Natural Wonders of the World
We live on a planet that is covered in amazing features, ranging from the red sands of the Sahara Desert to the blue ice of glaciers such as Vatnajökull, and from the grandeur of mountain chains like the Himalayas to the simple beauty of a bluebell wood. You might think that your list of things to see and do is complete, but once you start to look through the pages of this book you will realize there is so much more to discover and be inspired to go travelling once again.
However, you don’t have to leave the comforts of home to enjoy the wonders of our world. Through this book you can see Earth’s most active and dangerous volcanoes without taking any risks. If you don’t have a head for heights but want to know what the view from the top of Angel Falls looks like, then turn to the entry on South America’s Guiana Highlands. Even if you can’t swim, you’ll be able to find out what it would be like to dive inside the Great Blue Hole off the coast of Belize. And if you don’t have time to trek through the Congo, you can still marvel at the wildlife in its rainforest. This book will also take you to places where access is severely limited—only a few people each year are given permission to explore the Hang Son Doong cave system in Vietnam but you will see what it is like to stand in its vast interior.
While the book is filled with fantastic photographs, it isn’t just a celebration of the beauty of our natural world. It reveals the geology— and sometimes physics, chemistry, botany, and zoology—behind different features. Even the most complicated processes are explained in simple terms. And incredibly detailed artworks, constructed from satellite terrain data and imagery, reveal aspects of the natural wonders that cannot be seen with the naked eye. By viewing from above, going underneath, and looking inside, the artworks explain major geological formations and the processes behind them, including rift valleys, river canyons, island arcs, and stratovolcanoes.
This book doesn’t just focus on geological wonders—the key animals and plants that live in, on, or around the different features are included, and the world’s most important forests and grasslands are given their own entries. And if you want to know how a supercell storm forms, then turn to the section on extreme weather. In all, more than 240 of Earth’s natural wonders are covered in detail and a further 230 are described in the directory. But rather than reading from cover to cover, this is a book that you will pick up, put down, and then return to time and time again as you try to decide where next in the world you wish to travel to.
EARTH’S STRUCTURE Our planet has a diverse surface, with a huge variety of landscapes and materials, ranging from water and gases, to living matter, soil, ice, and rock. However, its interior is much less varied, consisting mainly of just rock, metal, and some water.
Internal structure Much of what is known about Earth’s structure has been established from the study of seismic waves created by earthquakes. Internally, our planet has three main layers: the core, mantle, and crust. The core is made mainly of iron, with some nickel, and is itself doublelayered—a liquid outer core enclosing a solid inner core. Surrounding the core is the mantle, which consists mainly of solid (but in parts, deformable) silicate rock. Its top part is called the upper mantle and has two layers. The uppermost of these, sometimes called lithospheric mantle, is solid and brittle and is fused to the crust. Below it is a more deformable layer, the asthenosphere. Below the upper mantle is a region called the transition zone, now known to contain significant amounts of water “locked into” its rocks. Beneath this is the thickest of all of Earth’s internal layers, the lower mantle. The outermost layer is the crust. This is solid, and there are two different kinds.
Thick continental crust, made up of many different rock types, forms the land surface, while thin, relatively dense oceanic crust—containing just a few rock types—lies under the oceans. Both types are fused to the underlying lithospheric mantle, forming a combined rigid shell, the lithosphere.
Mantle convection A slow, gradual circulation of rock occurs in the mantle. This circulation is called mantle convection and is driven by heat flowing out of Earth’s core. The mantle is also thought to contain plumes of rising, hotter, semisolid or liquid rock, called mantle plumes. Where they penetrate into the crust, these create surface hotspots (see p.13). Earth holds a colossal amount of internal heat energy, much of it generated during the planet’s original formation and still trapped inside. This energy is continually added to by the radioactive decay of unstable isotopes of various chemical elements scattered throughout the interior. Earth’s energy is constantly trying to escape, and mantle convection and mantle plumes, as well as phenomena such as earthquakes, are an expression of this. Although our planet’s mantle and crust are predominantly solid, as a result of processes connected to mantle convection and plumes both contain collections of hot melted rock and dissolved gases, called magma. The presence of magma in the crust is associated with volcanic and geothermal activity (such as hot springs and geysers) at Earth’s surface.
