What is a rock? A purely descriptive definition is that a rock is - A naturally occurring aggregate of minerals and other solid material. - Usually, there are several minerals in the aggregate, though some rocks may have only one. The other materials may include natural glasses, organic material, or fossils.
Sedimentology - reconstructing the the environmental context
Geologists usually think of rocks in a second important way, however. Please and recite it like a mantra:
A rock is a record of the environment in which it formed.
The three major rock types:
The Rock Cycle: Consider the three basic rock types and how they form:
- Igneous: Rocks that form from the cooling and solidification of magma. Igneous rocks are generally have interlocking crystals that show no preferred orientation. There are two types:
- Intrusive (also "plutonic"): Igneous rocks that formed underground from slowly cooling magma. These have large visible crystals. E.g.: Granite.
- Extrusive (also "volcanic"): Igneous rocks that formed near the surface from rapidly cooling magma. These have small, sometimes even microscopic, crystals. E.g.: Basalt.
- Metamorphic: Rocks that form from the recrystallization of preexisting rocks under extreme heat and/or pressure. E.g.: Gneiss
- Sedimentary: Rocks that form from transported fragments of preexisting rocks. E.G.: Gypsum, Limestone, or Sandstone.
- Igneous - solidified from molten material
- Metamorphic - recrystallized by heat and/or pressure.
- Sedimentary - composed of remains of preexisting rocks.
The material that makes up any rock might have a complex history.
- The grains of quartz sand in a sandstone might have been wethered from a quartz vein in a metamorphic rock that prior to being metamorphosed, had been an igneous granite.
- That same sandstone may, in the future, become buried deep in the crust, undergo metamorphism, melt, and resolidify as an igneous rock.
Geologists describe this range of possible histories as the Rock Cycle. As the schematic shows, it actually encompasses many possible cycles.
Sedimentary Rocks: Typically boring and ugly compared to igneous or metamorphic, but they have great utility because they speak to us about the history of the Earth's surface, where life lives.
Sedimentary rock - rock composed of the transported remains of pre-existing rocks, i.e. sediment.
- Remember, any rock is a record of the environment in which it formed. An igneous rock is, thus, a record of the chemistry of its parent mantle rock, the chemistry of the rocks through which it passed as magma, and the circumstances under which it solidified - i.e., they record events and conditions of the deep Earth. Sedimentary rocks, in contrast, are records of conditions at the Earth's surface. Additionally, because sedimentary rocks tend to form in layers, they represent something more like a movie, in which each layer represents a frame, as opposed to a simple snap-shot.
- Furthermore, because sedimentary rocks form at the Earth's surface, so they are easy to locate and study and they reflect ancient environmental conditions in environments that are of direct concern to humans.
Sediment: material derived from the weathering of preexisting rock.
General life history of sediment: In order to make a sedimentary rock, four things need to happen:
Weathering: The chemical and mechanical breakdown of preexisting rocks.
Transport: Running water, gravity, ice, wind. Material may be transported as fragments or in solution.
Deposition: The process of depositing the sediment. For materials in solution, this involves precipitation. For larger fragments, it's just a matter of being dumped.
Diagenesis: Physical and chemical changes that result in the lithification of the sediment into a solid rock, and maybe other subsequent changes.
Each of these processes leave their signature on the resulting rock, with the result that we can learn a great deal about:
- The source rock from which sediments were weathered
- How far and by what means they were transported
- In what environment they were deposited
- The physical changes that have occurred in the depositional environment since deposition.
- And of course, what the fossils that they contain say about the history of life and the nature of the ancient environment in which the sediments were deposited.
Before we can do this, we have to establish a vocabulary that we can use to talk about sedimentary rocks.
Classification: Sediment may be particles such as gravel or sand, the remains of plants and animals, or chemicals in solution. This encompasses a great variety of rock types. We try to make this variety manageable by classifying sedimentary rocks into the following types:
Of course, within each type, grains and other materials can be described further according to a number of criteria that we needn't go into. You should be aware of thebasic breakdown of clastic sedimentary rocks based on clast size:
- Clastic sedimentary rocks: Rocks composed of solid fragments of preexisting rock.
Clast: a single such fragment.
- Chemical sedimentary rocks: Rocks composed of material that was transported in solution and directly precipitated from solution.
- Biogenic sedimentary rocks: Precipitated first in the tissues of organisms, then deposited when the organism dies. E.G.
- limestone - calcite from the shells of marine organisms
- coal - carbon from compressed remains of land plants.
- Conglomerate: Sedimentary rocks made of gravel sized (2 mm.) clasts or larger.
- Sandstone: consists of sand sized (1/16-2mm.) grains.
- Mudrocks: Consists of clasts < 1/16 mm. I.e. smaller than the eye can readily distinguish.
Diagenesis: The chemical alteration of sedimentary rock after its deposition. Let's flesh out this concept, now that we have a basic sedimentological vocabulary:
- When clastic sediment has been deposited, we start with a pile of fragments that enclose a great deal of pore space.
- As sediment continues to accumulate, the sediment undergoes compaction: The geometric arrangement of grains changes so that pore space is reduced.
- In principle, compacted sediment can sit there forever without becoming a rock. Normally, this doesn't happen.
As groundwater moves through the pore space, it brings with it ions in solution. Often, these ions precipitate out in the pore space to form a cement that binds the clasts together. This cement provides mechanical cohesiveness and gives us our rock.
Common cements include:
Cementation is the first stage in a more general process of diagenesis. The chemical alteration of sedimentary rock after its deposition. This can include the dissolution, pressure solution, or in situ alteration of material, also. Thus, diagenesis acts both to preserve or to alter and destroy fossils.
