L1a – Uniformitarianism and Other Early Philosophies in Geology
Ideas developed during the mid-1700’s to mid-1800’s
Plutonism
Plutonists derived their name from Pluto, the Roman God of the Underworld. In this model, they assumed that the continents slowly eroded down to sea level – shedding sediment into the oceans. This was followed by volcanic activity that recycles ocean sediments back to land as lava that formed volcanic rocks. They tended to arm-wave the mechanism for how that recycling was generally done. Major proponents of this model included Anton Moro, James Hutton, John Playfair and Charles Lyell.
Neptunism
Neptunists derived their name from Neptune, the Roman God of the Sea. Their model assumed that the world was initially covered by a world ocean. The seawater was hot and charged with chemicals. Slowly as the Earth cooled, different types of rocks were precipitated or deposited from this ocean. Life comes in when the world ocean cooled enough to support life – and is recorded in more fossiliferous sedimentary rocks. During this cooling, the world experienced differential contraction – places where subsidence occurred became the modern ocean basins, while the raised portions became the continents. Neptunists (or sometimes referred to as Wernerians after Werner) viewed the continents as fixed in place. Major proponents of this model included Abraham Gottlob Werner and his students, and the Baron Georges Cuvier.
Plutonism vs. Neptunism
Both models actually have elements that have survived to the present. Plutonists may not have had a thorough understanding of the mechanisms yet but were well on the way to a modern understanding of the rock cycle. Neptunists recognized that some rocks did precipitate from seawater (i.e., some inorganic limestones like oolitic limestones, rock salt, rock gypsum). Both also had some stuff wrong – but they set the stage for active debate on how the Earth works. At the same time, geologists were debating two other philosophies: Catastrophism and Uniformitarianism. Most Catastrophists were Neptunists, while most Uniformitarians were Plutonists
Catastrophism
Idea promoted by Baron Georges Cuvier. He recognized that species could go extinct after careful examination of the fossil record and comparing those fossil animals to living animals. Cuvier studied the geology of the Paris Basin which records Mesozoic and Cenozoic rocks – he noted that many species tended to die out at the same time in Extinction events – notably at the boundary between the Mesozoic and the Cenozoic. He thought that the geologic record was punctuated by major catastrophes that wiped out many species – these events were not uniform or cyclic in nature.
Uniformitarianism
James Hutton proposed this idea, which was then popularized by his friend John Playfair and later by Charles Lyell. They recognized that geologic processes occurring today also operated in the past and that these processes were governed by the same chemical and physical laws. So, if we see specific types of sediments or sedimentary structures in ancient rocks, we can compare them to modern sediments or structures to understand how they formed.
Modern View – Actualism
Uniformitarianism is generally the guiding principle for modern geologists – but modern geologists recognize two complications:
The rates and intensities of some processes may have changed over time. For example, the change in atmospheric chemistry from reducing to oxidizing during the Precambrian.
Cuvier had a good point – mass extinctions have occurred and there are rare special events that punctuate the geologic record – so we do need to account for those.
L1b – The Geologic Time Scale – Lecture Notes
The people involved with the debate between Catastrophism and Uniformitarianism were starting to recognize that the Earth was old – the question then became how old was old! In Western Europe, Geologists started to piece the record of geologic events together by using a mix of old and new tools – their recognition of packages of rock that represented specific intervals of time slowly crystallized into what we now call the modern Geologic Time Scale.
The tools they used included:
Nicholas Steno’s principles
Superposition – the principle that in a stack of undeformed layers of rock, the oldest layers are at the base of the stack and the layers become younger up the stack.
Original Horizontality – sedimentary rocks and volcanic rocks (lava flows and ash deposits) are laid down as horizontal layers.
Lateral Continuity – these horizontal layers extend in all directions until either a.) the current runs out of sediment to lay down, so the layer pinches to zero thickness, or b.) the current runs into a barrier like the edge of the basin.
Inclusions (though the workers who compiled the Geologic Time Scale used the more modern formulation developed by Charles Lyell) – If a layer of rock has embedded in it, clasts (eroded pieces) of rocks. Those clasts are older than the hosting rock layer and were derived from weathering and erosion of pre-existing rock units.
