Discovering Planet Earth

A guide to the world's terrain and the forces that made it

Geordie Torr

• Éditeur: Arcturus
• Année: 2021
• ISBN:
  9781398816978
• Sujet: Sciences physiques
• Sous-sujet: Géologie et sciences de la Terre

From icy polar tundra to lush tropical rainforests, readers can explore the wonders of the planet we call home in this spectacular visual guide. This beautiful jacketed hardcover introduces the many landscapes and systems that make up Planet Earth, from its molten core and plate tectonics to the different landscapes which make up its surface. Readers can explore the Amazon basin, taiga forests across the frozen wastes of Siberia and vast deserts on almost every continent.Includes:
• Land: volcanoes, glaciers, caves, wetlands...
• Air: the geomagnetic field, weather, the auroras...
• Sea: tides, coral reefs, fjords...The text is brought to life by superb full-color photos, charts, maps and infographics to reveal the planet in all its splendor. A fascinating guide to the world which can be enjoyed by the whole family. ABOUT THE SERIES: Arcturus' Discovering... series brings together spectacular hardback guides which explore the science behind our world, brought to life by eye-catching photography.

THE LAND

Although there are rock outcrops that date back to the Earth’s early days, most of the land is dynamic and ever-changing, constantly being worn down by erosion, pushed up into towering mountain ranges by tectonic and other forces, and even newly created by volcanoes. Those same tectonic forces are also moving the land around the globe, the continents taking part in a slow, majestic dance – coming together to form vast supercontinents and then drifting apart again. Consequently, the land displays a remarkable diversity of forms: rivers and streams, and glaciers and icecaps have carved out deep valleys and canyons, and created fertile deltas; the ocean’s power has battered coastlines, sculpting them into distinctive landforms. The land affects both our climate and our weather, playing a role in determining where rain falls and winds blow, where plants grow and deserts form. The Earth’s total land area is roughly 150 million square kilometres (58 million square miles), or about 29 per cent of its total surface. For much of human history, most of that land was wilderness, dominated by vast forests and grasslands. But over the past few centuries, we have changed the land beyond recognition, cutting down forests, planting crops, digging mines, building cities and much more.

The Carpathian Mountains, Ukraine. Covering an area of about 200,000 square kilometres (77,220 square miles), the Carpathians form the eastward continuation of the Alps. A geologically young mountain chain, they were relatively unaffected by glaciation during the last ice age and have mostly been shaped by running water.

// The origin of the Earth

The Earth formed out of a cloud of cosmic dust known as a solar nebula about 4.5 billion years ago through a process called accretion.

At some point, more than 4.6 billion years ago, static electricity caused particles of dust to begin to stick to one another, forming tiny objects known as particulates. As the particulates’ mass grew, their gravity caused them to clump together with other particulates to form pebble-sized rocks that clumped together to form larger rocks, and so on. Eventually, this process of accretion led to the formation of tiny planets, about 1–10 kilometres (0.6–6 miles) in diameter, known as planetesimals.

The planetesimals collided to form larger bodies, one of which grew larger than the others and became the Earth. Over a period of some 120–150 million years, the nascent Earth was bombarded by more planetesimals, slowly enlarging further and further.

As the Earth grew, its gravitational attraction became stronger, drawing in more material and causing the material that was already there to compress more tightly. Compression causes materials to heat up. Several other processes, including radioactive decay of elements such as uranium and collisions with comets and asteroids, made the Earth heat further, to the point where most of its constituent material melted and the planet was essentially a ball of lava floating in space. This caused a ‘sorting’ of the constituent parts, with the less dense silicate materials rising and eventually cooling to form the rocky exterior or crust, while the heavier, denser metals – mostly iron and nickel – sank to form the Earth’s solid core. Materials with densities in between remained more or less molten, forming the intermediate layer, known as the mantle.

The formation of the Earth began with dust and small rock fragments sticking together until the resulting bodies, known as planetesimals, were large enough for gravity to become the dominant force. The protoplanet then grew swiftly, eventually becoming large enough for its surface to flatten out and an atmosphere to form. The final globe illustrated here shows the ancient supercontinent of Rodinia, which formed during the Precambrian period about 650 million years ago.