Rocks and the rock cycle Earth’s crust, the part of our planet that has been shaped into a myriad of landscapes and physical features, is made up mostly of rocks. Rocks are assemblages of chemical substances called minerals, and are of three main types: igneous, sedimentary, and metamorphic. Igneous rocks are the result of magma from deep within the planet being erupted at the surface or injected into the crust and then cooling and solidifying. Sedimentary rocks are formed by the deposition and cementation of mineral or rock particles created by the weathering or breakdown of other rocks. Metamorphic rocks are formed by the alteration of other types of rocks by high temperature, pressure, or a combination of the two. A variety of processes occurring within Earth’s crust and surface continually change crustal rocks from one type into another, in a never-ending succession of events known as the rock cycle. The various components of the rock cycle, which include, for example, volcanism and numerous erosional and depositional processes, are extremely important in shaping Earth’s surface landscapes.
Earth’s atmosphere The atmosphere is part of our planet’s overall structure and has several layers. Air circulates only in the lowest layer, which is called the troposphere, and it is only in this layer that weather occurs—taking the form of, for example, winds, precipitation (rain, hail, and snow), and changes in temperature, pressure, and humidity. Weather is driven largely by a combination of energy coming from the Sun and from Earth’s rotation. The averaged-out weather in a region over a long period of time is termed that region’s climate. Both weather and climate, and the connected processes of the water cycle (which include, for example, precipitation, the flow of water over Earth’s land surface in streams and rivers, and the evaporation of water from the sea) are other vital factors in shaping landscapes. Changes in Earth’s climate, in particular a persistent warming caused by increased levels of carbon dioxide in the atmosphere, are having a widespread effect on surface landscapes. For example, a general retreat and shrinkage of glaciers is occurring worldwide.
Numerous features and events at Earth’s surface, from volcanic activity to earthquakes and mountain-chain formation, can be explained on the basis that our planet’s outer shell is split into fragments called tectonic plates, which slowly move around on the surface.
Earth’s plates Earth’s crust is fused to the top layer of the underlying mantle (see p.10), forming a shell-like structure called the lithosphere. The lithosphere is split into several chunks, called tectonic plates, which slowly move around on Earth’s surface, driven by convection in the mantle. There are seven large and numerous medium-sized and smaller plates. As they move, Earth’s continents are slowly shuffled around. The rate of movement is small, but over tens or hundreds of millions of years, it can produce major rearrangements. During Earth’s long history, continents carried by different plates have collided from time to time, pushing up mountains, or combined to form supercontinents. Conversely, large landmasses have sometimes split into smaller chunks in a process called rifting.
Plate boundaries The science of plate tectonics largely revolves around the study of what happens near the edges of plates as they move relative to each other. These plate boundaries are dynamic places where important landscape-altering events take place, such as volcanic activity, mountain-building, island formation, rifting, and earthquakes. Plate boundaries fall into three main classes. The first, called divergent boundaries, occur where two plates are moving apart. At this type of boundary, material continuously welling up from the mantle fills the gap and creates new plate. Divergent boundaries are found extensively on the ocean floors, in the form of mid-ocean spreading ridges. But they also occur, among other places, in Iceland and in East Africa, along what is called the Great Rift Valley (see pp.184–87). They are always associated with volcanic activity. The second class of boundaries, convergent boundaries, are found where two plates move toward each other. At these boundaries, all or some of one plate moves down, or subducts, under the other and is destroyed. Where both plates carry continental crust, mountains form as chunks of crust are forced together. This process accounts for the origin of many mountain chains, such as the Himalayas. Other instances involve a plate carrying oceanic crust subducting the other plate. Boundaries of this type are characterized by features such as a deep-sea trench along the line of the boundary; a chain of volcanoes, always on the side of the plate that is not subducting; and the frequent occurrence of major earthquakes.
The third class of boundaries, called transform boundaries, occur where plate edges push past each other, without new plate being created or existing plate destroyed. These boundaries are also a site and source of earthquakes. Examples can be found in California (along the famous San Andreas Fault, see pp.30–31), in New Zealand’s South Island, and elsewhere, including extensively on the ocean floors.
