Lecture 4: Plate Tectonics and Zone of Volcanism

Lithosphere: Comprises the crust and the uppermost part of the mantle, extending approximately 100 km into the Earth.
Asthenosphere: Covers the remainder of the upper mantle, ranging from approximately 100 km to 660 km deep.

A theory proposing that Earth’s surface is divided into several large, thick plates that are in constant motion, altering their size and position over time.
When two tectonic plates meet, they interact, leading to various geologic activities.
Types of plate interactions include:
- Divergent Boundaries: Plates move apart.
  - Ocean Ridges: Elevated regions of the sea floor characterized by high heat flow and volcanism.
  - Example: Mid-ocean ridge, which is entirely volcanic.
  - Rift Valleys: Linear depression found between oceanic ridges.
  - Seamounts: Conical-shaped volcanoes formed on ocean ridges.
- Convergent Boundaries: Plates move towards each other.
  - The crust is destroyed in this interaction.
- Transform Boundaries: Plates slide past each other without generating or destroying magma.

Deep Ocean Trenches: Narrow, elongated depressions found along ocean floors.
Subduction Zones: Areas where one plate descends into the mantle, with the descending plate referred to as the "slab."
Volcanic Island Arcs: Arc-shaped chains of volcanic islands that are parallel to deep ocean trenches.
- Example locations include the Cascades (e.g., Mount St. Helens), the Japan Arc (e.g., Mount Fuji), and the Aleutian Arc.

Magma Formation: Most commonly generated at plate boundaries or hot spots where the upper mantle melts.
The mantle is primarily solid, not having sufficient heat to melt entirely. Partial melting occurs, resulting in magma of different compositions compared to the solid rock from which it was derived.

The type of volcanism is determined by the type of crust involved, with three primary zones identified:
- Divergent Zones: New crust is generated as plates pull apart.
- Convergent Zones: Typically more explosive volcanism due to the destruction of crustal material as one plate subducts under another.
- Hot Spots: Areas where magma can generate independently of plate boundaries, often resulting in intraplate volcanism.

Magma Production: Can occur in the middle of tectonic plates.
Mantle Plume: A fixed, narrow column of rising hot mantle rock responsible for melting and generating magma. It remains stationary relative to tectonic plate movement.
Hot Spot: The volcanic expression on Earth’s surface from a mantle plume, leading to a chain of volcanoes known as a Hot Spot Track.
Common characteristics of intraplate volcanism include:
- Lava Composition: Intermediate and felsic lavas including rocks such as andesite, rhyolite, pumice, obsidian, and tuffs.
- Eruption Style: Typically explosive due to the magma type.
- Melting Mechanism: Flux melting occurs at these zones.
- Magma Origin: Often sourced from the upper mantle.

The nature of volcanism that occurs at a given location depends on the specific crustal context and tectonic setting.