The bulk of the Earth is made from iron, oxygen, magnesium and silicon.
More than 80 chemical elements occur naturally in the Earth and its atmosphere.
The crust is made mostly from oxygen and silicon, with aluminum, iron, calcium, magnesium, sodium, potassium, titanium and traces of 64 other elements.
The upper mantle is made up of iron and magnesium silicates; the lower is silicon and magnesium sulphides and oxides.
The core is mostly iron, with a little nickel and traces of sulphur, carbon, oxygen and potassium.
Evidence for the Earth’s chemistry comes from analyzing densities with the help of earthquake waves, and from studying stars, meteorites and other planets.
When the Earth was still semi-molten, dense elements such as iron sank to form the core. Lighter elements such as oxygen floated up to form the crust.
Some heavy elements, such as uranium, ended up in the crust because they easily make compounds with oxygen and silicon.
Large blobs of elements that combine easily with sulphur, such as zinc and lead, spread through the mantle.
Elements that combine with iron, such as gold and nickel, sank to the core.
This diagram shows the percentages of the chemical elements that make up the Earth. 215 Sulphur 2.7% Nickel 2.7% Calcium 0.6% Silicon 13% Aluminum 0.4% Magnesium 17%.
The Earth formed 4570 million years ago (mya) but the first animals with shells and bones appeared less than 600 mya. It is mainly with the help of their fossils that geologists have learned about the Earth’s history since then. We know very little about the 4000 million years before, known as Precambrian Time.
Just as days are divided into hours and minutes, so geologists divide the Earth’s history into time periods. The longest are eons, thousands of millions of years long. The shortest are chrons, a few thousand years long. In between come eras, periods, epochs and ages.
The years since Precambrian Time are split into three eras: Palaeozoic, Mesozoic and Cenozoic.
Different plants and animals lived at different times, so geologists can tell from the fossils in rocks how long ago the rocks formed. Using fossils, they have divided the Earth’s history since Precambrian Time into 11 periods.
Layers of rock form on top of each other, so the oldest rocks are usually at the bottom and the youngest at the top, unless they have been disturbed. The order of layers from top to bottom is known as the geological column.
By looking for certain fossils geologists can tell if one layer of rock is older than another.
Fossils can only show if a rock is older or younger than another; they cannot give a date in years. Also, many rocks contain no fossils. To give an absolute date, radiocarbon dating is used.
Radiocarbon dating allows the oldest rocks to be dated. After certain substances, such as uranium and rubidium, form in rocks, their atoms break down into different atoms. This sends out rays, or radioactivity. By assessing how many atoms in a rock have changed, geologists work out the rock’s age.
Breaks in the sequence of the geological column are called unconformities.
The Earth’s crust (see crust) is a thin hard outer shell of rock which is a few dozen kilometers thick.
Under the crust, there is a deep layer of hot soft rock called the mantle (see core and mantle).
The crust and upper mantle can be divided into three layers according to their rigidity: the lithosphere, the asthenosphere and the mesosphere.
Beneath the mantle is a core of hot iron and nickel. The outer core is so hot – climbing from 4500°C to 6000°C – that it is always molten. The inner core is even hotter (up to 7000°C) but it stays solid because the pressure is 6000 times greater than on the surface.
The inner core contains 1.7 percent of the Earth’s mass, the outer core 30.8 percent; the core–mantle boundary 3 percent; the lower mantle 49 percent; the upper mantle 15 percent; the ocean crust 0.099 percent and the continental crust 0.374 percent.
Satellite measurements are so accurate they can detect slight lumps and dents in the Earth’s surface. These indicate where gravity is stronger or weaker because of differences in rock density. Variations in gravity reveal things such as mantle plumes (see hot-spot volcanoes).
Hot material from the Earth’s interior often bursts on to the surface from volcanoes. 216
Our knowledge of the Earth’s interior comes mainly from studying how earthquake waves move through different kinds of rock.
Analysis of how earthquake waves are deflected reveals where different materials occur in the interior. S (secondary) waves pass only through the mantle. P (primary) waves pass through the core as well. P waves passing through the core are deflected, leaving a shadow zone where no waves reach the far side of the Earth.
The speed of earthquake waves reveals how dense the rocky materials are. Cold, hard rock transmits waves more quickly than hot, soft rock.
The Solar System was created when the gas cloud left over from a giant supernova explosion started to collapse in on itself and spin.
About 4.55 billion years ago there was just a vast, hot cloud of dust and gas circling a new star, our Sun.
The Earth probably began when tiny pieces of space debris (called planetesimals) began to clump together, pulled together by each other’s gravity.
As the Earth formed, more space debris kept on smashing into it, adding new material. This debris included ice from the edges of the Solar System.
About 4.5 billion years ago, a rock the size of Mars crashed into Earth. Splashes of material from this crash clumped together to form the Moon.
The collision that formed the Moon made the Earth very hot.
Radioactive decay heated the Earth even further.
For a long time the surface of the Earth was a mass of erupting volcanoes.
Iron and nickel melted and sank to form the core.
Aluminum, oxygen and silicon floated up and cooled to form the crust.
Rotation is the normal motion (movement) of most space objects. Rotate means ‘spin’.
Stars spin, planets spin, moons spin and galaxies spin — even atoms spin.
Moons rotate around planets, and planets rotate around stars.
The Earth rotates once every 23.93 hours. This is called its rotation period.
We do not feel the Earth’s rotation — that it is hurtling around the Sun, while the Sun whizzes around the galaxy — because we are moving with it.
Things rotate because they have kinetic (movement) energy. They cannot fly away because they are held in place by gravity, and the only place they can go is round.
The fastest rotating planet is Saturn, which turns right around once every 10.23 hours.
The slowest rotating planet is Venus, which takes 243.01 days to turn round.
The Sun takes 25.4 days to rotate, but since the Earth is going around it too, it seems to take 27 days.
The study of the shape of the Earth is called geodesy. In the past, geodesy depended on ground-based surveys. Today, satellites play a major role.
The Earth is not a perfect sphere. It is a unique shape called a geoid, which means ‘Earth shaped’.
The Earth spins faster at the Equator than at the Poles, because the Equator is farther from the Earth’s spinning axis.
The extra speed of the Earth at the Equator flings it out in a bulge, while it is flattened at the Poles.
Equatorial bulge was predicted in 1687 by Isaac Newton.
The equatorial bulge was confirmed 70 years after Newton – by French surveys in Peru by Charles de La Condamine, and in Lapland by Pierre de Maupertuis.
The Earth’s diameter at the Equator is 12,758 km. This is larger, by 43 km, than the vertical diameter from North Pole to South Pole.
The official measurement of the Earth’s radius at the Equator is 6,376,136 m plus or minus 1 m.
The Lageos (Laser Geodynamic) satellite launched in 1976 has measured gravitational differences with extreme precision. It has revealed bumps up to 100 m high, notably just south of India.
The Seasat satellite confirmed the ocean surfaces are geoid. It took millions of measurements of the height of the ocean surface, accurate to within a few centimeters.