Lecture Notes: Divergent Plate Boundaries

Introduction

• Earth is over four billion years old.
• Ocean basin rocks are much younger, forming less than 200 million years ago.
• Ocean basins are recycled starting with new oceanic lithosphere formation at divergent plate boundaries.

Divergent Plate Boundaries

• Plates move away from each other along divergent boundaries.
• Examples found in all major ocean basins.
• Characterized by:
  • Continuous oceanic ridges forming submarine mountain ranges.
  • Shallow earthquake activity.
  • Presence of the youngest oceanic lithosphere.

Observations

• Bathymetry: Depth and topography of ocean features.
• Divergent boundaries have:
  • Oceanic ridges higher than surrounding sea floor.
  • Volcanic activity sources.
  • Younger ocean floor near the ridge, older as you move away.

Formation of Ocean Basins

• Start without an ocean; examine a continent splitting apart.
• Rift valleys form from stretching and thinning of continental crust.
  • Marked by volcanoes or lava flows
  • Example: African continent splitting (e.g., Red Sea region)
• Rift valley lowers below sea level, forming a narrow ocean as it floods.

Historical Context

• Atlantic Ocean formed post-Pangaea breakup.
• Red Sea opened 30 million years ago, seawater filled 5 million years ago.
• Evidence of volcanism through newly formed volcanic islands.

Evolution of Ocean Basins

• Ocean basins can expand from hundreds to thousands of kilometers.
• Width depends on plate motion rates and duration of process.

Summary of Plate Separation Process

• Separation occurs in rift zones associated with oceanic ridges.
• Mantle convection drives the process.
• Rift zone faulting causes shallow earthquakes.
• Magma rises, forming new oceanic crust.
  • Surface magma forms basaltic lava.
  • Subsurface magma solidifies into plutonic rock like gabbro.
• High heat flow results in higher ridge elevations.
• Oldest crust near continents, youngest at ridges.

Learning Objectives

• Understand features and processes of divergent plate boundaries.
• Practice by drawing a labeled sketch.

Reflection

• Reflect on learning objectives and personal understanding.⭕️