoscience feature or aspect of our Earth. ... Visitors to Earthcaches can see how our planet has been shaped by geological processes, how we manage the resources and how scientists gather evidence to learn about the Earth.”
~ Please read the cache page to assure you have what you need to verify your answers.
This cache requires that you complete tests in the field.
To assist in your answering the required questions, please bring a nail or something you can use to scratch the surface of a rock.
The following are among the sedimentary rocks that can be found on Guam: Biolithite, Fossiliferous limestone, Chalk, Crystalline limestone, Travertine, Carbonate breccia, Carbonate conglomerate, Carbonate sandstone, Micrite, Argillaceous limestone, Lithic sandstone, and Conglomerate. Today’s, focus is about Limestone, a sedimentary rock found on Guam.
“In the rock cycle, there are three different types of rocks: sedimentary, igneous, and metamorphic. Sedimentary and igneous rocks began as something other than rock. Sedimentary rocks were originally sediments, which were compacted under high pressure.”
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface, in contrast to metamorphic and igneous rocks, which are formed deep within the Earth and often have distinctive layering or bedding. The most important geological processes that lead to the creation of sedimentary rocks are erosion, weathering, dissolution, precipitation, and lithification (the conversion of loosely arranged sediments into solid rock). Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. The smallest grains are called clay, then silt, then sand, and grains larger than 2 millimeters are called pebbles. Common sedimentary rocks include sandstone, limestone, and shale. Biologic sedimentary rocks form when large numbers of living things die. Chert is a example for this type of rock, and this is one of the ways limestone can form. Limestone can also form by precipitating out of the water.
Sedimentary rocks can be organized into two categories. The first is detrital rock, which comes from the erosion and accumulation of rock fragments, sediment, or other materials - categorized in total as detritus, or debris. The other is chemical rock, produced from the dissolution and precipitation of minerals.
Detritus can be either organic or inorganic. Organic detrital rocks form when parts of plants and animals decay in the ground, leaving behind biological material that is compressed and becomes rock. Coal is a sedimentary rock formed over millions of years from compressed plants. Inorganic detrital rocks, on the other hand, are formed from broken up pieces of other rocks, not from living things. These rocks are often called clastic sedimentary rocks.
Chemical sedimentary rocks can be found in many places, from the ocean to deserts to caves. For instance, most limestone forms at the bottom of the ocean from the precipitation of calcium carbonate and the remains of marine animals with shells. If limestone is found on land, it can be assumed that the area used to be under water. The limestone of Guam is mostly in the crystalline form called calcite. Calcite is calcium carbonate (CaCO3), which is dissolved by acids. Rain falling on the island is mildly acidic.
Sedimentary rock is also classified by the texture of the sediment particles that form the rock.
1) Sedimentary breccia - coarse-grained sedimentary rock formed by cementation of coarse, angular fragments of rubble
2) Conglomerate - coarse-grained sedimentary rock formed by cementation of rounded gravel
3) Sandstone - medium-grained sedimentary formed by cementation of sand grains
4) Shale, Siltsone, Mudstone - fine-grained sedimentary rock formed by cementation of silt and clay particles
Sedimentary rock can be classified a Clastic Rock (formed by lithification of fragments of pre-existing rock broken down by mechanical or chemical weathering) or Chemical rock (resulting from evaporation, precipitation, secretion by plants or animals, or organic accumulation under acidic environments). Most of the sedimentary rocks found on Guam are chemical rocks.
Limestone is a sedimentary rock composed mostly of calcite (CaCO3), often precipitated in shallow seawater by living organisms. Limestone may be precipitated directly as a solid rock in the core of a reef by corals and encrusting coralline algae. Such limestone would have crystalline texture and would contain fossil remains of organisms still in their growth position. These limestones are called framework limestones. Once the limestone is formed, it weathers. Rainwater and carbon dioxide from the air and soil mix to form a weak acid. As the slightly acidic water percolates through the joints and bedding planes, it dissolves the calcium carbonate in a process called karstification. Limestone is relatively soft and can easily be scratched.
To Log this Earthcache, please send your answers to the following questions to the Cache Owner’s profile listed at the top of this page.
Please Do Not include your answers in your online log.
Look around the area for a sample on the ground or the wall of the cliff to examine to determine if you it feel may be limestone.
Question #1: As you look at your sample, would you describe its appearance as having layers or as compacted particles? Would this sample be consistent with the qualities of limestone? Why or why not?
Question #2: Considering where you are finding this sample, would you consider it to be detrital or chemical rock? Why?
Question #3: Using a sharp implement, what happens when you scratch the sample? Are your results consistent with the qualities of limestone.
Please don’t be intimidated about answering the questions. They are meant to improve your understanding of the geological aspects of the earth and provide you with new knowledge and/or insights about the location. You don't have to write a term paper. Just provide enough information to show me that you were there and tried to observe the details. But, most importantly have fun. I hope you take away some new knowledge with a smile on your face!
The above information was compiled using information from the following resources:
National Geographic Resource Library
ROCKS and MINERALS of GUAM - A Guide to Identifying What You Find Around our Island, Brenna Lorenz
The Rock Cycle: Sedimentary Rock - SPREP
USGS, Science for a Changing World