8.2 The geologic time scale and major events
4 min read•july 24, 2024
The geologic time scale divides Earth's 4.6-billion-year history into eons, eras, periods, epochs, and ages. These divisions mark major shifts in life and environments, from the fiery to the diverse eon we live in today.
Mass extinctions have shaped life's evolution, wiping out up to 95% of species in short spans. Five major extinctions reset Earth's ecosystems, creating opportunities for new dominant groups. Understanding these events helps us grasp the current biodiversity crisis.
Geologic Time Scale Structure
Divisions of geologic time scale
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- Eons largest units span billions of years divide Earth's history into four major phases
- Hadean Earth's fiery formation and early cooling 4.6 to 4.0 billion years ago
- Archean first continents and primitive life forms 4.0 to 2.5 billion years ago
- Proterozoic oxygen-rich atmosphere and complex cells 2.5 billion to 541 million years ago
- Phanerozoic diverse multicellular life 541 million years ago to present (dinosaurs, mammals)
- Eras subdivide eons represent major shifts in Earth's life and environments
- marine life explosion to first reptiles 541 to 252 million years ago
- age of reptiles and dinosaurs 252 to 66 million years ago
- rise of mammals and modern flora 66 million years ago to present
- Periods further subdivide eras marked by significant geological or biological events
- Mesozoic era periods Triassic and Cretaceous each with distinct fauna and flora
- Epochs subdivide periods reflect more subtle environmental or evolutionary changes
- Quaternary period epochs (ice ages) and (current warm period)
- Ages smallest units fine-scale divisions within epochs
- Pleistocene epoch ages Calabrian and Gelasian represent specific climate phases
Boundaries between major eras
- Precambrian-Paleozoic boundary (541 million years ago)
- rapid diversification of complex multicellular life (trilobites, mollusks)
- Marks transition from microbial-dominated world to macroscopic animal life
- Paleozoic-Mesozoic boundary (252 million years ago)
- wiped out ~95% of marine species and 70% of terrestrial vertebrates
- Set stage for dinosaur dominance in Mesozoic era
- Mesozoic-Cenozoic boundary (66 million years ago)
- caused by asteroid impact ended reign of non-avian dinosaurs
- Paved way for mammalian diversification and eventual human evolution
Major Events in Earth's History
Events of Phanerozoic periods
- (541-485 million years ago)
- Emergence of most animal phyla established basic body plans (arthropods, chordates)
- Development of hard body parts and exoskeletons enabled fossil preservation
- Ordovician (485-444 million years ago)
- Diversification of marine invertebrates (crinoids, bryozoans)
- First vertebrates appeared as jawless fish (ostracoderms)
- Silurian (444-419 million years ago)
- First land plants colonized shorelines (mosses, liverworts)
- Formation of created new marine ecosystems
- Devonian (419-359 million years ago)
- Age of Fishes saw diversification of jawed and bony fish (sharks, ray-finned fish)
- First amphibians and insects ventured onto land
- Carboniferous (359-299 million years ago)
- Extensive coal forests dominated by giant ferns and horsetails
- Diversification of amphibians and early reptiles in swampy environments
- Permian (299-252 million years ago)
- Formation of Pangaea supercontinent altered global climate and ocean circulation
- Diversification of reptiles including ancestors of mammals (synapsids)
- Triassic (252-201 million years ago)
- Recovery from Permian extinction led to new ecological niches
- Appearance of first dinosaurs and mammals as small, nocturnal creatures
- Jurassic (201-145 million years ago)
- Dominance of dinosaurs saw rise of long-necked sauropods and theropods
- First birds evolved from theropod dinosaurs (Archaeopteryx)
- Cretaceous (145-66 million years ago)
- Peak of dinosaur diversity included famous species (Tyrannosaurus, Triceratops)
- Diversification of flowering plants revolutionized terrestrial ecosystems
- Paleogene (66-23 million years ago)
- Rapid evolution of mammals filled niches left by extinct dinosaurs
- Formation of modern continents through continued
- Neogene (23-2.6 million years ago)
- Evolution of hominids in Africa led to human ancestors
- Global cooling and expansion of grasslands shaped modern mammal fauna
- Quaternary (2.6 million years ago to present)
- Ice ages and glacial cycles dramatically reshaped landscapes
- Evolution and spread of modern humans led to current Anthropocene era
Mass extinctions in Earth's history
- Mass extinctions rapid loss of significant percentage of plant and animal species within short geological timeframe
- Five major mass extinctions recognized in Earth's history
- (444 million years ago) eliminated ~85% of marine species
- (375-360 million years ago) wiped out ~75% of species over extended period
- (252 million years ago) largest known extinction eliminated ~95% of marine species
- (201 million years ago) cleared way for dinosaur dominance
- (66 million years ago) famously ended non-avian dinosaur reign
- Significance of mass extinctions
- Dramatic shifts in ecosystems and biodiversity reset evolutionary trajectories
- Creation of evolutionary opportunities for surviving species led to new dominant groups
- Alteration of global biogeochemical cycles changed Earth's carbon and nutrient flows
- Potential causes of mass extinctions
- Asteroid or comet impacts (Chicxulub crater linked to End-Cretaceous event)
- Volcanic eruptions and flood basalt events released massive amounts of greenhouse gases
- Rapid climate change warming or cooling stressed ecosystems beyond adaptability
- Changes in atmospheric or oceanic composition altered habitats (ocean acidification)
- Sea level fluctuations exposed or flooded crucial habitats
- Modern relevance
- Study of past extinctions informs understanding of current biodiversity crisis and potential outcomes
- Insights into ecosystem resilience and recovery guide conservation efforts
Key Terms to Review (26)
Appearance of Dinosaurs: The appearance of dinosaurs refers to the emergence of these diverse reptiles during the Mesozoic Era, particularly in the Triassic period, around 230 million years ago. This key event marks a significant shift in Earth's biological landscape, as dinosaurs became the dominant terrestrial vertebrates, leading to a wide variety of forms and adaptations that would define the era.