James Hutton’s Unconformity Concept – the recognition that there are gaps in the rock record where sediment was not deposited or after deposition was eroded away.
Cross-cutting relationships – mainly from Charles Lyell
If a layer has been folded or tilted – that deformation occurred after the layer of rock was deposited.
If magma is intruded across a layer to form a new body of intrusive igneous rock – that intrusive rock is younger than the layers it intrudes through.
If a fault cuts across several layers of rock, the fault is younger than the rocks.
Principle of Floral and Faunal Succession (independently developed by William Smith and Baron Georges Cuvier). Recognition that species evolve, survive for some amount of time, then go extinct. After they go extinct, they are not found in younger rocks (with 2 exceptions – 1. in a few cases, fossils can be eroded out of older rocks and redeposited with new sediments and 2.) sometimes we find new fossils that do not show signs of erosion and transport that are slightly younger than the range for members of the species – in this case we extend the known temporal range of the species). The fossil record then shows a trajectory from simpler organisms to more complex organisms over time and when we find specific fossil species, we can estimate the relative age of a rock unit.
Geologic Time Units
Geologists use much larger time units than you are used to – we consider spans of time that represent millions or even billions of years. Just as we subdivide time into smaller units – months, weeks, days, hours, minutes, seconds, Geologists have a series of time units.
Two abbreviations that you need to know because I will use them:
Ma – mega annum. 1 Ma represents 1 million years
Ga – giga annum. 1 Ga represents 1 billion years.
How do we use these abbreviations? Instead of writing: “The dinosaurs died out 65 million years ago”, we could write the shorter sentence: “The dinosaurs died out at 65 Ma.” Another example: “The iron ore deposits in the Upper Peninsula of Michigan were deposited between 1.8 and 1.9 billion years ago” could be written as “The iron ore deposits of the Upper Peninsula of Michigan were deposited between 1.8 and 1.9 Ga.”
The biggest unit of geologic time is the supereon – currently there is one named supereon – the Precambrian which is an informal term. The Precambrian started 4.54 billion years ago and ended at 542 million years ago – so it represents approximately 4 billion years of geologic time (90% of our planet’s history).
Geologists have formally recognized Eons – Eons are smaller than supereons. The Precambrian is split into three eons – the Hadean Eon (4.54 Ga to 4.0 Ga), the Archean Eon (4.0 to 2.5 Ga) and the Proterozoic Eon (2.5 to 0.54 Ga). The span of time from 0.54 Ga to the present is a separate Eon – the Phanerozoic Eon.
Each Eon is split into Eras, for this class – you should memorize the eras of the Phanerozoic Eon. The Phanerozoic is split into the Paleozoic (542 Ma to 252 Ma), the Mesozoic (252 Ma to 65 Ma) and the Cenozoic (65 Ma to the present).
Each Era is split into Periods. Each Period is split into Epochs. Each Epoch is split Ages. In terms of the Geologic Time scale – please memorize all of the periods of the Phanerozoic and all of the epochs of the Cenozoic.
The last thing to note – our basic time units that we use in our daily lives have set amounts of time and are hierarchical. A second represents a specific amount of time and there are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day, and so on. Geologic time units are hierarchical, but do not represent set amounts of time – so for example the eons range from 1.5 billion years long to as short at 542 million years long. Eons though are made up of Eras, Eras made up of periods, and so on.
