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The formation of Earth

 

Geography is a very interesting subject. The word ‘Geography’ comes from the Latin word ‘Geographia’. Geographia is derived from the Greek words ‘ge’ which means Earth, and ‘graphia’ which means to describe or write. So, Geography means to write about Earth. The term was first used by Eratosthenes, a Greek scholar.

 

It is hard to believe, but once upon a time, there was nothing - no universe, no stars, no Earth, no people. According to cosmologists, people who study the origins (a synonym for genesis) and evolution of the universe, the universe is 13.8 billion years old. It all began with the Big Bang, a huge explosion of energy.

 

Our Sun has existed in a galaxy called the Milky Way for 4.6 billion years. Earth formed around 4.5 billion years ago when swirling cosmic dust and gases surrounding the newly formed Sun were pulled together by gravity to form a sphere of rock.

 

Soon after the Earth formed, while it was still hot soft rock, other rocks smashed into it. This gradual (which means over time) accumulation (which means build up) of material is called accretion. The impact of the rocks smashing into the newly formed Earth converted into heat and the planet began to melt. At this point there was no land, no air, no water.

 

When the planet was still forming, a smaller planet called Theia crashed into it. Although this increased the size of the early Earth, lots of rock and dust blasted out into Space, but gravity later pulled it together to form a ball – our moon. Meanwhile, the heat generated by the impact melted any rock that was forming, turning Earth’s surface into a swirling ocean of magma.  

 

Eventually, accretion started slowing down and the Earth’s surface began to cool. Most of its rock solidified forming a hard outer layer known as the crust. Often, the crust was split by boiling magma from below. Magma is molten (melted) rock. Where molten rock burst through the Earth’s newly formed crust, erupted lava formed the Earth’s first volcanoes.

The structure of Earth?

 

When Earth started forming 4.5 billion years ago it developed a layered internal structure. We know that Earth increased in size due to the gradual accumulation of rocks and that this process is known as accretion. We also know that the impact of these rocks smashing into each other created intense heat that caused the early Earth to melt, becoming a ball of hot soft rock. The rock contained elements, which are substances that cannot be broken down into any other substance. Examples of elements found in rocks include iron, aluminium, magnesium, and silicon.

 

During the meltdown of early Earth, especially after the Mars-sized planet Theia smashed into it, gravity pulled most of Earth’s heaviest, metallic elements down and the lightest elements remained close the Earth’s surface. This separation of the elements created layers. Earth has four layers which are named the inner core, outer core, mantle, and crust, or lithosphere. Lithosphere comes from the Greek word ‘lithos’ meaning rocky or stone, and ‘sphere’ meaning space.

 

The meltdown of Earth caused gravity to pull all the heavy metals down to form the Earth’s core, a metal ball at the centre of Earth, roughly the same size as Mars. The core is made up of two different states, the inner core is a solid ball of iron and nickel about 760 miles thick with a temperature as hot as the surface of the Sun. The inner core is between 5000 and 6000°C.

 

Surrounding the inner core is the outer core. The outer core is made up liquid iron, nickel, and sulphur and is about 1,400 miles thick. Although still very hot, the outer core is not as hot as the inner core. The outer core is between 4000 and 5000°C.

 

During the cooling of early Earth, the mantle cooled. The mantle is made up of molten (melted) and semi-molten rock which contains magnesium, silicon, iron, aluminium and calcium, amongst others. The mantle is about 1,800 miles thick making it the thickest layer. The mantle is so big that it is divided into the lower mantle and upper mantle. The temperature of the mantle varies from being up to 4000°C at its boundary with the outer core to 1000°C at its boundary with the crust.

 

The Earth’s crust, or lithosphere is the solid and rigid (not flexible) outermost layer of Earth. There are two types of crust, continental crust, and oceanic crust. Volcanic processes led to the formation of volcanic islands that joined to form the first continents. The first continents did not look like the continents of today and we will explore this in future lessons. Continental crust is mainly composed of granite and is about 27 miles thick whereas oceanic crust is 2-6 miles thick and mainly composed of basalt. The crust is by far the coolest layer of Earth with temperature increasing with depth.  

Life of Earth

 

When Earth accreted (grew by accumulation) from solid particles 4.5 billion years ago, its surface was molten rock and it had almost no atmosphere. Atmosphere comes from the Greek words ‘atmos’ which means vapour (gas), and ‘sphaira’ which means ball or globe.

 

Earth’s earliest atmosphere consisted of mainly methane, ammonia, water vapour, and neon. There was very little free oxygen and there was no liquid water. During this time, there was no life on Earth.

 

As the Earth cooled and the crust formed, it was split by the boiling magma in the mantle. These openings allowed lava to erupt at the Earth’s surface to form the world’s first volcanoes. These volcanoes emitted methane, sulphur and large quantities of water vapour and carbon dioxide. The air was very toxic.

 

The Earth continued to cool, and water vapour emitted from volcanoes condensed into water droplets that joined to create clouds. By 4 billion years ago, if not sooner, rain began falling from the young sky forming our oceans. Scattered throughout the oceans were tiny volcanic islands formed by lava that had erupted, solidified, and built up in height. In the future, those volcanic islands join to form the first continents. Earth started to look more familiar with water and land, but it was still a dangerous place with toxic air and scorching temperatures.

