1 Geological Time scale and Geological Processes

Gurmeet Kaur and Shashi Kad

Module 1: Geological time scale and geologic processes

Objectives:

1. Geological Time Scale

1.1 Why do we need a geological time scale and how the geological time scale evolved?

1.2 Understanding the modern geological time scale and the basis of its divisions.

2. Geological Processes

2.1. Geological processes and their role in understanding Earth

2.2. Concept of Geological processes vis-a-vis Geological Time Scale

2.3. Internal and External Geological Processes

1. Geological Time Scale

1.1 Why do we need a Geological time scale and how the Geological Time Scale evolved. Introduction: Our planet Earth, the only known habitable planet in the known Universe, continues to be a dynamic system. Just as we write and the reader reads, the internal and external physical, biological and chemical processes are continuously reshaping the Earth. Geological processes in general are very slow that we can hardly notice during our life span while some are abrupt and catastrophic. What we observe today on the surface of the Earth is a snapshot resulting from complex  interplay of many internal processes, and by understanding these processes; the Geologists have extrapolated the information to reveal the story of the Earth from its inception.

Earth as a Jigsaw: How do we understand the story of the Earth? How do we fit these small pieces of this complex jigsaw puzzle to spatially and temporally validate our understanding of the processes today? The reader may appreciate that even to tell the story of the world for the last one hundred years; historians would often refer to the global events such as Pre-World War I, or post World War II or first oil crisis or second oil crisis. Some others may be familiar with the Facebook timeline where a person is choosing to tell his/her story in a reverse chronological way. Similarly, to tell the story of the Earth, geologists look for global events that can have significant relevance and presence across the planet.

Early work on Geological Time Scale: The initial studies that contributed to the understanding of the geological processes were limited in scope to a region or a field; to correlate and construct the story of the Earth has been a very formidable and complex task. It may be important to mention that from the time when interest in this subject appeared the age of the Earth was not known and information on natural processes was limited to unverified hypotheses. The early efforts to construct this story started in 1600’s.

One of the earliest attempts to calculate the age of the Earth was 6000 years by Archbishop James Ussher mentioned in Genesis (Annals of the World, A.D. 1658)

Nicolaus Steno, a 17th century Danish scientist, pioneered the field of stratigraphy (study of strata). Contemporary geoscientists to date rely on the timeless principles viz., The principle of original horizontality and the principle of superposition, proposed by Steno to comprehend sedimentary rocks. William Smith, British surveyor, developed the concept of faunal succession. Smith’s Principle of faunal succession on the basis of characteristic fossils in different strata helps in identifying strata of the same age in different outcrops at different locations.

Nicolaus Steno in 1667 found shark tooth while studying the sedimentary strata in Tuscany, what were then called “tongue stones” (Figure 1.1). The concept promoted by Nicolaus that these were remains of sharks left in the rock was not accepted at that time. William Smith, worked on understanding the distinct rock types with distinct fossils, that helped him create a sequence that he was able to validate in other parts of England as well. This stratigraphic sequencing was one of the earliest attempts at creating a geological order for assigning relative ages to the rock formations.

Charles Darwin’s assumptions on age of the Earth was 6000 years, a common belief during that time, however based on some geology and his own work on natural selection he thought that the age of Earth could be several hundred million years, which was not what Lord Kelvin believed. Lord Kelvin’s work based on thermodynamics and Earth’s thermal gradient predicted a much younger age.

There were multiple geological evidence pouring in to support a backward revision in the age of the Earth.

At this stage a better understanding on deposition of sedimentary rocks was developed. While observing the then rate of deposition, estimates on time it may have taken to deposit a thick rock succession was made. As an example, the age of thickest sedimentary sequence such as Grand Canyon was placed at 100 Ma, which supported the theory that Earth is much older than the other estimates that were being made.

To simplify our understanding of the story of the Earth, geologists made attempts at the widest divisions of rock strata into eons, periods and epochs. One of the big events that kept shifting the boundary of these divisions was the presence and absence of life, and of simple and complex life.

