Geology and Tectonics

Geology and Tectonics Study Notes

Required Reading

• Chapters 12 & 13 of Geosystems The Dynamic Planet

  • Topics to cover:

  • The Pace of Change

  • Geologic Time

  • Earth’s Structure and Internal Energy

  • The Geologic Cycle

  • Plate Tectonics

Geologic Time Scale

• Age of the Earth: 4.6 billion years (4.6 Ga)

  • (Ga = Giga = 10^9)

The Pace of Change

• Uniformitarianism:

  • The assumption that physical processes active today have been operating at the same pace and intensity as they did throughout geologic time.

• Catastrophism:

  • The assumption that rapid, dramatic changes occurred episodically throughout the geologic record.

Earth’s Interior Structure

• Seismic Discoveries:

  • Discovered in 1909 by a Croatian seismologist.

  • Seismic waves are diffracted through the mantle, indicating different layers within the Earth.

Continental Drift

• Alfred Wegener (1912):

  • Proposed that all landmasses migrate and published Origin of the Continents and Oceans.

  • Regarded as the father of plate tectonics, which he initially termed continental drift.

• Pangaea:

  • Wegener concluded that approximately 225 million years ago (m.y.a.), all landmasses formed one supercontinent called Pangaea, meaning “all Earth.”

Plate Tectonics

• Lithosphere:

  • This structure is divided into a number of plates that float over the mantle.

  • New crust formation, mountain building, and seismic activity (earthquakes) occur at the boundaries of these plates.

Testing the Plate Tectonics Model

• Key evidence includes:

  • Seafloor spreading

  • Magnetic reversals

  • Age of the seafloor

  • Volcanic activity

  • Hot spots

  • Earthquakes

Divergent Boundaries

• Occur in areas of seafloor spreading:

  • Upwelling material from the mantle forms new seafloor and lithospheric plates spread apart in a constructional process.

  • Examples include the Great Rift Valley of East Africa, where continental crust is rifting apart.

Magnetic Reversals

• Earth's magnetic polarity undergoes reversals at irregular intervals, with no fixed schedule.

Age of the Seafloor

• Involves studying earthquake and volcanic activity to understand the age distribution of oceanic crust.

Settings for Volcanic Activity

• Hot Spots: Locations where magma rises from the mantle to form volcanoes, typically not at plate boundaries.

• Example: Iceland, located at the Mid-Atlantic Ridge.

Types of Volcanoes

• Shield Volcanoes:

  • Formed at divergent plate boundaries and hotspots.

  • Characterized by broad, gently sloped layers due to low-viscosity lava that can flow over long distances.

  • Eruptions are generally effusive.

• Composite Volcanoes:

  • Occur at convergent plate boundaries or subduction zones.

  • Noted for their steep conic shape and high-silica, highly viscous lava.

  • Eruptions tend to be explosive and destructive.

Volcanic Hazards

• Statistics: Approximately 50 to 60 volcanoes erupt worldwide each year.

• Regions particularly vulnerable include Japan, Mexico, the Philippines, and Indonesia.

Crustal Thickness and Isostatic Adjustment

• Isostatic Adjustment:

  • Refers to the gravitational equilibrium state between the lithosphere and asthenosphere, whereby tectonic plates float based on their thickness and density.

Convergent Boundaries

• Characterized by crustal collision and subduction:

  • Continental and oceanic lithosphere meet, where compression occurs, and crust is lost into the mantle.

• Examples:

  • India and Asia collision (continental)

  • Deep trenches in the western Pacific Ocean (oceanic collisions).

Orogeny

• The term refers to “mountain generation” processes during tectonic plate interactions.

Types of Tectonic Convergence

• Oceanic-Continental Collision: Characterized by the subduction of the oceanic plate beneath a continental plate.

• Oceanic-Oceanic Collision: One oceanic plate subducts beneath another, forming island arcs.

• Continental-Continental Collision: Collision of two continental plates results in mountain building without subduction.

Continental Shields Definitions

• Craton: An old and stable part of the continental crust.

• Continental Shield: An area where a craton is exposed at the surface.

• Sedimentary Platform: A region with relatively flat or gently tilted sedimentary strata.

• Crystalline Basement: A region beneath the sedimentary platform or cover.

Stress and Strain in Rocks

• Rocks experience stress from tectonic forces, gravity, and overlying pressure.

• Three Types of Stress:

  • Tension Stress: Causes stretching strain.

  • Compression Stress: Causes shortening strain.

  • Shear Stress: Causes twisting (lateral) strain.

• Strain Responses:

  • Folding: Rocks bend due to stress.

  • Faulting: Rocks break and shift due to stress.

Folding

• Anticline: An arch-shaped upward fold where strata slope downward away from the center axis.

• Syncline: A trough-shaped downward fold; strata slope upward away from the center.

• Hinge: The horizontal line defining the fold's curvature.

Faulting

• Occurs when rocks on either side of a fracture shift relative to each other.

• Fault Types:

  • Normal Fault: Caused by tensional stress; rocks move apart.

  • Reverse Fault: Caused by compressional stress; rocks move toward each other.

  • Strike-Slip Fault: Caused by lateral shear stress; horizontal displacement.

Earthquake Introduction

• Approximately 1 million earthquakes occur annually.

• The energy released is described as the magnitude of the earthquake, while the intensity of shaking is referred to as ground motion.

Interplate and Intraplate Earthquakes

• Interplate Earthquakes: Occur at boundaries between two tectonic plates, leading to linear or curvilinear zones.

• Intraplate Earthquakes: Often large, occur within a single plate far from boundaries.

Earthquake Terminology

• Focus: The subsurface area along the fault plane where seismic waves are initiated.

• Epicenter: The surface area directly above the focus.

• Aftershock: Occurs after the main earthquake shock.

• Foreshock: Precedes the main shock.

Earthquake Magnitude Scales

• Richter Scale: Based on the amplitude of seismic waves related to energy released.

• Moment Magnitude Scale: More accurate; based on fault slippage, surface area ruptured, and material properties affected.

Earthquake Intensity and Frequency Table

The Rock Cycle

• Process: The slowest of Earth’s cyclic processes wherein rocks are recycled over millions of years, concentrating nonrenewable resources.

• Key processes in the rock cycle include:

  • Weathering: The breakdown of rocks.

  • Erosion and Transport: The movement of weathered material.

  • Lithification: The process when sediments compact and cement to form sedimentary rocks.

  • Metamorphism: Changes in the mineralogy and texture of rocks under heat and pressure.

  • Melting and Crystallization: The process leading to igneous rock formation.

Types of Rock

• Igneous Rocks: Form from cooled magma or lava.

• Sedimentary Rocks: Formed through the accumulation of sediments.

  • Clastic Sedimentary Rock: Compacted fragments of other rocks.

  • Example: Conglomerate, Aztec Sandstone.

  • Bio/Chemical Sedimentary Rock: Formed from biological activity or precipitation of minerals from water.

  • Examples: Limestone from calcium-rich shells; Gypsum from evaporation.

• Metamorphic Rocks: Result from the alteration of pre-existing rocks under temperature and pressure.

Geological Features and Structures

• Geological Transect: Transects across southern Ontario showing the geological layering.

• Imaginary Example: A hypothetical folded and faulted mountain range as it may have developed in the Grenville Orogeny in the Bancroft area.