Earth

EARTH

Earth is the planet we live on. Earth is a huge sphere, or ball, that goes around the sun in a circle. It is covered with water, rock, and soil and surrounded by air. Animals and plants live almost everywhere on Earth's surface. They can live on Earth because it is just the right distance from the sun. Living things need the sun's warmth and light. But if Earth were closer to the sun, it would be too hot for living things. If Earth were farther from the sun, it would be too cold for anything to live. Also, Earth has plenty of water. Most living things need water.

Earth is one of the nine planets that travel through space around the sun, and it is always moving. It spins like a top, and at the same time it travels around the sun. Human beings use these movements to measure the length of days and years on Earth. One day is the time it takes Earth to spin around once. One year is the time it takes Earth to travel once around the sun. Earth is not perfectly round. It is a little bit flattened at the poles. The North Pole is at Earth's top, and the South Pole is at its bottom. Halfway between the poles is an imaginary circle around Earth's center. This circle is called the equator. The equator cuts Earth into two halves called the Northern Hemisphere and the Southern Hemisphere.

The air that surrounds Earth is called the atmosphere. An invisible gas called nitrogen makes up most of the atmosphere. Oxygen, the gas that people and animals need to breathe, makes up most of the rest. Earth's surface is about 70 percent water. Almost all of this water is in Earth's oceans. These oceans form one great body of water called the world ocean or global ocean. Land makes up about 30 percent of Earth. The largest pieces of land are called continents. There are seven continents. They are Asia, Africa, Antarctica, Australia, Europe, North America, and South America.

Earth is covered with a rocky skin called the crust. The crust is about 5 miles thick under the oceans, and about 25 miles thick under the continents. The crust is made up of about 30 huge pieces, called plates. Underneath the crust, Earth has three layers of hot rock and metal. These layers are the mantle, the outer core, and the inner core. The mantle is a layer of hot, melted rock below the crust. It is about 1,800 miles thick. The outer core lies beneath the mantle. It is made mostly of melted iron, and it is even hotter than the mantle. The inner core is the hottest part of Earth. Scientists believe that the inner core is a ball of solid iron. The crust varies in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The core is composed of iron. The temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene, calcium and aluminum. Most of this is only from seismic techniques; samples from the upper mantle reach to the surface as lava from volcanoes but the majority of the Earth is unreacheable. The crust is made out of quartz and other silicates like feldspar. The Earth's chemical components is by mass:

34.6% Iron

29.5% Oxygen

15.2% Silicon

12.7% Magnesium

2.4% Nickel

1.9% Sulfur

0.05% Titanium

The Earth is the densest major body in the solar system.

The other terrestrial planets have similar structures and compositions with some differences: the Moon has a small core; Mercury has an extra large core; the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores.

Earth's crust is divided into separate solid plates which float around on top of the hot mantle, it is known as plate tectonics. Spreading occurs when two plates move away from each other and new crust is created by magma. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also a diagonal motion at some plate boundaries and impact between continental plates.

Scientists believe that Earth is at least 4.5 billion years old. The oldest fossils of living organisms are less than 3.9 billion years old. They think Earth began as a dry ball of rock covered by a cloud of gas. The rock began to heat up, and the inside of Earth melted. Heavy materials, like iron, sank to Earth's middle and formed the core. The lighter rocks moved to the top and formed the first crust. The heating of the inside of Earth caused chemicals to rise to the surface. Some of these chemicals formed water. Others formed the gases that make up the atmosphere. Over millions of years, the water slowly collected in low places on the crust and formed the oceans.

Earth's surface is always changing. For example, earthquakes break hillsides apart and cause cracks in the ground. Floods wash away soil and form new lakes and rivers. Volcanoes erupt, or explode, burying huge areas in rock. Other changes happen slowly. For example, water