Relative ages: Smith’s and Steno’s principles were applied to outcrops across the globe in the nineteenth century. Similar fossils from formations in different parts of the world were found. Fossil assemblages also matched up for these formations along with other cross cutting relationships. At the end of the century, the geologists could put a time scale together which was the first geologic time scale based on relative ages.

Absolute age: The story of absolute ages started when the radiometric dating began. Rutherford in 1905 deduced that radioactivity could be used to give an accurate and absolute age for a rock. This started the trend of isotopic dating that could provide absolute ages. However, till about 1953, the age of the Earth was nowhere close to what we know now.

Claire Patterson, American geochemist, calculated age of the Earth to be 4.55 billion years in the year 1956 and this age of earth calculated on the basis of geochronology has remained largely unchanged since then.

1.2. Understanding the Modern Geological Time Scale and the basis of its subdivisions:

Modern Geological Time Scale is a result of many centuries of work put together by geologists that includes understanding stratigraphic successions in the field and studying correlations to comprehend relative ages, and determining absolute ages using radiometric dating.

Intervals of Geologic Time

What is fundamental to the primary divisions of the Geological Time Scale is the presence or absence of fossils and/or distinct set of fossils. The end of a division and start of another demarcates a distinct presence or absence of a particular set of fossils. Eon is the longest subdivision of the Geologic time and two or more Era’s form an Eon. Period is the basic unit of Geologic time in which a typical set of rock unit is made, and is usually named after a locality or the distinguishing characteristics of the rocks reported. Devonian, as an example, is named after a locality in England and Carboniferous period for coal bearing sedimentary rocks. However, Palaeogene and Neogene are an exception and mean old and new respectively.

International Commission on Stratigraphy v 2016/04 (Figure 1.2) is an agreed upon and elaborate description of the Modern Geological Time Scale, which offers numerical age for each Eon, Era, Period, Epoch and Stage. This represents no finality as numerical ages are subject to further refinement etc.

Primary Divisions of the Geological Time Scale: To understand the Geological Time Scale, one can start with its broadest divisions called eons. There are four eons: Hadean, Archean, Proterozoic and Phanerozoic. The Geological Time Scale in its tabular form is presented in a reverse chronological scheme, with the most recent at the top. Hadean (4600 to 4000 Ma) is the oldest eon and is at the base while Phanerozoic the youngest at the top. Hadean: The term ‘Hadean’ comes from the Greek God of the underworld. This represents a time with absolutely no record of rocks present. One can say that Hadean eon represents those 600 million years of young earth about which we have no records available.

Archean: Archean eon (4000 to 2500 Ma) was like a new chapter in Earth’s history as the Earth is thought to have cooled off sufficiently to allow formation of continents during this eon. Readers may note during the earlier versions of Geological Time Scale that Archean was considered as the oldest (Greek meaning ‘beginning). To envision the conditions during Archean is to see the planet as a time of high volcanic activity and so the surviving rocks of the eon are metamorphic and igneous. The readers must also note that owing to the long-time gap between the beginning and end of the period, there must have been considerable variation in the conditions of the Earth at the beginning of the eon and at its end. Archean is now divided into four era’s, Eo-Archean, Palaeo-Archean, Meso-Archean and Neo-Archean. Eo-Archean (4000 to 3600 Ma) is characterised by earliest crust and greenstone belts in certain shield areas. Palaeo-Archean is younger than Eo-Archean (3600 Ma to 3200 Ma), where the first evidence of fossils ever, dated at 3500 Ma and called stromatolites, were found in Western Australia.

