Rock Formation & The Rock Cycle
The Rock Cycle
Planet earth is made up of a molten rock interior, with cooler outer regions terminating in a thin, solidified skin called the "crust," or lithosphere. Earth's outer crust is thinner underneath the ocean - called the "oceanic crust" - with an average thickness of 10 kilometers, while the portions of the crust that contain land masses - called the "continental crust" - have an average thickness of 20 to 80 kilometers.
The outer crust of the earth is fractured into 14 individual sheets of rock called "plates," that are floating on a sea of molten rock. As these floating islands of rock move along a molten sea they separate and collide with each other in a process called "plate tectonics."
Through gravitational and inertial forces, the individual plates are in a constant state of motion, causing the cyclical opening and closing of ocean basins. Over 130 million years ago, all of the continents on earth today started out as one giant land mass called "Pangaea." As the plates separate or collide with each other they create three distinct types of "boundaries" which are called:
- Convergent Boundaries Plates colliding with each other, causing subduction (Himalayas)
- Divergent Boundaries One plate that is separating or tearing into two plates (Rift Valley)
- Transform Boundaries Plates slide against each-other along transform fault (San Andreas)
When two plates collide the subordinate plate will be consumed underneath the dominant plate in a process called "subduction." The subduction process within the "collision zone" of a convergent boundary creates enormous upward pressure, causing mountain-building activity through a process called "uplifting" or "collision orogeny," and may also cause volcanic activity.
Mountains that are created by tectonic orogeny can occur as a single mountain range (ie. the Urals) or as a "thrust belt" consisting of several mountain ranges (ie. North American Cordillera - Rocky Mountains, Sierra Nevada, and Cascades).
Subduction also drags surface material and sediments deep underground along with the subordinate crust in a process called "deep burial," where the surface material is re-congealed into molten rock.
Basic Rock Types
The earth and its geology is referred to as a "closed system," meaning that all rock and minerals are constantly being recycled in a process of evolution called the "rock cycle." Starting with its initial formation as cooled and solidified lava, rock is worn away by ice, rain, and wind (weathering), then transported by erosion, until the material reaches its final resting place (deposition). During this evolutionary process there are three basic states of being (forms) that a rock can have:
- Igneous Parent Rock (felsic, intermediate, mafic or ultramafic)
- Metamorphic Formed by Heat and Pressure
- Sedimentary Formed by Deposition
When the earth was formed several billion years ago, there was only molten material (Igneous rock), which was the "parent" to all other rock varieties. Each of the three types of rock can evolve into the other in a bi-directional process (see chart at top of page).
All gemstones are found in roughly four types of situations or occurrences, which describe their physical location at the time of their discovery. When prospecting for precious metals, ore bodies, or gemstones, each of these unique situations calls for a different method of exploration, identification and extraction.
- In Situ Gems found "in place" in their parent, or host rock
- Eluvial Gems exposed by weathering or erosion
- Colluvial Gems found in loose deposit of debris at base of slope
- Alluvial Gems that are transported and redeposited by erosion, or running water
Gems that are found in situ, which is Latin for "in place," can occur in both igneous pegmatites and kimberlites, or in metamorphic host rock, which are considered to be a "primary" deposit. Gems that have remained roughly in place, but the surrounding soil and/or rock has eroded/weathered away are considered to be "eluvial," or "eluvium." When a gem is part of a debris field that was deposited by a landslide or "spill" it is considered to be "colluvial." Lastly, when the gem has been completely transported to a new, secondary location by running water, and redeposited within sediments, sands, or river gravels it is considered to be "alluvial," or "alluvium."
Gemstones are typically hard, highly resistant to weathering, and have a specific gravity that is higher than that of common minerals, rock or sedimentary soil, therefore, they tend to concentrate in alluvial deposits in much the same way as gold placers develop.
Bibliography and Reference on Gem & Rock Formation
1. Paul R. Shaffer, Herbert S. Zim, Raymond Perlman, Rocks, Gems and Minerals . Martin's Press
2. UC Berkeley, Pegmatites . ist-socrates.berkeley.edu
3. Merguerian, Geologic Structure - A Primer . people.hofstra.edu
4. Walter Schumann, Gemstones of the World . NAG Press; 2Rev Ed edition