Cambrian: The Cambrian is a geological period that occurred approximately 541 to 485 million years ago, marking the first period of the Paleozoic Era. This period is notable for the 'Cambrian Explosion,' a rapid diversification of life forms in the oceans, which led to the emergence of many major groups of animals that still exist today.
Cambrian Explosion: The Cambrian Explosion refers to a significant evolutionary event that occurred approximately 541 million years ago, characterized by a rapid diversification of life forms in Earth's oceans. During this period, most major animal phyla first appeared in the fossil record, leading to a dramatic increase in biodiversity. This event marks the beginning of the Paleozoic Era and is crucial for understanding the evolution of complex life on Earth.
Cenozoic: The Cenozoic is the most recent geological era, spanning from about 66 million years ago to the present day. This era is characterized by significant changes in the Earth's climate, biodiversity, and landforms, leading to the dominance of mammals and birds. The Cenozoic is divided into three periods: the Paleogene, Neogene, and Quaternary, which encompass major evolutionary and geological events.
Coral reefs: Coral reefs are underwater structures made up of calcium carbonate produced by living organisms, primarily corals. They are one of the most diverse ecosystems on Earth, supporting a wide variety of marine life and playing a crucial role in coastal protection and marine biodiversity. Their formation and evolution are closely linked to major geological events over millions of years.
Cretaceous-Paleogene extinction event: The Cretaceous-Paleogene extinction event was a major global extinction event that occurred around 66 million years ago, marking the end of the Cretaceous period and the beginning of the Paleogene period. This event is most famously associated with the mass extinction of dinosaurs, along with numerous other species, leading to significant changes in Earth's biodiversity and ecosystems.
End-cretaceous: The end-cretaceous refers to the significant geological boundary that marks the end of the Cretaceous Period, around 66 million years ago. This boundary is most famous for the mass extinction event that led to the demise of approximately 75% of all species on Earth, including the non-avian dinosaurs. This event is a critical marker in the geologic time scale, representing major changes in Earth's biodiversity and climate.
End-Ordovician: The end-Ordovician refers to a significant geological time marker that denotes the end of the Ordovician Period, approximately 445 million years ago, characterized by one of the largest mass extinction events in Earth's history. This event led to the loss of around 85% of marine species, reshaping marine biodiversity and ecosystems dramatically. It is often associated with rapid climatic changes, including glaciation, which impacted sea levels and habitats.
End-Permian: The end-Permian, also known as the Permian-Triassic extinction event, marks the boundary between the Permian and Triassic geological periods around 252 million years ago. It is recognized as the most severe mass extinction in Earth’s history, resulting in the loss of approximately 90% of marine species and 70% of terrestrial vertebrate species. This event reshaped ecosystems and set the stage for the rise of dinosaurs in the following Triassic period.
End-Triassic: The end-Triassic refers to a significant geologic boundary that marks the transition from the Triassic Period to the Jurassic Period, approximately 201 million years ago. This boundary is characterized by a major mass extinction event that led to the loss of many species, particularly marine reptiles, and significant changes in terrestrial ecosystems. Understanding this period helps highlight critical shifts in Earth's biodiversity and climate that influenced subsequent geological time.
Fossil record: The fossil record refers to the preserved remains and traces of ancient life, including bones, shells, imprints, and traces like footprints, found in sedimentary rock layers. It serves as a crucial tool for understanding the history of life on Earth, revealing patterns of evolution, extinction events, and the relationships between different organisms over geological time.
Glaciation: Glaciation refers to the process during which large areas of the Earth's surface become covered by glaciers and ice sheets, significantly shaping the landscape and influencing climate. This process has occurred repeatedly throughout Earth's history, marked by periods of extensive ice coverage followed by warmer interglacial periods. Glaciation not only affects landforms and ecosystems but also plays a crucial role in understanding the geologic time scale and past climatic changes.
