Sedimentary Rocks: Clastic, Chemical & Organic: Sixth Grade Science
Sedimentary rocks form at or near Earth's surface through the weathering, erosion, deposition, compaction, and cementation of mineral sediments and biological remains.
The Formation of Sedimentary Rocks
Sedimentary rocks cover roughly 75 percent of Earth's continental land surface, forming the layered strata that record the history of ancient environments, climate shifts, and prehistoric life. All sedimentary rocks form through a five-stage geological process known as the sedimentary cycle: weathering, erosion, deposition, compaction, and cementation (lithification).
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| THE SEDIMENTARY ROCK CYCLE |
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| 1. WEATHERING: Physical breaking and chemical decomposition of rocks. |
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| 2. EROSION & TRANSPORT: Water, wind, or ice carry sediments away. |
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| 3. DEPOSITION: Sediments settle out into horizontal layers (strata). |
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| 4. COMPACTION: Overlying sediment weight squeezes pore water out. |
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| 5. CEMENTATION: Dissolved minerals precipitate, gluing grains solid! |
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Lithification: Compaction and Cementation
The transformation of loose, soft sediment into solid rock is called lithification:
- Compaction: As meters and kilometers of new sediment accumulate over millions of years, the immense weight of the overlying layers presses downward. Sand and mud grains are squeezed tightly together, reducing pore space and squeezing out trapped water.
- Cementation: Groundwater percolates through the remaining microscopic pore spaces. The water carries dissolved mineral ions (primarily calcite, silica, and iron oxide). As conditions change, these dissolved minerals precipitate out of the water, crystallizing around the sediment grains and acting as natural rock glue that binds the particles into hard, solid rock.
The Three Classes of Sedimentary Rocks
Geologists classify sedimentary rocks into three primary categories based on the source and nature of their sediment: clastic, chemical, and organic.
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| THE THREE SEDIMENTARY ROCK CLASSES |
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| Classification | Sediment Origin | Diagnostic Examples |
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| Clastic | Solid rock & | Shale (mud), Sandstone (sand), |
| | mineral fragments | Conglomerate (rounded gravel), |
| | cemented together | Breccia (jagged angular gravel) |
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| Chemical | Dissolved minerals| Rock Salt (halite evaporite), |
| | precipitating out | Rock Gypsum, Travertine limestone |
| | of water solutions| chert (microcrystalline silica) |
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| Organic | Remains of once- | Fossiliferous Limestone (shells), |
| (Biochemical) | living plants and | Chalk (microscopic coccoliths), |
| | animal skeletons | Bituminous Coal (swamp vegetation)|
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Clastic Sedimentary Rocks: Classified by Grain Size
Clastic sedimentary rocks are formed from broken rock fragments (clasts) produced by mechanical weathering, classified strictly by the physical diameter of their grains:
- Shale: Formed from microscopic clay and mud particles (< 0.002 mm) deposited in calm, tranquil waters (lakes, deep ocean basins). Shale splits easily along thin, delicate layers (fissility).
- Sandstone: Formed from sand grains (0.06 to 2 mm), primarily resistant quartz, deposited in high-energy environments like ocean beaches, river sandbars, and desert sand dunes.
- Conglomerate: Formed from large, rounded gravel pebbles (> 2 mm) smoothed by turbulent tumbling in fast-moving mountain rivers.
- Breccia: Formed from sharp, jagged, angular gravel fragments, indicating the sediment was deposited very close to its weathering source (such as rockslides or fault zones) without being tumbled smooth.
Chemical Sedimentary Rocks: Evaporites and Precipitates
Chemical sedimentary rocks form when dissolved mineral ions precipitate out of water:
- Evaporites: When shallow desert lakes or enclosed ocean seas evaporate under hot climates, dissolved salts reach saturation and crystallize onto the lake bed. Examples include rock salt (halite, NaCl) and rock gypsum (CaSO4).
- Precipitates: Chemical reactions cause solid minerals to fall directly out of solution, forming rocks like travertine in limestone caves and hot springs.
Organic (Biochemical) Sedimentary Rocks
Organic sedimentary rocks are built from the accumulated biological remains of ancient organisms:
- Fossiliferous Limestone: Composed of calcium carbonate (CaCO3) shells, coral skeletons, and marine fossils cemented together.
- Coal: Formed in stagnant, oxygen-poor prehistoric swamp environments. Dense layers of dead plant matter accumulated without decaying, gradually being compressed over millions of years into black, carbon-rich combustible rock.
Stratification and the Fossil Record
Sedimentary rocks are the only class of rocks that routinely preserve fossils:
- Igneous rocks are formed from scorching molten magma that destroys biological remains.
- Metamorphic rocks are subjected to intense heat and crushing pressure that crushes and distorts fossils.
- In sedimentary basins, dead organisms are gently buried beneath mud and sand, sealing them from scavengers and oxygen, allowing delicate shells, bones, and plant imprints to mineralize and endure across hundreds of millions of years.
Exercises and Solutions
Exercise 1: Decoding Depositional Environments from Clastic Grain Shape. A field geologist collects two gravel-bearing sedimentary rock specimens: Specimen 1 is a conglomerate composed of smooth, oval, highly rounded quartz pebbles cemented in sand. Specimen 2 is a breccia composed of sharp, jagged, angular limestone shards with razor edges cemented in fine silt. Reconstruct the depositional environments and transport histories of both rocks based on clastic grain geometry.
Solution 1: In Specimen 1 (conglomerate), the smooth, highly rounded pebbles provide evidence of long-distance transport in a high-energy aquatic environment, such as a turbulent mountain river or wave-pounded ocean beach. As the gravel tumbled along the riverbed over miles, violent collisions ground down sharp corners and smoothed the edges through mechanical abrasion. In Specimen 2 (breccia), the sharp, jagged angular fragments indicate that the rock was deposited almost immediately at the source of weathering with virtually zero water transport. It formed in a low-travel environment such as a catastrophic mountain rockslide, talus scree slope at the base of a cliff, or an active fault shear zone where fragments were shattered and quickly cemented without abrasive tumbling.
Exercise 2: Explaining Why Fossils Are Unique to Sedimentary Rocks. A paleontology museum displays dinosaur bones, fossilized fish impressions, and ancient fern leaves. An exhibit guide explains that more than 99 percent of all fossils on Earth are discovered exclusively within sedimentary rock strata. Explain why fossils are preserved in sedimentary rocks while being absent from igneous and metamorphic rocks.
Solution 2: Fossils require rapid, gentle burial in protective sediments to shield dead organisms from scavengers, decomposers, and atmospheric oxidation. Sedimentary deposition in river deltas, lake bottoms, floodplains, and shallow seas gently covers organic remains with mud, silt, and sand under mild temperatures and low pressures, allowing mineral-rich groundwater to gradually replace bones and shells with stable stone. In contrast, igneous rocks form from molten magma and lava at extreme temperatures (700 to 1,200 degrees Celsius) that incinerate and melt organic tissue instantly. Metamorphic rocks undergo intense directed tectonic pressures and superheated temperatures that crush, recrystallize, and obliterate biological structures, making sedimentary rocks the unique geologic archive of prehistoric life.