Pigeon rock is a type of igneous rock found in many parts of the world. It gets its name from its grayish-white color and the similarities it bears to actual pigeon droppings. There has been some debate among geologists as to whether pigeon rock should be classified as a sedimentary or igneous rock. In this article, we will examine the characteristics of pigeon rock and compare them to the defining features of sedimentary and igneous rocks in order to determine where pigeon rock fits best in the rock classification system.
What are sedimentary rocks?
Sedimentary rocks are formed through the accumulation and cementation of sediments. Sediments are broken down bits of other rocks, minerals, organic matter, and other materials that are deposited by wind, water, or ice. Over long periods of time, usually thousands to millions of years, these sediments are compressed and cemented together to form sedimentary rock.
There are three main types of sedimentary rocks:
- Clastic sedimentary rocks such as conglomerate, sandstone, siltstone, and shale that form from mechanical weathering debris.
- Chemical sedimentary rocks such as rock salt, iron ore, chert, flint, some dolomites, and some limestones that form when dissolved materials precipitate from solution.
- Organic sedimentary rocks such as coal, some limestones, and some dolomites that form from the accumulation of plant or animal debris.
Some key features of sedimentary rocks include:
- They are formed from the accumulation and cementation of sediment.
- They often contain distinct layers or strata.
- They may contain fossils of plants, animals, and other organisms.
- They form primarily on or near Earth’s surface, not deep underground.
- They often have a clastic texture from the fragments of older weathered rock.
If a rock displays some or all of these features, it is likely a sedimentary rock.
What are igneous rocks?
Igneous rocks form from the cooling and crystallization of magma or lava. Magma exists deep below Earth’s surface and lava erupts onto the surface from volcanoes. When magma or lava cools, it solidifies into igneous rock. The rate of cooling determines the crystal size and texture of the rock. Slower cooling underground leads to large mineral crystals while fast cooling above ground results in small crystals.
There are two main types of igneous rocks:
- Intrusive igneous rocks such as granite, diorite, and gabbro that cool underground.
- Extrusive igneous rocks such as basalt, andesite, obsidian, and pumice that cool above ground.
Some key features of igneous rocks include:
- They originate from magma or lava.
- They often have interlocking mineral crystals.
- They do not contain layers or fossils.
- Intrusive igneous rocks have larger crystals while extrusive igneous rocks have smaller crystals.
- Their chemical composition relates to where the magma formed in Earth’s crust or mantle.
Any rock that forms from the cooling and crystallization of magma or lava can be classified as igneous as long as it exhibits these definitive characteristics.
The composition and texture of pigeon rock
Now that we have reviewed the key features of sedimentary and igneous rocks, we can examine pigeon rock itself to determine which category it fits into. Here are the basic characteristics of pigeon rock:
- Chemical composition: Pigeon rock consists primarily of the minerals potassium feldspar, quartz, and hornblende.
- Color: Grayish-white, resembling pigeon droppings.
- Texture: Phaneritic texture with visible interlocked mineral grains.
- Grain Size: Medium-grained.
- Occurrence: Found in dikes, sills, and small intrusions.
The chemical composition and phaneritic texture of pigeon rock tells us that it is likely an intrusive igneous rock. It formed underground from slowly cooled magma, allowing the potassic feldspar, quartz, and hornblende crystals to grow relatively large and interlock. The medium grain size also indicates slow subterranean cooling.
Does pigeon rock contain sedimentary features?
To confirm that pigeon rock is indeed igneous, we should check that it lacks major sedimentary rock features:
- Formed from sediment – Pigeon rock does not contain sediment and is not formed by the accumulation and cementation of sediment.
- Layered strata – Pigeon rock does not contain layers or strata. The mineral grains are interlocking and uniform throughout.
- Fossils – There are no visible fossils within pigeon rock.
- Clastic texture – Pigeon rock does not have a clastic texture composed of fragments of older weathered rock.
The absence of any observable sedimentary features in pigeon rock provides compelling evidence that it did not form through sedimentary processes and should not be classified as a sedimentary rock.
Does pigeon rock display igneous features?
Since we have ruled out pigeon rock being a sedimentary rock, the next step is confirming that it does indeed share the expected features of an igneous rock:
- Formed from magma – The chemical composition and texture of pigeon rock indicate crystallization from silica-rich magma.
- Interlocking mineral grains – Pigeon rock has visible interlocking feldspar, quartz, and hornblende crystals, typical of igneous rocks.
- No layers or fossils – The rock is uniform throughout with no strata or fossils.
- Intrusive texture – The medium-grained phaneritic texture signals slow cooling within a dike or sill.
The definitive igneous features present in pigeon rock provide strong evidence that it formed through the cooling and crystallization of magma deep below Earth’s surface and should be classified as an intrusive igneous rock.