The readers may note that newer scientific discoveries would continue to refine and alter our current understanding of the Geological Time Scale. One such recent discovery of stromatolites from Greenland has taken the stromatolite appearance to 3700 Ma (2016), in the Eo-Archean. Meso-Archean (3600 to 2800 Ma) is characterised by Banded Iron Formations, when oxygen levels on the planet were still low. Some of the earliest super continents formed during this time. Stromatolites diversified during Meso-Archean. Neo-Archean (2800 Ma to 2500 Ma) is when continents started to shape up and continued to fragment and assemble through various supercontinent cycles to acquire present day configuration.

Proterozoic, in Greek means “early life” and the Eon extends from 2500 Ma to 541 Ma; subdivided into three eras, namely Palaeo-Proterozoic (2500 to 1600), Meso-Proterozoic (1600-1000 Ma) and Neo-Proterozoic (1000-541).

Earth’s atmosphere changed substantially in the Proterozoic, with increased oxygen levels, referred to as the Great Oxidation Event Although oxygen did show up in the atmosphere in Archean eon, it is only now for the first time that oxygen build up happened to a certain degree that was conducive for many life processes to follow.

Oxygen was a by-product of photosynthesis by cyanobacteria. In its initial stages, whatever oxygen was produced was taken by dissolved iron or organic matter. At one point these sinks became saturated when no more oxygen could be taken up, at which point it started getting released into the atmosphere. It is also argued that free oxygen in the atmosphere may have been detrimental to the anaerobic bacteria of that time, so in that sense, while it paved way for oxygen that is so required for life, it may have caused the first mass extinction. The other thing that this oxygen did was convert methane into carbon dioxide (less potent greenhouse gas and water) which started the Huronian Glaciation, one of the earliest glaciations events in the history of the Earth that led to snowball earth.

In this way, Banded Iron Formations and later red beds coloured by haematite, imply an increase in oxygen levels. The other most significant events in Proterozoic are the formation of the supercontinents and further evolution of life, while stromatolites proliferated, eukaryotes or complex organisms showed up for the first time on the Earth.

Precambrian- this term was used for everything before Cambrian, and if everyone is wondering what is the most often used name Precambrian refers to, it can be called a super eon that represents ‘stuff before Cambrian” and combines Hadean, Archean and Proterozoic.

Phanerozoic is the current geologic eon. Phanerozoic in Greek means visible life, and it represents the last 541 Ma on the Earth, from the start of the hard-shelled organisms to the dominance of humans. The 541 Ma years have the most records in terms of life on the Earth and other expressions of geological processes.

Phanerozoic eon is divided into Palaeozoic, Mesozoic and Cenozoic eras. Palaeozoic started with the appearance of first complex organisms and it is divided into Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. Cambrian (541 to 485 Ma) which symbolizes the ‘explosion of life’ is known for proliferation of trilobites. Ordovician (485 t0 440 Ma) was the time of primitive fishes. Tectonically, Gondwana the large southern amalgam of continental blocks was at the south pole in the Ordovician period. Glaciation in this period led to mass extinctions. Silurian (440 to 415 Ma) is the age when vascular plants as well as arthropods, fish, both jawed and jawless, became widespread. Devonian (415 Ma to 360 Ma) can be called the ‘the age of fishes’ first tetrapods, trees, seeds and amphibian evolution. At the end of Devonian another mass extinction happened. Carboniferous (360 and 300 Ma) started with high temperature conditions, trees created carbon that got buried as coal deposits. On the other hand amphibians were dominant. Later carboniferous underwent a period of glaciation. Permian (300 to 250 Ma) is the time when all continents came

together  and  formed  Pangaea  supercontinent  and  the  ocean  surrounded  it  is  called  Panthalassa. Conifers also evolved during this period and this period ended with mass extinctions.

Mesozoic Era (252 Ma to 66 Ma): This is called the age of the dinosaurs and further subdivided into Triassic, Jurassic and Cretaceous periods. Pangaea fragmented during Triassic (250 million to 200 Ma) and Tethys Sea opened up, modern corals and dinosaurs, such as Coleophysis appeared. There was extinction at the start and end of this period.