Supereon | Eon | Eras | Period | Epoch5 | Name derivation and originator of Period | |
Phanerozoic4 (= visible life) George Halcott Chadwick (1930) | Cenozoic4 (= recent life) John Phillips (1840) | Quaternary4 | Holocene4 | Jules Desnoyer (1829) but based on the works of Giovanni Arduino (1759) and Abraham Gottlob Werner (1787) | ||
Pleistocene | ||||||
Neogene1 | Pliocene | Moritz Hörnes (1853) defined the Neogene. Neogene = new born | ||||
Miocene | ||||||
Paleogene1 | Oligocene | Naumann (1866) defined the Paleogene. Paleogene = ancient born | ||||
Eocene | ||||||
Paleocene | ||||||
Mesozoic (= Middle life) John Phillips (1840) | Cretaceous | Defined by Jean Baptiste Julien d’Omalius d’Halloy in 1822 for the chalk deposits of the Low Countries. Creta means chalk in Latin. | ||||
Jurassic | Attributed to Alexander von Humboldt (1799) for the Jura Mountains of France and Switzerland. Later formalized by Alexandre Brongniart (1829) | |||||
Triassic | Friedrich von Alberti (1834) based on the three major rock types found in Triassic rocks of Germany (red beds, chalk, black shale) | |||||
Paleozoic (=Ancient life) John Phillips (1840) | Permian | Sir Roderick Impey Murchison (1841) – after the kingdom of Permia, an ancient kingdom in the Urals region of Russia. | ||||
Carboniferous2 | Pennsylvanian | Carboniferous – William Coneybeare and William Phillips (1822). Mississippian – Alexander Winchell (1871) for rocks in the Mississippi Valley. Pennsylvanian – Henry Shaler Williams (1891) for rocks in Pennsylvania. | ||||
Mississippian | ||||||
Devonian | Adam Sedgwick and Sir Roderick Impey Murchison (1839) for rocks exposed near Devonshire | |||||
Silurian | Sir Roderick Impey Murchison (1835) for the Latin name of a Celtic tribe from Ancient Wales | |||||
Ordovician | Charles Lapworth (1879) for the Latin name of a Celtic tribe from ancient Wales | |||||
Cambrian | Adam Sedgwick (1835) for the Latin name for Wales | |||||
Precambrian3 ( = prior to the Cambrian; Adam Sedgwick, 1835 and Charles Darwin, 1859) | Proterozoic | Proterozoic Eon – means earlier life. Defined by Samuel Franklin Emmons in 1888. | ||||
Archaean | Archaean Eon – meaning earliest time by James Dwight Dana in 1872. | |||||
Hadean | Hadean Eon – named for Hades (the Greek Underworld) by Preston Cloud in 1972. | |||||
Notes
1Older works (and geologists!) will refer to the Tertiary Period – the Tertiary Period corresponds to what is now known as the Paleogene and Neogene Periods. Like Quaternary it is an artifact of the Neptunian Model of Earth History.
2T.C. Chamberlain and R.D. Salisbury in their 1906 Geology textbook lifted the Mississippian and Pennsylvanian to the rank of period – noting the presence of an unconformity between the 2 systems in the rock record. Later workers correlated the Mississippian to the coal-poor Lower Carboniferous and the Pennsylvanian to the coal-rich Upper Carboniferous. This dichotomy in the Time Scales persists to the present – North American geologists tend to use Mississippian and Pennsylvanian while other workers tend to use Carboniferous.
3The Precambrian is an informal term popularized by Adam Sedgwick and his student Charles Darwin. It is often ranked as a “supereon”, another informal term. The weight of informal usage over the past ~2 centuries means that this word is firmly fixed in the geological lexicon. Precambrian refers to the time prior to the Cambrian.
4By definition the Phanerozoic, Cenozoic, Quaternary, and Holocene are open-ended terms – they all have a fixed base but extend into the future until such a time that an event occurs to warrant their closure. What does this mean? Today, we are living in the Phanerozoic, in the Cenozoic, in the Quaternary, and in the Holocene – tomorrow we will still be in the Phanerozoic, Cenozoic, Quaternary, Holocene, at the end of the semester we will very likely still be in the Phanerozoic, Cenozoic, Quaternary, Holocene… Most boundaries in the geologic time scale represent major extinction events – for the Holocene to close, we would need to define the end of a massive extinction event or some other significant event.
One caveat, there is a subset of the Geological community that wants to split off a portion of the Holocene to make a new Epoch, the Anthropocene – the epoch where humans are a geologic agent (think mining, harbor construction, migrating animal and plant species all over the world, etc.). These workers would set an upper boundary for the Holocene and have all time from that boundary to the present be in the Anthropocene. The really difficult question is where to put this boundary, proposals include:
First use of fire (when exactly was that???)
