Chapter 1A Main ideas

1. Earth’s layers

Earth is divided into layers based on chemical composition or mechanical behavior. The compositional layers are a rocky crust and mantle, and a core consisting largely of iron. The mechanical layers are the lithosphere (crust and the rigid mantle together), which is broken into tectonic plates, and the asthenosphere, which is soft and slowly flows allowing the plates to move.

2. Plates and Plate Boundaries

The evidence for continental drift in the early 20th century included the matching of continental shapes on either side of the Atlantic, and the geological and fossil matchups between continents that are now thousands of kilometers apart. Giant strides were made in understanding Earth during the middle decades of the 20th century, such as mapping the topography of the ocean floor. By the mid-1960s, the fundamentals of the theory of plate tectonics were in place.

Earth’s lithosphere is made up of over 20 plates that are moving independently of each other at rates of between 1 cm/y to greater than 10 cm/y. They interact at their boundaries, where most geologic activity (earthquakes, volcanoes, mountain building, etc.) occur. There are three types of plate boundaries: divergent, convergent, and transform.

3. Divergent Boundaries

At divergent boundaries plates move away from each other. These boundary types are mostly found in the ocean and lead to the formation of new oceanic crust and a high mountainous ridge called a Mid-Ocean Ridge.  Volcanism and earthquakes are common.

4. Convergent Boundaries

At convergent boundaries plates move towards each other. These are subdivided into ocean-ocean convergent, ocean-continent convergent (collectively referred to as subduction zones), and continent-continent collision. The patterns of volcanism, earthquakes, and seafloor topography are very distinct at subduction zones. At continent-continent collision areas there is no subduction, but very high mountains are formed.

5. Transform Boundaries

At transform boundaries plates slide past each other on the surface of the earth. Shallow earthquakes are common on these boundaries.

6. Mechanisms for Plate Motion

The gravitational forces of ridge-push and slab-pull are the main mechanisms for plate motion. They work together with the thermal factor of mantle convection, which creates the conditions necessary for ridge-push and slab-pull.