In the Jurassic (200 Ma to145 Ma) period, primary vertebrates in sea were fish and marine reptiles and ichthyosaurs, plesiosaurs, pliosaurs, crocodiles and turtles proliferated. In Cretaceous (145 to 66 Ma) period, first flowering plants evolved and sauropods became extinct along with the ichthyosaurs. Deccan volcanism in India and a large meteorite impact in Chicxulub marked the end of Cretaceous period (Cretaceous – Paleogene boundary, previously known as the K-T boundary)

Cenozoic Era is the age of mammals, and divided into Palaeogene, Neogene and Quaternary. Palaeogene (66 Ma to 23 Ma) comprises Palaeocene, Eocene and Oligocene epochs. India and Asia were in the position of collision, all continents started to drift towards their present position. Australian plate separated from Antarctica during this time. One of the significant events in this time was initiation of Antarctic Circumpolar current which cooled the Earth. There is an exceptionally high temperature period in between, called Palaeo-eocene thermal maxima. This period saw the spread of cetaceans, primates and birds.

Neogene (23 Ma to 2.58 Ma) comprises Miocene and Pliocene epochs and during this period, mammals and birds continued to evolve. The movement of plates resulted in North and South America getting connected at the Isthmus of Panama in the Pliocene epoch. The impact of this was that warm pacific waters were transferred to Pacific only through the Gulf Stream. This led to global cooling and to several subsequent glaciation episodes. This was also the time when grasses spread which was significant for the evolution of many grazing herbivores that we see today. This is also the time when first hominids evolved in Africa and moved to Eurasia.

Quaternary (2.58 Ma to the present) is the time when there is no major change, glaciation episodes were periodic in this time period. Many animals such as saber toothed, wooly mammoths and mastodons became extinct. Quaternary subdivisions are based on biostratigraphy and the two epochs are Pleistocene (2.58 Ma to 0.0117 Ma) and Holocene that begun at 0.0117 Ma to Recent (11700 years ago or 9700 B.C.).

It has been scientifically established that the Earth was born between 4.6-4.5 billion years ago. The current estimated age of the Earth is based on rocks brought back from the Moon (4.4 Ga) and meteorites (4.6 Ga) that are thought to be good representatives of the primary and undifferentiated chemical systems, unlike the Earth where rocks have been recycled several times. This data suggests that the present chemical composition of the crust must have evolved for more than 4.5 Ga.

Oldest dated terrestrial minerals and rocks on the Earth: To date, the detrital zircons from a conglomerate horizon in Jack Hills, Western Australia are the oldest dated terrestrial minerals (4,363 ± 20 Ma).

The 4.03–3.94 Ga granitoids and diorites from the Acasta Gneiss Complex, Slave Craton, Canada is the so far, oldest rock.

2. Geological Processes

Geological processes form mountains in place of oceans and vice versa and how earth scientists unearth the Earth’s story!

“They look at mud and see mountains, in mountains oceans, in oceans mountains to be. They go up to some rock and figure out a story, another rock, another story, and as the stories compile through time they connect-and long case histories are constructed and written from interpreted patterns of data and clues. This is detective work on a scale unimaginable to most detectives, with the notable exception of Sherlock Holmes”

John McPhee, Annals of the Former World

.1 Geological processes and their role in understanding Earth

Geological features that adorn the Earth’s surface, both on continents and oceans, in the present form are result of dynamic geological processes that have been operative inside the Earth since the birth of this unique and the only life sustaining planet of the Solar System, almost since last 4.56 billion years. Geological features and events on the Earth are the outcome of a complex interplay of internal and external processes; the former is driven by the internal heat and the latter by the solar heat and atmosphere.

James Hutton, Scottish geologist, gave the concept of ‘Principle of uniformitarianism’.

Principle of uniformitarianism: The geological processes we see in action on Earth today have analogy to the geological processes which operated throughout the geologic past. This important concept is known as the ‘principle of uniformitarianism’.