First agriculture and domestication of wild animals (another when exactly was that???)
1492 A.D. – Columbus made quite the impact
1950 A.D. – Nuclear Test Bans after several years of above-ground nuclear bomb testing (this one is probably the one that left the biggest mark – since anything organic formed after this time is enriched in bomb 14C)
5Most of the Epochs of the Cenozoic were named by Charles Lyell in his Principles of Geology (1833). Wilhelm Philipp Schimper defined the Paleogene (1874). Lyell named the Eocene, Miocene and Pliocene (1833) – he also defined the New Pliocene, which would later become the Holocene. The Oligocene was named by Heinrich Ernst Beyrich in 1854. The Pleistocene was defined by Charles Lyell in 1839. The Holocene was proposed by Paul Gervais in 1867. The International Commission on Stratigraphy currently works on formalizing the epochs of other eras and for the Precambrian – the periods and eras.
L1c: Relative Dating Lecture Notes
Geologic Time and the Geologic Record Part 1
How do Geologists determine how old a rock is? Geologists have two methods of determining the timing of deposition or precipitation of a rock.
Relative Dating – the tools in this category allow us to determine the sequence of events that occurred to produce a sequence of rocks. It does not give us precise ages for specific events, just the order of when 1 rock unit was formed relative to another.
Absolute or Radiometric Dating – using chemical properties of a rock (presence of a radioactive isotope and its daughter products) to determine a specific time of formation.
Radiometric dating can determine the age of an igneous rock (volcanic or plutonic) – as the radiometric isotopes were bound into the crystalline structures of minerals during cooling and crystallization of minerals from magma or lava.
Radiometric dating can determine the timing of metamorphism for a metamorphic rock.
Radiometric dating is rarely done directly on sedimentary rocks – there are some special cases where we can age date cements. But most of the time age dating a sedimentary rock is not going to give us information about the rock but may provide information that constrains the sources of sediments that were deposited to form a rock.
In this lecture, we will focus on Relative Dating.
Nicolas Steno’s Principles
Nicolas Steno was a scientist and physician in the late 1600’s who made some observations of sedimentary strata that led to him describing several principles:
Principle of Lateral Horizontality – sedimentary layers are laid down as horizontal sheets during deposition. If we see layers that are not horizontal, that change in orientation had to happen after the layer was deposited.
Principle of Original Lateral Continuity – that horizontal layer extends out in all directions, until it either a.) thins to zero (pinches out) or b.) terminates against some barrier - usually the edge of the basin.
Principle of Superposition – in a stack of undeformed layers, the oldest layer is on the bottom of the stack, and the layers become progressively younger up the stack.
He also proposed a basic formulation of what is now known as the Principle of Inclusions, though Charles Lyell refined the idea in his textbook, Principles of Geology. Inclusions are objects preserved in a sedimentary rock unit – they could be fossils or clasts (pebbles) of an older rock. These inclusions are features that were formed prior to deposition of the sediment that makes up the layer. The fossils were organisms living in the environment – after death, their bodies settle to the seafloor and accumulate with sediment. Pebbles are fragments of older rocks that were weathered and eroded from those rocks – then transported before eventually being deposited with sediment.
Cross-cutting Relationships
Charles Lyell summarized some ideas that he (and James Hutton) had in his book, Principles of Geology to understand specific events and their timing. Cross-cutting relationships is a set of events that include intrusions (emplacement of magma that cools and crystallizes to form new rock), deformation and the development of folds and faults, and erosion that shapes the land surface (forming an unconformity) prior to renewed deposition. We will look at these 3 categories in some detail:
Igneous intrusions – magma is buoyant – it rises by flowing through fractures in the subsurface. It can widen out those fractures by melting the surrounding rock (creating a void space – or magma chamber). As the magma cools, it becomes less buoyant and eventually starts to crystallize in the void space. Because this magma is emplaced into pre-existing rocks, the cooled intrusion that forms is younger than the surrounding rock that had not melted. The surrounding rock can be altered – by being metamorphosed.