Don't forget that sedimentary rocks preserve both the fossil record and a record of the environment in which the fossil was deposited. The physical characteristics and geographical location determines the type of sediment that will normally be deposited. In broad strokes, geologists classify depositional environments as:
- Continental: Deposited on land or in fresh water.
- Fluvial: stream or river
- Alluvial: Deposits of flash floods and intermittent streams in mountainous environments, such as alluvial fans
- Glacial: Deposited by glaciers
- Eolian: Deposited by wind (in deserts)
- Lake: Dominated by finely laminated clastic sediments.
- Swamp: Rich in organic material.
- Transitional: Deposited in an environment showing influence of both fresh water or air and marine water.
- Deltaic: Deposits at the mouths of large rivers.
- Esturine: Deposits in valleys drowned by rising sea level.
- Lagoonal: Deposits in the waters separating barrier islands from the shore.
- Intertidal: Deposits between high and low tide lines.
- Beach: Deposits in shallowest marine water influenced by waves.
- Marine: Only influenced by sea water.
- Shallow marine clastics: Regions near the mouths of rivers are usually clastic dominated because the critters that secrete CaCO3 tend to have trouble living in muddy water.
- Carbonate shelf: Regions with clear water shallow enough to be penetrated by sunlight are often dominated by the skeletons of marine organisms.
- Continental slope: Dominated by the deposition of submarine landslides.
- Deep marine: Very thin sediments formed by the slow accumulation of skeletons and clasts dropped into the ocean by wind.
Two patterns should bit you in the face:
- Not all environments on Earth are depositional. In mountains and rapidly flowing streams, fossils will be destroyed, not preserved. Some depositional environments are more prone to destruction than others. E. G. : We expect the continuous subduction of deep ocean crust to destroy deep marine fossils.
- Different depositional environments will preserve different parts of the biota to different degrees. Thus, terrestrial organisms may be fossilized in fluvial or deltaic environments more often than in deep marine ones whereas continental shelf environments are more likely to preserve reef organisms.
The Geologic Time Scale
We haven't always understood the significance of sedimentary rocks. Interestingly, fossils played a big part in the discovery of the Earth's age and how it could be inferred from the rock record. Indeed, the science of Biology didn't take shape until the discovery of geologic time (deep time).
- James Ussher, Bishop of Armagh (1581-1656) Calculated biblical Creation of Earth to have been in 4004 BC.
- Nevertheless, by Ussher's time, significant steps had been taken:
- Nicholas Steno: (Danish) Critically examined the current belief among intellectuals that fossils were not the remains of organisms but, rather, "sports of nature" whose resemblance to living things was coincidental. Looking at "tongue stones" which suspiciously resembled sharks' teeth, he noted that they not only looked exactly like sharks' teeth, but that he could even say what species of shark and what part of the mouth. Steno concluded that fossils were, indeed, the remains of organisms. But this led to a problem:
Fossils are found in rocks. What was a shark tooth doing inside a rock?
This led Steno to study how the rocks (which we today recognize as sedimentary) formed. He recognized them to be composed of lithified remains of sediment deposited in layers (or strata) and proposed a set of several principles of stratigraphy by which one coud distinguish younger and older sediments. His results were published in Prodromus, In 1668. The two most significant principles are:
- Original horizontality. Sediments originally deposited in horizontal layers. Therefore when horizontal and disturbed layers are found together, the horizontal ones must be younger, or else whatever disturbed the disturbed layers would have disturbed them, too.
- Superposition. In undisturbed strata, older layers lie beneath younger ones because they must ahve already bee present for the younger ones to be deposited on top of them.
Using these, it began to be possible to say what order the separate rock layers had formed in, provided they could be seen in association. More importantly, Steno was the first to grasp that rocks could be formed from fragments of preexisting rocks that had been transported as sediments and that different rocks formed at different times. Of course, Steno thought that all of this deposition had occurred during the 40 days and nights of Noah's flood.
- In the late 18th century, James Hutton (the founder of modern geology) added a new and powerful principle of stratigraphy to Steno's: Cross-cutting relations. If one structure cuts across another, then the one that is being cut must be oldest. Hutton had his epiphany at Siccar Point in Scotland. Which rocks are younger, the ones above or below?
- In 1796 William Smith, a British civil engineer, added a fourth principle: Faunal succession, noting that different groups of fossil organisms were preserved in different rock units. By this means it became possible to say that one rock was the same age as another rock halfway around the world.
However, you have to use the right fossils:
- Index fossils: Fossils of organisms that existed for short perios of geologic time but were geographically widespread. Example: Ammonites, Shelled cephalopods that evolved quickly (so each species lasted only a few million years, but whose remains were distributed worldwide in many environments.
- Facies fossils: Fossils of organisms that endured for long periods of geologic time but were linked to a specific environment. Example: Lingula, a brachiopod living only in lagoonal mud-flats that has changed very little in the last 500 million years.
The Geologic Time Scale: Using Steno's and Smith's principles, Geologists gradually developed, a standardized a Geologic Time scale developed. It's major features:
But don't say I'm not helpful. Here are the mnemonic devices I promised earlier.
Periods of the Phanerozoic Eon
(You know, Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, Tertiary, Quaternary.)
Epochs of the Cenozoic Era
- Cold oysters seldom develop many precious pearls, their juices congeal too quickly.
- Come over some day, maybe play poker. Three jacks can take queens.
(You know, Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, Holocene (or Recent).)
If you have a strong stomach, there are many more memorable mnemonics out there.
- Pigeon Egg Omelets Make People Puke Heartily.
- Phooey! Even old men play polo, right?.