Charles Lyell, British geologist, in his book ‘Principles of Geology, being an attempt to explain the former changes of the Earth’s surface, by reference to causes now in operation’ gave the famous punch-line “The present is the key to the past.”

scale. Geoscientists have built up an understanding of the present Earth and how it has shaped up and evolved through geologic time.

Lithosphere: Rigid outermost part of the Earth comprising the crust and upper mantle, resting on the weak and ductile asthenosphere.

Plate tectonics: Theory which got acclaim during 1960’s on the basis of sea floor spreading. The Earth’s surface has been divided into large and small lithospheric plates (Figure 2.1). The plates are in motion and their relative motion with respect to each other determines the type of boundary: convergent, divergent or transform.

Continental drift: Alfred Wegener, a German meteorologist and explorer, formally proposed the breakup and drifting of continents. His theory was based on his observations of rock types, coastline fit, geological structures and fossils on both the sides of the Atlantic Ocean. In his book entitled ‘The Origin of Continents and Oceans’ he proposed that the landmasses of the Earth almost fit together like a jigsaw puzzle (Figure 2.2).

2.2.   Concept of Geological processes vis-à-vis Geological Time Scale

The concept of geological time as discussed in detail in the above section is the greatest geologic discovery that has revolutionized the way earth scientists look at the dynamic Earth in the contemporary times. Pioneering geological field and laboratory observations of scientists such as James Hutton, Charles Lyell, Nicolas Steno, William Smith, Alexander von Humboldt, Alfred Wegner, Claire Patterson to name a few, logically convinced us that the present Erath is an outcome of the past geological processes which operated on multiple time scales since 4.56 billion years. The temporal variation is on a large-to- small scale i.e. million of years to few years to a fraction of seconds! These geological processes are still operative and will continue to operate in future as well. Thus the face of Earth is ever changing and evolving vis-à-vis the geological time

The principle of uniformitarianism does not imply that all geologic phenomena proceed at the same gradual pace but it gives us clues to understand the basic idea how the Earth has evolved since its inception. Geologic processes can occur in a matter of seconds (volcanic eruptions e.g. Colima volcano, earthquakes e.g. Ring of fire), time span of human life i.e. few years (movement of plates e.g. Rising of Himalaya up to 5 mm every year, migration of sand dunes etc.) or take millions of years (Plate tectonics: Assembly and breaking up of supercontinents e.g. formation of the Himalaya; formation of sedimentary rocks; Figure 2.3 and 2.4) to shape the Earth’s exterior. Let us consider examples of two geological processes operative on short and long time scales. The example of a short time scale is of drop in altitude of a segment of the Himalaya post-Nepal earthquake, which occurred on 25th April, 2015. “The primary stretch that had its height dropped is a 80-100 km stretch of the Langtang Himal (to the northwest of the capital, Kathmandu),” said (Richard Briggs, a research geologist with the United States Geological Survey (USGS) quoted in ‘Himalayan drop after Nepal quake’ published by BBC world service on 8th May, 2015 in the Science and Environment section).

The example of a geological process on longer time scale is the formation of the Himalaya (Figure 2.4). John McPhee has beautifully summarised the formation of Himalaya in place of the Tethys ocean by movement of plates (a geological process on a large time scale) and how once the marine fossilferrous limestone which formed in the Tethys ocean now constitutes the summit of Mount Everest (Box below)!

Figure 2.3. Formation and breakup of supercontinents during the Geological time scale.

(Source: Precambrian research journal)

The best way to exemplify John McPhee’s quote is to look at the summit of Mt. Everest the highest peak of the world, which is made up of The Qomolangma Formation (Ordovician-limestone, 488-443 million years old) in which many marine fossils such as trilobites, crinoids, and ostracods are entombed (Figure 2.5).