Hotspot volcanism Although two of the three main types of plate boundary are frequently linked to volcanism, not all volcanoes develop at plate boundaries. Some appear in the middle of plates. This type of volcanism is usually accounted for by the presence of mantle hotspots—locations at the top of Earth’s mantle that appear to be the source of peculiarly large amounts of energy. The movement of a plate across a hotspot (whose position is fixed) can, over a long period of time, create a chain of volcanic features at the surface. This explains, for example, evidence of much ancient volcanic activity in a line to the southwest of Yellowstone National Park (due to movement of the North American Plate over this hotspot). It is also regarded as the best explanation for the formation of some chains of volcanic islands, including the Hawaiian Islands (see pp.316–19).
The landscape features we see on Earth today are the result of events and processes stretching back more than 4 billion years. It is only in the last few hundred years, however, that scientists have pieced together some of the main details of the story.
Earth’s age and origins A precursor body of what is now planet Earth formed around 4.55 billion years ago, from collisions of smaller bodies in a disk of material spinning around the Sun. When it was perhaps 40 million years old, the protoEarth is thought to have experienced one final collision, leading to the formation of what we now call Earth and its natural satellite, the Moon. The early Earth was probably born in an extremely hot, molten state. Heavier materials, mainly iron, sank to the center, with lighter materials forming layers around this. For around the first 150 million years, no solid crust formed because there were continuing impacts from comets and asteroids, while high volcanic activity continually reworked the surface. Around 4.37 billion years ago, the oceans had begun to form, through condensation of water released into the atmosphere by ancient volcanoes. By 4 billion years ago, the first pieces of continental crust had also formed. Movements of early tectonic plates caused landmasses to collide and merge, gradually forming the ancient cores of today’s continents.
The geological timescale The fact that Earth is extremely ancient, and that the sedimentary rock layers (strata) of its continental crust were laid down in sequence over a vast period of time, first became clear to scientists between the 17th and 19th centuries. They noticed that many rock strata contain fossils—the remains of ancient, apparently extinct, animals and plants. They divided the fossilbearing rock strata—and thus Earth’s history as a life-bearing planet—into three eras: the Paleozoic (meaning ancient life), Mesozoic (middle life), and Cenozoic (recent life). Later, these eras were subdivided into geological periods. In rock strata deeper and older than the fossil-bearing rocks, there seemed at first to be no life. These rock strata were later subdivided into three extremely long time intervals called eons—the Hadean followed by the Archean and Proterozoic eons. It is now known that simple organisms called prokaryotes (single-celled bacteria-like organisms) first evolved during the Hadean. Some 2 billion years then passed before more complex single-celled organisms called eukaryotes evolved (see pp.16–17).
Agents of change Geologists have argued for centuries about the nature of the processes, operating through Earth’s past, that have brought about the varied landscapes seen today. In the late 18th century, support grew for the argument that Earth’s features were mostly formed by slow, gradual, and ongoing changes over time. It was clear, for example, that erosion had shaped the planet everywhere. This view contrasted with the opposing doctrine, which proposed that a series of cataclysms was responsible for most of the Earth’s surface features. Today, it is agreed that most landforms on Earth now are the result of gradual processes. Nevertheless, there have been catastrophes: for example, an asteroid or comet impact about 67 million years ago (MYA) is thought to have caused fires, massive earthquakes, worldwide darkness, and possibly wiped out the dinosaurs
Changes in climate and sea level
Earth’s climate has changed quite drastically over long periods of time. At one extreme, there have been times when there were no ice caps and temperate deciduous forests grew at the poles. At the other, it seems probable that Earth was at least once completely frozen over. Similarly, sea level has fluctuated dramatically. For much of the past 540 million years, it has been higher than it is today. But, for the last 2.5 million years or so, sea level has mostly been lower, since during this time Earth has been in an ice age with much of its water locked up in polar ice sheets. Over the past 17,000 years or so, Earth has been in a warming period within this ice age, called an interglacial, and sea level has risen again. Since the late 18th century, the planet has warmed by around 1.8°F (1°C), due to increased atmospheric carbon dioxide, and sea level has risen by about 12 in (30 cm).
|March 15, 2020
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