Gravity pulled the Earth into a roughly spherical shape. However, its rotation caused it to bulge slightly at the equator, forming what’s termed an oblate spheroid (the Earth’s circumference is 21 kilometres [13 miles] – or about 0.3 per cent – longer around the equator than it is from pole to pole).

By about 4.5 billion years ago, the Earth had grown large enough that its gravitational field was strong enough to hold gas atoms to it, and it began to build an atmosphere (see page 130). Around this time, it was struck by a Mars-sized planet, known as Theia, whose metal core merged with the Earth’s. The collision released an enormous amount of debris, which eventually coalesced to form the Moon, as well as a great deal of heat.

Further collisions with comets and asteroids over several million years deposited water on the young Earth’s surface, while also creating deposits of metals and other heavy elements in the crust.

Not long after the Earth formed, it was struck by Theia, a protoplanet about the size of Mars. Some of the debris from the impact went into orbit and coalesced to form the Moon, while much of the remainder rained down on the Earth.

// The structure of the Earth

The Earth is made up of three main layers – the core, mantle and crust – with very different compositions and behaviours.

The Earth’s outermost layer, which accounts for less than 1 per cent of its mass, is a rocky shell called the crust. It is rigid, brittle and cold compared to what lies beneath.

The crust is mostly made of the relatively light elements silicon, aluminium and oxygen. There are two types of crust: oceanic and continental. Oceanic crust is younger than continental crust; it consists primarily of basalt that is continuously being created at mid-ocean ridges and destroyed in ocean trenches (see page 104). Continental crust, in contrast, is made up of a wide range of older igneous, metamorphic and sedimentary rocks, the most common of which is granite. Oceanic crust is denser than continental crust, causing it to sink lower into the mantle and thereby form the basins that house the Earth’s oceans. When the two types of crustal material collide, it is the denser oceanic crust that is forced downwards.

Crustal thickness is highly variable: beneath oceans it may be as little as 5 kilometres (3 miles); beneath continents, as much as 80 kilometres (50 miles) – the thickest part lies under the Himalaya. On average, oceanic crust is about 6.5 kilometres (4 miles) thick and continental crust about 35 kilometres (22 miles) thick.

The next layer down is called the mantle. At close to 3,000 kilometres (1,900 miles) thick, it is the largest layer, comprising 83 per cent of the Earth’s volume. It is also relatively dense, making up about 68 per cent of the Earth’s mass. It consists mostly of oxides of iron, magnesium and silicon. In the upper mantle, the dominant rock is a mineral called peridotite.

The upper mantle consists of two layers: topmost is the cooler, rigid lithosphere, a region that includes the crust; below is the hot asthenosphere, which is semi-molten and hence capable of flowing slowly. On average, oceanic lithosphere is about 100 kilometres (60 miles) thick. It thickens as it ages and cools, adding material from below. Continental lithosphere is roughly twice as thick, although its thickness also varies. The lithosphere is broken up into a jigsaw puzzle of tectonic plates (see page 14).

Below the upper mantle lies the transition zone, where rocks neither melt nor disintegrate, instead becoming extremely dense. It’s believed that this zone prevents material moving into the lower mantle, a region of solid rock that is hotter and denser than the upper mantle. The intense heat of the lower mantle creates convection currents in the asthenosphere that help to move the tectonic plates around. In general, the deeper one goes into the Earth, the less detail is known for sure, and much from the lower mantle onwards is open to conjecture.

At the Earth’s centre is the core, which makes up about 30 per cent of the planet and is almost twice as dense as the mantle. The core is roughly 80 per cent iron and 20 per cent nickel, although a few other elements are also present, including gold, platinum, cobalt and sulphur. It consists of two layers: the dense, solid inner core, which has a radius of roughly 1,220 kilometres (760 miles); and the liquid outer core, which is about 2,200 kilometres (1,400 miles) thick. There may also be an inner inner core that consists almost entirely of iron.

The Earth is made up of a number of different layers, each with distinct compositions and properties.

The temperature at the boundary between the inner and outer core has been estimated to be about 6,000°C (10,800°F), while the pressure is some 3.3 million times the atmospheric pressure at sea level.

Radioactive decay in the inner core, mostly of ...

Table des matières

• Cover
• Title
• Contents
• Introduction
• Part 1: The Land
• Part 2: The Sea
• Part 3: The Air
• Feedbacks and tipping points
• Impacts
• Further Reading
• Picture credits
• Copyright