Earth's outermost layer, called the lithosphere, is composed of rigid tectonic plates that have been in motion for millions of years. While approximately 29% of the Earth's surface is land made up of continents and islands; The remaining 71%, which is covered with water, is called the hydrosphere formed by oceans, lakes, streams and other fresh waters. Today, most of the Earth's polar regions are covered with ice. The Earth's interior consists of the still active inner core of solid iron and nickel at the center, the liquid outer core that makes up the planet's magnetic field, and the mantle that causes the tectonic plates to move.