L18 - Oceanic Lithosphere

Ocean Floor Overview

  • Ocean Floor Characteristics
    • Covers 70% of Earth's surface.
    • Oceanic lithosphere created at mid-ocean ridges (MORs) and recycled into the mantle at subduction zones.
    • Age typically less than 200 million years (Ma).

Studying the Ocean Floor

  • Methods of Exploration:
    1. Submersibles:
    • Remotely operated vehicles (ROVs) and manned submersibles used to sample the ocean floor.
    • Example: Deepsea Challenger.
    1. Drilling:
    • Drill ships extract samples from below the seafloor; e.g., JOIDES Resolution collects geological data over millions of years.
    1. Ophiolites:
    • These are exposures of oceanic crust on land (e.g., Vancouver Island) and allow for study of oceanic plate cross-sections.

Key Questions in Ocean Floor Research

  • What is the topography of the seafloor?
  • Effects of mid-ocean ridges on oceanic plates.
  • How oceanic plates change with age and distance from MORs.
  • Definition and significance of ophiolites.

Seafloor Components and Features

  • Bathymetry:
    • Major features include:
    • Mid-Ocean Ridge: Continuous ridges 2500 m above the seafloor.
    • Abyssal Plain: Deep seafloor with depths of 3 to 6 km, limited tectonic activity, and fine sediments accumulation.
    • Continental Margins: Divided into shelf & slope.

Sediment Types

  • Terrigenous:
    • Coarser material derived from land.
    • Limited large grain sizes as distance from continents increases.
  • Abyssal Plains:
    • Composed primarily of fine sediment with minimal life.

Mid-Ocean Ridges and Seafloor Spreading

  • Seafloor Spreading:
    • Mechanism that forms new oceanic lithosphere via decompression melting and volcanic activity.
    • Most active volcanism occurs where spreading rates are highest.

Hydrothermal Activity

  • Hydrothermal Vents:
    • Occur at mid-ocean ridges where seawater interacts with hot rock, creating unique ecosystems through chemosynthesis.
    • Vents can reach 350°C and produce metal-rich fluids that precipitate at vents, forming chimney-like structures.

Age and Depth of Oceanic Crust

  • As oceanic crust moves away from MORs, it cools, increases in density, and sinks, leading to deeper seafloor.
    • Seafloor depth increases with the square root of age.

Ophiolites Explained

  • Definition: Ophiolites are sections of oceanic lithosphere that have been uplifted and exposed on land.
    • Useful for studying ocean crust composition.
    • Typically consist of various layers, including:
    • Layer 1: Sediments (mudstone, chert, limestone).
    • Layer 2: Pillow lavas.
    • Layer 3: Sheeted dikes.
    • Layer 4: Gabbro.
    • Layer 5: Mantle rock (peridotite).

Chemical Processes and Environment

  • Below the Calcite Compensation Depth (CCD), calcareous matter dissolves, leading to clay or chert formation.
  • Ophiolite Examples: Occur in regions such as Metchosin and Sooke, providing important geological samples from oceanic processes.

Visual Aids and Further Learning

  • Refer to provided links for animations and additional resources to gain a clearer understanding of ocean floor geology, including mid-ocean ridge formation and associated geological features.