Summary
Highlights
The Earth is predominantly covered by water, accounting for 71% of its surface, with 29% being land. However, 97% of this water is saltwater, leaving only 3% as fresh water. A significant portion of this fresh water is locked in glaciers or deep underground, making the accessible fresh water for rivers, lakes, and streams a tiny fraction.
Scientists classify Earth's interior by composition into three layers: the crust, mantle, and core. The crust is the thin outer skin, similar to an eggshell. The mantle is a thick, rocky layer, comparable to the egg white, making up most of the Earth's volume. The core is a super-hot, dense ball of metal at the center, like the yolk.
Mechanically, the Earth is divided into five layers: the lithosphere (rigid outer shell including the crust and upper mantle), the asthenosphere (hot, soft, flowing upper mantle), the mesosphere (strong, rigid lower mantle), the outer core (liquid iron and nickel generating Earth's magnetic field), and the inner core (solid, dense iron).
There are two types of crust: continental and oceanic. Continental crust is thick (25-70 km), made of lighter, buoyant granite, and can be billions of years old. Oceanic crust is thin (7-10 km), made of darker, heavier basalt, and is constantly recycled, rarely exceeding 200 million years in age.
The lithosphere, composed of the crust and the uppermost mantle, behaves as a single rigid shell fractured into tectonic plates. These plates move on top of the asthenosphere, a softer, flowing layer of the mantle. This relationship is crucial for understanding plate tectonics, analogous to a rigid chocolate bar (lithosphere) floating on a warm, pliable one (asthenosphere).
The fundamental interaction between the rigid lithosphere and the flowing asthenosphere drives continental drift, the opening and closing of oceans, and the formation of mountains, volcanoes, and earthquakes. This continuous motion sculpts our world and is the source of geological power.