D.1.1 Plate Tectonics and Mechanisms of Plate Movement Notes

Understanding Geological Processes Driving Geophysical Events

1. Imagine standing on a beach in Hawaii, watching lava flow into the ocean.
2. This dramatic scene is a result of geological processes that have been shaping our planet for billions of years.
3. These processes drive geophysical events like earthquakes, volcanic eruptions, and the formation of mountains.

Internal Heating: The Engine of Earth's Activity

1. At the heart of these processes lies internal heating.
2. Within the Earth's mantle and crust, heat is generated by the radioactive decay of elements like uranium, thorium, and potassium.

How Does Internal Heating Work?

The Earth is composed of several layers:

1. Core: The innermost layer divided into solid inner core and hot molten outer core, where temperatures can reach up to 5,500°C.
   1. The outer core moves around the inner core and this creates the Earth's magnetic field.
2. Mantle: The thickest layer of semi-solid rock surrounding the core. A
   1. The Asthenosphere: At a depth of between approximately 700 km and 100 km, the upper part of the mantle becomes hotter and more fluid.
   2. The Lithosphere: Above the asthenosphere the mantle becomes solid.
3. Crust: The thin, outermost layer where we live, and the upper part of the lithosphere. There are two types of crust:
   1. Oceanic Crust: A relatively thin (6-10 km) and young (~200 million years) layer of dense basalt.
   2. Continental Crust: Thicker (30-70 km) and older (~1,500 million years) layer of less dense granitic rock.

Heat is being transferred within the Earth:

1. Heat from the core radiates outward, warming the mantle.
2. This heat causes the mantle's rocks to become less dense and rise toward the surface.
3. As they cool, they sink back down, creating a continuous cycle known as convection currents.

Convection Currents: The Force Behind Tectonic Plates Movement

1. Convection currents in the mantle are the driving force behind the movement of tectonic plates.
2. These currents create zones of tension and compression in the Earth's crust, leading to various geophysical events.

There are Three Types of Tectonic Plate Margins

1. Destructive (Convergent) Plate Margin: Where plates collide.
   1. Subduction Zone: Where dense oceanic plate plunges beneath a less dense continental plate, e.g. west coast of South America.
   2. Collision Zone: Where two continental plates collide, e.g. the Himalayas.
2. Constructive (Divergent) Plate Margin: Where plates move apart, e.g. along the mid Atlantic Ocean.
3. Conservative (Transform) Plate Margin: Where plates slide past each other, e.g. San Andreas Fault in California.

How Do Convection Currents Work?

1. Rising Material: Hot, less dense material in the mantle rises toward the surface.
2. Spreading: As it reaches the upper mantle, the material spreads out, pushing tectonic plates apart.
3. Cooling and Sinking: The material cools, becomes denser, and sinks back toward the core.
4. Recycling: This cycle repeats, continuously moving the plates.

Example

The Mid-Atlantic Ridge is a prime example of a constructive plate margin and convection currents in action. Here, rising magma pushes the Eurasian and North American plates apart, creating new oceanic crust.

Plumes and Hotspots: Creating Volcanic Islands

1. In addition to convection currents, mantle plumes play a crucial role in geological activity.
2. These are columns of hot, rising magma that originate deep within the mantle and create hotspots on the Earth's surface.

How Do Plumes Work?

1. Rising Magma: A plume of hot magma rises through the mantle.
2. Surface Breakthrough: The magma breaks through the crust, forming a volcanic hotspot.
3. Island Formation: Over time, repeated eruptions build up volcanic islands.

Example

Hawaii is a classic example of a hotspot. As the Pacific Plate moves over a stationary mantle plume, a chain of volcanic islands forms.

Subduction Zones: The Birthplace of Earthquakes and Volcanoes

1. This process generates intense geological activity, including earthquakes and volcanic eruptions.

How Do Subduction Zones Work?

1. Plate Collision: An oceanic plate collides with a continental plate creating a zone of earthquakes.
2. Sinking: The denser oceanic plate sinks into the mantle.
3. Melting: As the plate descends, it melts, forming magma.
4. Volcanic Eruptions: The magma rises through the crust, creating volcanoes.

Example

The Pacific Ring of Fire is an area of multiple subduction zones, with frequent earthquakes and active volcanoes like Mount St. Helens (USA) and Mount Fuji (Japan).

Rifting: When Plates Pull Apart

How Does Rifting Work?

1. Tension: Convection currents pull plates apart, creating tension in the crust.
2. Cracking: The crust cracks and forms a rift valley.
3. Magma Upwelling: Magma rises through the cracks, leading to volcanic eruptions.

Example

1. The East African Rift is a prime example of rifting.
2. Here, the African Plate is splitting into two smaller plates, creating a series of rift valleys and active volcanoes.

Why Do These Processes Matter?

Understanding geological processes helps us predict and mitigate the impacts of geophysical events.

For example:

1. Earthquake Preparedness: Identifying subduction zones can help communities develop early warning systems.
2. Volcanic Monitoring: Studying hotspots and rift zones allows scientists to monitor volcanic activity and issue timely alerts.
3. Resource Management: Rift valleys and subduction zones are rich in minerals and geothermal energy, offering economic opportunities.