Which statement best describes the theory of plate tectonics? Plates sometimes move closer to one another. Sections of Earth that shift are part of Earth's asthenosphere. Sections of Earth's continents are in slow, constant movement. Parts of the ocean floor move as they are carried by rift valleys.

The **theory of plate tectonics** explains how Earth's lithosphere is divided into several large and small plates that float on the semi-fluid asthenosphere beneath them. These plates move relative to one another, leading to various geological phenomena such as earthquakes, volcanic activity, and the formation of mountain ranges. Let's evaluate each statement: 1. **Plates sometimes move closer to one another.** - **True.** This describes convergent boundaries where plates collide, leading to mountain formation, earthquakes, or the creation of volcanic activity. 2. **Sections of Earth that shift are part of Earth's asthenosphere.** - **False.** The shifting sections are part of the **lithosphere**, which includes the crust and the uppermost mantle. The asthenosphere acts as a semi-fluid layer beneath the lithosphere, facilitating the movement of tectonic plates. 3. **Sections of Earth's continents are in slow, constant movement.** - **True.** This captures the essence of **continental drift**, a key component of plate tectonics, where continents move at rates of a few centimeters per year. 4. **Parts of the ocean floor move as they are carried by rift valleys.** - **Partially True.** Rift valleys are associated with divergent boundaries where new crust is formed as plates move apart, primarily along mid-ocean ridges in the ocean floor. **Best Description:** While multiple statements touch on aspects of plate tectonics, the statement that **"Sections of Earth's continents are in slow, constant movement"** best encapsulates the overarching concept of the theory, emphasizing the dynamic nature of Earth's surface. **Answer:** Sections of Earth’s continents are in slow, constant movement.