Earth History: Ecological Succession and Plate Tectonics
Ecological Succession and Climax Communities
Ecological succession is the process by which certain populations are replaced by new ones to eventually form a climax community. A climax community occurs when a population reaches a stable composition and no longer undergoes significant changes.
Types of Succession
- Primary Succession: Occurs on virgin terrain where no previous community existed.
- Secondary Succession: Established on an existing biological community that has been eliminated by events such as fire or logging.
Fossils and the Process of Fossilization
Fossils are the remnants of living organisms or biological activity preserved in sedimentary rocks. Fossilization is the specific process whereby an organic body becomes a fossil.
Dating and Information
Fossils allow for dating (providing a specific chronological context). To provide useful information, organisms must have lived for a short geological period, possessed a wide distribution, and been abundant. Examples include:
- Trilobites: Primary (Paleozoic).
- Ammonites: Secondary (Mesozoic).
- Nummulites: Tertiary (Cenozoic).
Facies fossils are those that allow scientists to date the specific environment in which the sedimentary rock was formed.
The Three Major Geological Eras
The Paleozoic Era
Geology: Pangea fragmented, leaving a single mass in the hemisphere known as Gondwana, which later collided with Laurasia to form Pangea II. This era saw the Caledonian orogeny and the Hercynian orogeny (mountain building). Significant coal deposits were formed during this time.
Biology: A massive expansion in life forms occurred, known as the Cambrian explosion. There was an abundance of invertebrates; vertebrates appeared, including the first fish. Flying insects, plants, amphibians, and reptiles emerged. By the end of this period, 95% of life became extinct.
Climate: The climate was mild during the Ordovician, followed by six glaciations in the Silurian. Gondwana experienced temperate and tropical conditions, while cooling and glaciation occurred during the Carboniferous.
The Mesozoic Era
Geology: This era is marked by the emergence of dinosaurs and is divided into three stages: Triassic, Jurassic, and Cretaceous. Pangea II fragmented, resulting in the formation of modern oceans and continents.
Biology: Marine fauna diversified, and mollusks expanded. By the end of this era, turtles, snakes, lizards, and crocodiles were established. The first birds appeared in the Jurassic, followed by the first placental mammals and early seed plants. Approximately 75% of life disappeared at the end of this era.
Climate: The weather was hot and humid, with tropical conditions at low latitudes and temperate conditions at high latitudes. Temperatures were approximately 15ºC higher than they are today.
The Cenozoic Era
Geology: Divided into three periods, this era covers the last 65 million years. It features the continued separation of continents and ongoing Alpine folding due to collision and subduction processes.
Biology: Invertebrates remained abundant, particularly Nummulites. Birds, mammals, and plants continued to develop. The genus Australopithecus appeared, followed by the genus Homo, ending with Homo sapiens.
Climate: Initially, the warmer weather continued, but an ice age occurred 40 million years ago. The Quaternary period was marked by several glaciations.
Scientific Evidence for Plate Tectonics
- Paleontological: Similar animal and plant life found in geographically distinct locations.
- Geographic: The physiographic coincidence between the coasts of Africa and South America.
- Seismic: Earthquake distribution is not random; seismic lines define the edges of tectonic plates.
- Oceanic Topography: Features include mid-ocean ridges (dorsales) in central ocean areas, troughs (narrow, deep trenches), and rifts (central sulcus).
- Oceanic Composition: Magmatic rocks formed from mantle magma rising in the rifts.
- Oceanic Age: The oldest ocean floors are 180 million years old and are typically located near continents.
- Paleoclimate: Evidence varies depending on the historical direction of ice movement.
Fundamental Principles of Plate Tectonics
- The lithosphere is divided into large blocks called plates that cover the Earth’s surface and fit together like a puzzle.
- The majority of geological and biological activity (volcanoes, earthquakes) occurs at the edges of the plates, while the interiors are relatively stable.
- The oceanic lithosphere is thinner and denser than the continental lithosphere.
- Plates move and drag continents, interacting with each other. The separation of two plates generates new oceans, while collisions form mountain ridges.