Hadean: The Hadean is the earliest eon in Earth's history, spanning from the formation of the planet about 4.6 billion years ago to roughly 4 billion years ago. It is characterized by the planet's initial formation, extreme volcanic activity, and the development of a primordial crust. This eon lays the foundation for our understanding of Earth's formation and subsequent geological developments.
Holocene: The Holocene is the current geological epoch that began approximately 11,700 years ago after the last major ice age, marking a significant period of climate stability and the development of human civilizations. It follows the Pleistocene epoch and is characterized by the warming of the Earth, leading to rising sea levels and the flourishing of flora and fauna, which greatly influenced human settlement and activities.
Jurassic: The Jurassic is a geologic period that lasted from about 201 million to 145 million years ago, forming the middle period of the Mesozoic Era. This time is known for significant geological and biological changes, including the dominance of dinosaurs and the appearance of the first birds and mammals.
Late Devonian: The Late Devonian is a specific time interval in the Devonian period, occurring approximately 382 to 359 million years ago, characterized by significant geological, climatic, and biological changes. This period witnessed the diversification of fish, the first appearance of amphibians, and major changes in marine ecosystems, which are pivotal for understanding the development of life on Earth during the Paleozoic Era.
Mass extinction: Mass extinction refers to a significant and rapid decrease in the diversity and abundance of life on Earth, marked by the loss of a large percentage of species in a relatively short geological time frame. These events are often associated with dramatic changes in environmental conditions, such as volcanic eruptions, asteroid impacts, or drastic climate shifts, which can disrupt ecosystems and lead to widespread extinction. Understanding mass extinctions is crucial as they have shaped the evolution of life and influenced the geologic time scale.
Mesozoic: The Mesozoic Era is a geological time frame that lasted from about 252 to 66 million years ago, often called the 'Age of Reptiles' due to the dominance of dinosaurs during this period. This era is divided into three major periods: the Triassic, Jurassic, and Cretaceous, each marked by significant evolutionary advancements and environmental changes that shaped the Earth's biosphere.
Origin of life: The origin of life refers to the process by which living organisms first emerged on Earth, marking a significant milestone in the history of our planet. This event is crucial to understanding the geologic time scale, as it sets the stage for the evolution of complex life forms and major biological developments that followed. The origin of life intertwines with various geological events, shaping ecosystems and influencing Earth's atmospheric conditions.
Paleozoic: The Paleozoic Era is a major division of the geologic time scale that spans from approximately 541 to 252 million years ago. It is characterized by significant geological, climatic, and biological developments, including the emergence of complex life forms and the first appearance of vertebrates. This era is crucial for understanding the evolution of life on Earth and includes several major extinction events.
Permian-Triassic Extinction Event: The Permian-Triassic extinction event, occurring around 252 million years ago, is recognized as the most severe mass extinction in Earth's history, eliminating approximately 90-96% of marine species and about 70% of terrestrial vertebrate species. This catastrophic event marked the boundary between the Permian and Triassic periods, leading to significant ecological and evolutionary changes that shaped the future of life on Earth.
Phanerozoic: The Phanerozoic is the current geological eon that began approximately 541 million years ago and continues to the present day. This eon is characterized by an abundance of fossil evidence, showcasing the development and evolution of complex life forms, including plants, animals, and fungi, leading to significant biological diversity. The Phanerozoic is divided into three main eras: the Paleozoic, Mesozoic, and Cenozoic, each marked by major events in Earth's history.
Plate Tectonics: Plate tectonics is the scientific theory that describes the large-scale movements of Earth's lithosphere, which is broken into tectonic plates that float on the semi-fluid asthenosphere beneath. This theory explains how these plates interact at their boundaries, leading to geological phenomena such as earthquakes, volcanic activity, and the formation of mountains and oceanic trenches.
Pleistocene: The Pleistocene is a geological epoch that lasted from about 2.6 million years ago to approximately 11,700 years ago, characterized by repeated glaciations and significant changes in climate and sea levels. This period saw the development of major ice sheets in the Northern Hemisphere and is marked by the emergence and evolution of various species, including early humans. The Pleistocene's glacial and interglacial cycles shaped much of today's landscape and influenced ecological patterns.
Radiometric dating: Radiometric dating is a method used to determine the age of rocks, fossils, and other geological materials by measuring the decay of radioactive isotopes within them. This technique connects to various processes in the rock cycle, as it helps understand when certain rocks formed or events occurred, providing a timeline for Earth's history. The method is essential for absolute dating, allowing geologists to quantify the age of geological formations and relate them to the geologic time scale, which outlines significant events in Earth's history.
Stratigraphy: Stratigraphy is the branch of geology that studies rock layers (strata) and layering (stratification). It plays a critical role in understanding Earth's history, as it helps scientists interpret the sequence of geological events and changes over time, linking it to geologic processes and the rock cycle, as well as the geologic time scale and major events in Earth's history.