2.3.         Internal and External Geological Processes

The internal and external geological processes are instrumental in keeping the Earth dynamic and geologically vibrant. These geological processes manifest on the surface in the form of volcanoes, earthquakes, drifting, movement of lithospheric plates, formation of supercontinents and their dispersal, sedimentary rocks, ore deposits, etc. (Figure 2.3). To understand these geological processes geologists track the information concealed in various rocks. There are limitations to the accumulation of this data because of the dynamic nature of the Earth. It is not possible to retrieve all the records preserved in the rocks in the past, and records are obliterated due to weathering and erosion of rocks and subduction of earlier rocks both in oceanic and continental environments. Almost all oceanic crust older than 180 million years has been subducted back into the mantle. To understand the geological story of Earth it is important to have an understanding of the pace of these processes and their driving mechanisms.

Sources of heat for Internal processes

There are two principal sources of heat responsible for the internal geological processes: 1. Primordial heat – this heat has been escaping the Earth since its inception, resulting from the collision of the planetesimals and the later accretion and gravitational differentiation which led to three distinct layers of Earth viz. crust, mantle and core. 2. Radioactive elements – the heat generated by decay of radioactive elements contribute significantly to the Earth’s internal heat engine. The internal heat is responsible for generation of melts and convection currents in the mantle which are important in terms of large scale magmatism and plate motions (Figure 2.6).

Geological features as a result of internal geological process

Internal geological processes like plate convergence, plate divergence and volcanic eruptions tend to lift and modify Earth’s relief. The plate convergence/collision has resulted in mountain chains; the mighty Himalaya was created when the Indian and the Eurasian continental plates collided some 40 million years ago (Figure 2.4). All along the western coast of South America the plate convergence in the form of collision of oceanic and continental plates resulted in the formation of Andes Mountains. Volcanic eruptions have led to the formation of Deccan Plateau during the Cretaceous period when Indian plate passed over Reunion Plume during its northward journey. The divergence of lithospheric plates in the oceanic environment results in the formation of mammoth network of mid ocean ridges (Example: The Mid Atlantic Ridge, East Pacific Rise, 90 East Ridge, etc.). East Pacific Rise and Mid Atlantic Ridge are examples of relief on the ocean floors. Large scale magmatism in form of extensive and regional igneous provinces could be explained by intraplate magmatism in the rift environments and mantle plume activities (Example: the East African Rift system). The convergence of plates/subduction is responsible for opening of back arc basins giving rise to opening of seas (Example: The Japan sea). The internal geological processes significantly build the Earth’s relief.

Sources of heat for external processes

Solar energy is the most important source of heat for all external geological processes operating on the Earth’s surface. It creates variations in temperature and pressure in the atmosphere that generates wind, including the monsoon. These processes also complete the hydrologic cycle wherein the water changes its domain and forms; from the oceans, rivers lakes and even glaciers it gets evaporated and is reintroduced into the Earth system through various forms of precipitation such as rain and snow. The activity by rain, wind, snow, glaciers, rivers and oceans promote geological processes on the Earth’s surface.

Geological features as a result of external geological processes

External geological processes, primarily driven by the solar energy constantly shape the Earth’s surface at variable rates. External agents such as water, ice, wind and also the human beings are responsible for the changing face of the Earth and modify the existing relief. Highlands are eroded by various external geological agents and the eroded materials are transferred/transported to lowlands and thus result in infilling of the basins and depressions on Earth’s surface leading to the formation of sedimentary rocks. These activities over millions of years led to the formation of landforms such as deep valleys, deltas, sand bars, river terraces, beaches etc. Different places on the Earth experience disparate rates of external geological processes. The factors which control the formation of landforms are the climate, location (latitude and longitude), rate of weathering, erosion, transportation, deposition, prevalent lithology and relief of a given place. The processes may operate for millions of years before changing landforms. Sedimentary rocks, particularly those enclosing fossils are important in deciphering relative ages of rocks. These sedimentary rocks also preserve the earliest life records and are very crucial in understanding the Earth’s palaeo-environments and evolution of diverse life forms.