Geology and Earth History Lecture Notes

Geological Time Scale and Supercontinents

Introduction to Geological Eras

  • The geological time scale is divided into various eras, with notable events occurring from 541 to 252 million years ago (Ma).

  • During the Cambrian period, the Earth began to break into four primary landmasses:

    1. Gondwana

    2. Siberia

    3. Baltica

    4. Laurentia (North America)

Formation and Collision of Supercontinents

  • Gondwana fragmented into smaller pieces, such as Avalonia, which later collided with other cratons.

  • Throughout the Devonian period, Laurensia was positioned near the equator and was predominantly covered by warm seas.

  • During the late Paleozoic, continents reassembled to form the Pangea supercontinent, which was characterized by significant geological features and major mountain-building events globally.

  • Notable mountain-building events in the geological history of North America include:

    • Taconic Orogeny: Involving volcanic island arc addition during the Ordovician.

    • Avalonian Orogeny: Continental collision impacting the Devonian.

    • Appalachian/Alleghanian/Hercynian Orogeny: Forming during the Carboniferous and Permian periods.

Characteristics of Pangea

  • The Pangea supercontinent was largely desert-like during its existence.

  • The Permian period experienced the largest known extinction event marked by the eruption of the Siberian Traps, leading to significant changes in the fossil record.

Volcanism

Variability in Volcanic Eruptions

  • Volcanic eruptions can be categorized based on their explosiveness, which varies significantly:

    • Magma Composition:

    • Mafic: Low viscosity leading to runny lava (e.g., basaltic eruptions).

    • Felsic: High viscosity resulting in explosive eruptions.

    • Volatile Content: Particularly water content, which reduces viscosity and influences eruption style.

    • Gas Content: High levels lead to increased expansion as magma approaches the Earth's surface, contributing to explosive eruptions.

Types of Volcanoes

  1. Shield Volcanoes:

    • Characterized by their basaltic composition, generally low relief but large in size.

  2. Stratovolcanoes:

    • Exhibit classic volcanic morphology, with steep slopes and smaller landforms.

  3. Calderas:

    • Formed when a volcano's crater collapses into an underlying magma chamber (e.g., Crater Lake, Yellowstone).

  4. Volcanic Cones and Fissures.

Volcanic Ejecta

  • Ejecta from volcanic eruptions, classified as Tephra, can vary in size:

    • Bombs: Large fragments.

    • Pumice and Ash: Smaller to very fine fragments.

  • Tephra can travel hundreds of kilometers, with significant historical records of such events.

Hazards Associated with Volcanic Activity

  • Volcanic eruptions pose various hazards, including:

    • Lahar: Volcano-driven mudslides.

    • Pyroclastic Flows: Hot ash flows that hug the ground, as observed in the case of Pompeii.

  • Most known historic eruptions are minor when compared to those in geological history.

Proterozoic Era (1,000 Ma - 541 Ma)

Early Supercontinents

  • Significant supercontinents during the Proterozoic include:

    • Rodinia (1,000 Ma).

    • Pannotia (~600 Ma).

  • This era is marked by the occurrence of the second 'Snowball Earth,' where glacial deposits were widespread worldwide in rocks.

Notable Geological Features and Events

  • The planet likely experienced extensive ice coverage for 10 - 20 Ma during the Proterozoic.

  • The Ediacaran Period (635 - 541 Ma) is noted for the emergence of the earliest multicellular life forms.

Archean Era (4.0 - 2.5 Ga)

Development of Continental Crust

  • The Archean era saw the formation of continental crust, with a limited variety of rock types such as:

    1. Gneiss

    2. Granite

    3. Greenstone

    4. Greywacke

    5. Chert ± Limestone

  • Small continental masses were situated between different subduction zones.

  • Oldest known rocks, such as the Acasta Gneiss, formed at this time (4.0 Ga).

  • The oldest sedimentary rocks from this era include greywackes formed around 3.7 Ga.

Biological Developments

  • The first biological structures known to form during this period were stromatolites (3.4 Ga), created by blue-green algae mats.

  • By approximately 2.7 Ga, the majority of microplates had merged into cratons, leading to the development of the North American Craton during the Hudsonian orogeny (1.9 – 1.6 Ga).

  • The formation of Banded Iron Formations (2.4 Ga) signifies the rise of free oxygen in the atmosphere.

  • First instances of eukaryotic algae populated the Earth by 1.0 Ga.

Stress and Strain in Geology

Changes in Rock Geometry

  • Deformation: Change in shape or volume of rock due to stress:

    • Stress: Force applied per unit area in a given direction.

    • Can be isotropic (same in all directions) or vary by direction, leading to either compressional or extensional stress.

    • Strain: Rock's response to stress, can be elastic or plastic.

  • Layers of sediment can undergo folding due to tectonic forces:

    • Types of Folds:

    • Anticlines

    • Synclines

    • Monoclines

  • Fold characteristics are influenced by:

    • Layer thickness

    • Layer spacing

    • Layer strength

Faulting of Rock Layers

  • Faults are discontinuities with observable offsets and include:

    • Normal Faults

    • Reverse Faults

    • Strike-Slip Faults

  • Understanding of hanging walls and foot walls relative to faults is crucial in geology:

    • The block above the tilted surface is the Hanging Wall, while the block below is the Foot Wall.

Historical Geology and the Principle of Uniformitarianism

Development of Geological Principles

  • The geologic past has been structured based on principles established by James Hutton (1785), particularly the concept of Uniformitarianism:

    • "The present is the key to the past" suggests that geological processes occurring today have shaped the Earth through similar processes in the past.

  • Various methods used to determine the ages of sediments include:

    1. Law of Superposition

    2. Lateral Continuity

    3. Cross-Cutting Relationships

    4. Fossil Assemblages

Unconformities in the Geological Record

  • Unconformities represent gaps or discontinuities in the sedimentary record:

    • Angular Unconformity

    • Disconformity

    • Non-Conformity

  • A notable example of Great Unconformity is recorded in North America.

Radiometric Dating Techniques

  • The age of rocks can also be determined through radiogenic isotopes that decay at known rates.

  • The relationship between parent and daughter isotopes allows scientists to calculate the age of geological materials.

  • Examples of radiometric dating systems include Carbon-14 dating, specifically used for organic materials. This technique involves measuring the concentration of isotopes to yield the age of the sample.