Instructions

Read and take notes, especially where the learning objectives are addressed:

Learning Objectives

Distinguish between the terms tectonic plate (plate), plate boundary, and plate tectonics
Recognize Earth’s major landform features
Identify the layers of Earth
Describe some of the evidence used to show the continents and tectonic plates move
Recognize the difference between the hypothesis of continental drift and the theory of plate tectonics

Time Needed

About 45 minutes.

What Is a Tectonic Plate and What Are They Made From?

Many different lines of evidence suggest tectonic plates are moving. To build a theory, scientists need an explanation or a mechanism to explain the observed patterns. The theory of plate tectonics states that the outer rigid layer of the earth, the lithosphere, is broken into pieces called tectonic plates (Figure 5.2), and that these plates move independently above the flowing plastic-like portion of the mantle, the asthenosphere.

A map illustrating the major tectonic plates of Earth. — **Figure 5.2**: The tectonic plates divide the Earth's crust into distinct "plates" that are always slowly moving. Plate boundaries are the lines along which two plates meet. Earthquakes, volcanoes, and mountain building are concentrated along these plate boundaries. (Public Domain; USGS).

Tectonic plates are composed of the lithosphere: the crust and uppermost mantle that functions as a brittle solid. These plates are composed of oceanic crust, continental crust or a mixture of both. Oceanic crust is thinner and normally underlies the world’s oceans, while the continental crust is thicker and, as its name implies, consists of the continents. The interaction of these tectonic plates along plate boundaries is at the root of many geologic events and features, such that we need to understand the structure of the plates to better understand how they interact. The interaction of these plates is controlled by the relative motion of two plates (moving together, apart, or sliding past) as well as the composition of the crustal portion of the plate (continental or oceanic crust).

Continental crust has an overall composition similar to the igneous rock granite, which is a solid, silica-rich crystalline rock typically consisting of a mixture of pink (feldspar), milky white (feldspar), clear (quartz), and black (biotite) minerals. Oceanic crust is primarily composed of the igneous rock basalt, which is a solid, iron and magnesium-rich crystalline rock consisting of a mixture of black and dark gray minerals (pyroxene and feldspar). The difference in rock composition results in distinctive physical properties that you will determine in the next set of questions (Table 5.1).

**Table 5.1:** Major differences between continental and oceanic crust (CC-BY 4.0, Chloe Branciforte, own work).
Crustal Properties	Continental Crust	Oceanic Crust
Composition	Granite	Basalt
Relative thickness	Thick	Thin
Relative age	Old	Young
Relative density	Less dense	More dense
Relative buoyancy	More buoyant	Less buoyant

Textbook Reading

Chapter 2, Plate Tectonics:

Read from the start of the chapter and stop at 2.1.3 "Paleomagnetism" (stop just after the video)
Start again with 2.1. Did I Get it?