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.