Deformation is where stress is placed on pre-existing rock. That stress can act in two ways:
Weak rocks will fold under stress – they start out as horizontal layers that can be squeezed into arcuate layers.
Strong rocks will break under stress forming fractures. Later stress placed on that rock will cause rock one side of the fracture to move relative to the other side of the fracture. If movement or displacement occurred – then we call this fracture a fault. There are three types of faults:
Normal faults – the stresses placed on the rock act to pull it apart. The footwall block moves up relative to the hanging wall block in a normal fault.
Reverse faults – the stresses placed on the rock act to squeeze it together. The footwall block moves down relative to the hanging wall block in a reverse fault.
Strike-slip faults – the stresses cause shearing – the fault plane is generally vertical, and one block slides past the other.
Unconformities are surfaces in the rock record that represent what the land surface was at some point in the past – that surface was shaped by erosion and non-deposition and represents a period of missing time. After some period of time passes, deposition occurs in the area burying the unconformity surface with younger sediments. There are three basic types of unconformities:
Angular unconformity – the layers below the unconformity surface have a different orientation than the layers above the unconformity. The older layers started out as horizontal layers – that were then tilted or folded under stress. After tilting, erosion truncates the tilted layers. Some time later, new layers are laid down on top of the unconformity – which are horizontal.
Nonconformity – The rock material below the unconformity surface are crystalline rocks – igneous intrusions and metamorphic rocks. Erosion strips away the overlying materials, exposing these crystalline rocks – then new sedimentary layers are deposited on top of the crystalline rocks.
Disconformity – these unconformities are more subtle – usually requiring that we look at the fossils in the layers below the unconformity versus those in the layers above, because the fossils show that there was missing time. In this type of unconformity, sediments are laid down as horizontal sedimentary layers. Conditions change and the layers become partially eroded away – followed by renewed deposition of new horizontal layers above the unconformity. The layers on either side of the unconformity are relatively parallel to each other.
Fossils and Relative Dating
Fossils are also used to assess the age of a sedimentary unit (and in limited cases some igneous rocks (ash deposits) and low-grade metamorphic rocks). The basic principles are the principle of fossil correlation and the principle of faunal and floral succession. Both principles were independently developed by William Smith and the Baron Georges Cuvier.
Principle of Fossil Correlation – If we find a fossil species in rocks at one location, then the same species as fossils in rocks from a second location – those rocks are the same age.
Principle of Faunal and Floral Succession – Cuvier and Smith recognized that in undeformed sequences, they could follow rock units from older to younger and see changes in the composition of the flora and fauna in those rocks. Cuvier made the leap that ancient species could go extinct – and then recognized that the fossil record had a trajectory. Fossil species evolve and then eventually go extinct and are replaced by new species. An extinct species does not spontaneously re-appear at some time more recent (with one caveat – we can have fossils eroded out of older rocks – then deposited with newer sediment – but usually we can see evidence that the fossil had been weathered, eroded and transported).
Geologic Units
Early Geologists developed three sets of units – we have already covered one type, time units. The other two sets of units are rock units and time-rock units.
Time Units break up geologic time into manageable chunks of time. Units include Eons, Eras, Periods, Epochs and so on and are the basic units of the Geologic Time Scale.
Rock Units break up the rock record into packets of rock of similar composition and character (textures, sedimentary structures, etc.). At the time, these units were then assumed to have been deposited at the same time (though we will see in Lecture 6 that this is not truly the case). Rock units could then be used to map geology at the surface (as recorded in geologic maps) or in the subsurface (as recorded in cross sections).
Time-Rock Units are not used as much today, but I do want you to understand what they represent. These units are used for more regional or global studies or rocks laid down during a specific time interval – so for example all rocks worldwide deposited during the Silurian Period are part of the “Silurian System” (time-rock unit).
All three sets of units are hierarchical – so larger units are made up of smaller-scale units. They do not necessarily specific the same amount of time or the same amount of rock (unlike units of measurement we use in our daily lives – where a centimeter has a specific length or a second has a specific amount of time).
