Our Planet Earth ... the blue planet. It is roughly 93 million miles away from the Sun. It has a circumference of 24,902 miles along the Equator and a radius of 3,963.34 miles when measured from the Equator. It spins slowly around its axis, making one complete rotation every 24 hours. This rotation causes us to experience day and night. The side of the planet currently facing the Sun experiences day but as the earth spins, this side will eventually face away from the Sun and experience night. The Earth also revolves around the Sun. It travels around the Sun at a speed of roughly 69, 360 miles per hour and takes about 365 days and six hours (a solar year) to complete one revolution.
Most of Earth's surface (roughly 70%) is covered by water and most of this water is found in our oceans. Land covers the other 30% of the planet's surface. A mixture of gases known as the atmosphere surrounds the planet. The Earth is the only planet known to contain life. As Geography students we will be studying many of the processes which occur on this planet as well as the people which inhabit it. Let's start by finding out just what this planet is made of...
Layers of the Earth
The Earth is not solid all the way through. As a matter of fact, the Earth is made up of several layers, each quite different from the others. Examine the picture below which shows the concentric layers that the Earth is made of. We will examine each layer in some detail.
Let's start at the very center of the planet. This layer is known as the Earth's core. No human has ever made a journey to the core and it is highly unlikely that anyone ever will. The heat and pressure here are very intense. The core is made up of two layers; the inner core and the outer core. The inner core is believed to consist mostly of iron with some nickel as well as other metals. The temperature of the inner core is believed to be approximately 6000 degrees Celcius. Despite the extreme temperature, the inner core is believed to be solid due to the extreme pressure which exists there. The pressure in the inner core is so great that you would be crushed into a tiny ball (smaller than a table tennis ball) if you ever went there.
The inner core is suspended in a liquid outer core. The outer core, like the inner core, is made up mostly of iron with some nickel and smaller amounts of other elements. The pressure in the outer core is not as high as the pressure in the inner core, allowing the metals to exist in a liquid state. The movement of the material in the outer core is believed to create the Earth's magnetic field which shields us from the solar wind. The temperature of the outer core varies from about 6,000 degrees Celsius near the inner core to about 4,400 degrees Celsius near the mantle.
Surrounding the outer core is a layer known as the mantle. Most of the material which makes up our planet is found in this layer. The mantle is made up of rock and just like the core, it is divided into two layers; the upper mantle and lower mantle. The upper mantle consists of minerals such as olivine and pyroxene. The lower mantle consists of silicon, calcium, magnesium, oxygen, iron and aluminium. The temperature in the mantle is believed to be as high as 3800 degrees Celsius near the outer core. The temperature is so high that most of the rock which makes up the mantle exists in a semi-molten or "plastic" state. As one moves from the lower mantle toward the upper mantle the rock becomes less "plastic" and more stable. Indeed, some parts of the upper mantle are made of solid rock.
The outermost layer of the Earth is known as the crust. The Earth's crust is roughly 5km thick below the oceans and about 20 - 65 kilometers thick below the surface of our continents. Compared to the rest of the planet, the crust is very thin. If the Earth was the size of a breadfruit, the crust would be about as thick as the skin of that breadfruit. The crust consists of two types of material. Beneath the oceans lies a thin layer of dense rock called sima as it consists of silica and magnesium. This material makes up the oceanic crust. In many places a lighter material called sial lies above the sima, forming our continents. Sial which consists of silica and aluminium, makes up the upper part of our continental crust. We will be devoting much time to the study of the processes which occur on the Earth's crust.
Watch the video below which provides information about the layers of the earth.