The structure of the Earth appears to be unchanging - except during catastrophic events, such as earthquakes and volcanic eruptions. In fact, the Earth is a dynamic entity, characterized by continuous movement on and beneath the surface.

Core

At the center of the Earth is a core made primarily of iron and nickel.  The core is actually composed of two sections: a solid inner core that consists almost entirely of iron and a liquid outer core that is made primarily of a nickel-iron alloy.

Movement of the metals in the outer core creates electric currents that generate the Earth's magnetic field.  This field protects our atmosphere from dangerous solar radiation. Without it, life as we know it could not exist on Earth.

The Earth's core is extremely hot; it retains a great deal of the heat that existed when the planet was first formed. It is also heated by friction that occurs when dense material sinks as well as by radiation that comes from radioactive elements within the core.

Nevertheless, the core has been slowly cooling during our planet's long history. The Earth cools from the inside out; this has resulted in part of the outer core freezing and solidifying, leading to the inner core becoming larger.  The inner core is growing at a rate of about one millimeter every year.

At the same time, parts of the inner core are melting.

Mantle

Above the core is the mantle. Like the core, it is divided into two sections - a lower mantle and an upper mantle.  The mantle is made mostly of iron, oxygen, magnesium and silicon.  Although it is mostly solid, the mantle does contain viscous material, which flows extremely slowly.  The upper mantle tends to be more fluid than the lower mantle.

The process of convection - heat transfer - causes movement within the mantle. The heat from the Earth's core heats up material in the lower part of the mantle.  As it heats, it expands and therefore becomes less dense and rises upward.

As this material rises, it cools and becomes denser and then sinks back down toward the core.

Crust

The crust is the Earth's top layer.  The crust, which contains the ocean basins and the continents, consists of rocks made primarily of silicon, aluminum and oxygen.

The crust and the topmost portion of the mantle together form the lithosphere.

Plate Tectonics

The lithosphere is broken up into plates, known as tectonic plates that float around on top of the mantle. It is believed that convection that within the mantle is what causes tectonic plates to move.

Plate tectonics is the study of the movement of tectonic plates.

These plates can collide, and one plate can move under another plate. This can result in the formation of mountains and ocean trenches.  Earthquakes and volcanoes occur where tectonic plates converge and where they diverge.

Because of the movement of continental plates, the appearance of the Earth's surface is constantly changing. The continents did not always have the same positions on the Earth that they do today.

The movement of the continents throughout the Earth's history has played an important role in the evolution of life.