Time units – the largest unit is an eon. Eons are made up of Eras. Each Era is made up of Periods. Each Period is made up of Epochs and Epochs are made up of Ages.
Time-rock Units – the largest unit is an eonothem. Eonothems are made up Erathems. Erathems are made up of Systems. Each System is made up of Series and Series are made up of Stages. The Mesozoic Erathem represents all the rocks deposited or formed during the Mesozoic Era.
Rock Units
Rock Units are the basic units for mapping geology at the surface (maps) and in the subsurface (cross-sections and various types of subsurface maps). They are hierarchical. The biggest rock unit is the Supergroup. Supergroups (as they are treated in modern geology) represent large scale packages of rocks that formed under some of the same tectonic conditions. Supergroups are composed of groups. Groups are thought to have formed under similar environmental conditions (such as one cycle of sea level change or as the sediments deposited in several adjacent environments such as river system with swamps, river channels and floodplains). Groups are made up of Formations. Formations are the basic unit of geologic mapping. In practice, a formation is defined by lithologic characteristics that are uniform through the body of rock and can be mapped across a portion of the Earth’s surface or in the subsurface. If our Group represented the deposits of a river valley – the swamp sediments (coals) might represent one formation, the channel deposits (sandstones) might be defined as another formation, and the silts and muds of the floodplain may be mapped as yet another formation. Formations are made up of Members and Members are made up of individual layers of rock called Beds. In the example below, you will see that rock units are formally named – mostly after nearby locations.
Here is a Michigan Example. There is a package of sandstones, conglomerates, shales and basalts in the Upper Peninsula of Michigan called the Keweenaw Supergroup. This Supergroup is Precambrian in age (with age dates suggesting a temporal range of deposition between 1.2 and 1.05 billion years ago) and was deposited in an ancient rift valley. In terms of tectonic context, the Keweenaw Supergroup formed as the North America was being stretched. The Keweenaw Supergroup is divided into the lower Portage Lake Volcanics (a truly massive pile of basalt) and the Oronto Group. The Oronto Group is made up of several sedimentary rock units – the Copper Harbor Conglomerate, the Nonesuch Shale and the Freda Sandstone. These three parts of the Oronto Group are formations – and can be mapped over significant areas of the western Upper Peninsula because they have distinct rock properties. The Nonesuch Shale is a formation – it is composed of black to gray, shale and siltstone and was likely deposited as lake deposits in the rift valley. The Nonesuch Shale was named for the town of Nonesuch (a now abandoned mining town in the southern part of what is now the Porcupine Mountains State Park in western Ontonagon County).
Rock units do not equal time units. Time units often cut across formation boundaries – such that portions of formations neighboring each other are time equivalent (see figure 1 below).
Figure 1: Model of three formations mapped in a cross-section. Formation A consists of conglomerate, Formation B of Sandstone and Formation C of Shale. The black dashed lines show the time lines superimposed on the cross section – showing portions of these rocks that are the same age based on fossils. Time line 1 cuts across all three formations – and was mapped out by finding the same fossil species in all three formations at that horizon. In this case, each formation was laid down in a specific portion of the depositional environment – here we could be seeing an alluvial fan deposit (Fm. A), the downstream river system (Fm. B), and a lake (Fm. C). – and these environments may have shifted location over time.
Another related term is “facies”. Facies are packages of rock that have distinct characteristics – a formation can be made up of one facies or more than one facies. Facies are not formally named. Facies are defined by the characteristics of a rock and can be used to infer depositional environments. The characteristics that go into a facies description include:
Rock Color
Rock Texture – grain size, grain shape, grain sorting
Composition – minerals that make up the grains
Cement – type of cement and degree of cementation
Sedimentary structures present
Fossils present
Nature of Layering – thickness, nature of contacts with other layers
Rock Type (i.e., Sandstone, Limestone, etc.)
Geometric relationship with neighboring facies
Many of these features can be used to interpret the depositional environment that produced the sedimentary rock that we are studying. Formations are mappable units whereas Facies are generally smaller scale. We will come back to formations and facies in lectures 6 and 7.