Examples of pigeon rock formations
Now that we have determined pigeon rock is indeed an igneous rock, it is useful to examine some real-world examples of where it occurs to understand its geological context:
Continental dikes
Pigeon rock is found in some continental dikes, which are vertical igneous rock intrusions that cut across existing rock layers. The magma from deep within the continental crust intrudes into vertical cracks and slowly crystallizes into pigeon rock. Examples are found in dikes cutting across granite and gneiss in the Mojave Desert.
Batholiths
Large batholiths, which are massive igneous rock intrusions, can also contain zones of pigeon rock. This indicates the main batholith mass cooled slowly underground, allowing phaneritic textures like pigeon rock to form in places. Some occurrences are found in the Sierra Nevada Batholith.
Stratified intrusive sills
Intrusive sills formed by the forcible injection of magma between rock strata may show layering or gradients in texture and mineralogy. Pigeon rock is found in certain stratified sills, typically toward the slower cooled interior portions rather than the fine-grained chilled margins. The Bushveld Igneous Complex in South Africa provides examples.
Volcanic pipes and plugs
Volcanic pipes or plugs are built from the magma conduits that feed volcanoes. Slow cooling deep underground prior to eruption can create phaneritic igneous rocks like pigeon rock. Pigeon rock has been identified in the plugs associated with old volcanoes in Arizona and New Mexico.
How pigeon rock forms
Now that we have established that pigeon rock is an intrusive igneous rock and looked at some areas where it occurs, we can examine the specific process that creates it:
- Magma forms deep underground, likely at least 10-20 km down into the continental crust based on pigeon rock’s felsic mineralogy.
- The magma intrudes upward into fractures and conduits as a pluton or volcanic plug, or it pushes between rock layers as a dike or sill.
- The magma cools slowly within these intrusive bodies, taking thousands to millions of years to fully crystallize.
- The slow cooling allows large interlocking crystals of the minerals potassium feldspar, quartz, and hornblende to grow, creating the phaneritic texture.
- The combination of minerals gives pigeon rock its distinctive grayish-white color, similar to pigeon droppings.
This process explains how the igneous textural and mineralogical characteristics that define pigeon rock are formed. The intrusive emplacement provides the key slow cooling that generates its medium-grained phaneritic texture.
How to identify pigeon rock
Based on the above information, here are some tips for identifying pigeon rock in the field:
- Look for a medium grayish-white color resembling pigeon droppings.
- Look for visible intergrown feldspar, quartz, and hornblende crystals up to 1cm across.
- Check for a phaneritic texture without layers or clastic grains.
- Seek out dikes, sills, plugs, and plutons where intrusive cooling creates igneous textures.
- Look for associations with granitic batholiths or rhyolite lava domes.
- Remember it exhibits no sedimentary textures, layers, or fossils.
Following these guidelines will help properly identify pigeon rock in the field based on its characteristic intrusive igneous features.
Uses of pigeon rock
Pigeon rock has some uses, mainly in construction, owing to its hardness and durability:
- Aggregate – Crushed pigeon rock serves as a strong aggregate in concrete and asphalt.
- Dimension stone – Pigeon rock can be cut and polished into tiles, countertops, and building stones.
- Decorative gravel – Its interesting color makes pigeon rock gravel useful in landscaping.
- Reinforcing fill – Pigeon rock aggregates provide a strong fill for subgrade reinforcement.
However, pigeon rock’s limited abundance and unremarkable appearance mean it sees only minor commercial use relative to other igneous rocks. But in areas where pigeon rock is common, it often serves as a locally available construction material.
Key facts about pigeon rock
In summary, here are the key facts to remember about pigeon rock:
- Pigeon rock is an intrusive igneous rock.
- It has a phaneritic texture with visible feldspar, quartz, and hornblende crystals.
- The rock composition indicates origins from felsic continental crust magmas.
- Slow cooling underground allows the phaneritic texture to form.
- Typical occurrences are in dikes, sills, plutons, and volcanic conduits.
- It lacks any sedimentary textures, layers, or fossils.
These facts all support classifying pigeon rock as an intrusive igneous rock rather than a sedimentary rock. Careful field identification of texture, mineralogy, and geologic context is needed to distinguish pigeon rock from other light-hued igneous rocks with similar appearances.
Conclusion
Based on a detailed look at its mineral composition, texture, geological occurrences, and mode of formation, pigeon rock conclusively exhibits the defining characteristics of an intrusive igneous rock. It lacks sedimentary textures, layers, fossils, and clastic grains that would indicate a sedimentary origin. The interlocking phaneritic mineral grains and presence in dikes, sills, and plutons provide compelling evidence that pigeon rock consolidated from magma emplaced at depth. While casual visual similarities to sedimentary rocks exist, careful examination proves pigeon rock formed through igneous rather than sedimentary processes. This analysis shows that pigeon rock is unambiguously classified as a felsic intrusive igneous rock according to the principles of petrology and geological field relationships.