 

About 3.9 billion years ago, the newly formed oceans were battered by meteorites. Some theories suggest that these meteorites were responsible for the oceans. It is argued, they contained the water vapour that condensed to form the first rains that brought us our oceans by 4 billion years ago. Nevertheless, the meteorites dissolved and released minerals. Around 3.8 billion years ago, the minerals from the meteorites had allowed for life on Earth. Layers of stromatolites, single celled organisms started forming on the shallow sea floor. These stromatolites used the sunlight to turn carbon dioxide and water into food through the process of photosynthesis. Photosynthesis released oxygen that transformed our atmosphere.

 

It is believed that the amount of oxygen pumped into the atmosphere by photosynthesis offset greenhouse gases that were keeping Earth warm. As a result, some 2.3 billion years ago, Earth was covered in ice, and again 700 million years ago. We refer it as ‘Snowball Earth’. When the ice melted, it released more oxygen into our atmosphere. This allowed many species to flourish and evolve.

 

Around 550 million years ago, jellyfish and other soft-bodied animals started populating our oceans.

 

Then, by 540 million years ago, animals with shells such as trilobites and sea snails started to appear.

 

Earth has a history spanning 4.5 billion years, to break it down, we use the geological timescale. Life on Earth began to flourish at the start of the Cambrian period, sometimes known as the Cambrian explosion.

Geological Timescale

 

Earth has a history spanning 4.5 billion years, to break it down, we use the geological timescale. Geology comes from the same Greek root work as Geography ‘Ge’ which means Earth. Ology means the study of. So, Geology means the study of Earth, especially its rocks. Geologists use knowledge from rocks to understand how the Earth has arrived at its present state. For that reason, the geological timescale can be seen as the history of Earth.

 

The history of Earth is broken down into blocks called eons. The word eon comes from the Latin word ‘aeon’ and means a long period of time. There are four eons. The first three eons are often grouped together and referred to as the Precambrian Eon, so the time before life began to flourish some 540 million years ago. Separately, they are the Hadean, Archean, and the Protozoic eons. The eon we are in now is called the Phanerozoic Eon and it marks the beginning of the Cambrian explosion. Phanerozoic comes from the Greek words ‘phaneros’ which means visible, and ‘zoion’ which means animal. So, Phanerozoic means visible life.

 

The eons are broken down into eras. The word era comes from the Latin word ‘aera’ which means a division of time. There have been three eras. The Palaeozoic Era spanned the period of time between 540 and 250 million years ago. Palaeozoic comes from the Latin word ‘paleo’ which means old or ancient, and the Greek word ‘zoe’ which means life. So, the Palaeozoic Era means time of ancient life. It is divided into six periods including the Cambrian and Carboniferous periods. The Cambrian Period marked the beginning of life on Earth with animals with shells such as trilobites and sea snails starting to appear. The Carboniferous Period started about 348 million years ago. Carboniferous comes from the Latin words ‘carbo’ meaning coal, and ‘ferous’ meaning producing. It is believed this period of time was when coal formed, so it is named after it. During this time, giant insects such as dragonflies flew through the forests, and were hunted by some of the earliest land vertebrates. The Palaeozoic Era ended with the Permian Extinction that wiped out 96% of all life forms on Earth.

 

The Mesozoic Era lasted between 250 and 66 million years ago. Mesozoic comes from the Greek words ‘mesos’ meaning middle, and ‘zoe’ meaning life. So, the Mesozoic Era means time of middle life. It is divided into three periods. The Triassic Period marked the beginning of the dinosaurs and mammals that roamed the lush green forests. The Triassic Period ended with a mass extinction around 200 million years ago. But life recovered quickly, and more dinosaurs appeared along with pterosaurs and birds during the Jurassic Period. The Mesozoic Era ended with another mass extinction believed to have been caused by an asteroid bigger than Mount Everest. This marked the end of the dinosaurs some 66 million years ago.

 

Now we enter the third era, the Cenozoic Era spans the period of time from 66 million years ago to the present day. This is the era we live in today. Cenozoic comes from the Greek words ‘kanois’ meaning new, and ‘zoe’ meaning life. So, Cenozoic Era means time of new life. It is divided into three periods, the Paleogene, Neogene and Quaternary periods. During the Neogene, many birds, animals and reptiles evolved into the modern animals we have today such as rhinos, apes, horses and elephants. Then, 2.6 million years ago, the first human appeared in East Africa marking the beginning of the Quaternary Period. So, in that sense, we are all connected to Africa. The Quaternary Period has not ended, we are living in it.

Earth's continents

 

Before looking at the Earth’s continents, lets remind ourselves of what we know so far. Earth is 4.5 billion years old, and its history is broken down into eons, eras, and periods. The accretion of Earth, the meltdown, subsequent cooling, early atmosphere, and the oceans happened during the Precambrian Eon. Much of how Earth is today has happened during the Phanerozoic Eon. The Phanerozoic Eon is broken down into the Palaeozoic, Mesozoic, and Cenozoic Eras.

                                                                                            

During the Precambrian Eon, the Earth started to cool to form the crust, or lithosphere. As Earth’s crust cooled, boiling magma from inside the mantle created openings that separated the Earth’s crust into what we now call tectonic plates. Lava erupted between these plates forming underwater volcanoes which gradually increased in height until they emerged from the oceans to create volcanic islands, like the Azores. These island arcs (chains of islands) accreted (joined and got bigger) to form continents. It is believed by some that the first continents started forming around the same time as life on Earth, so 3.8 billion years ago. So, how did the volcanic islands merge?

 

The heat from inside the Earth caused the tectonic plates to float around on the molten mantle. This meant that lighter continental crust on top of the tectonic plates moved too. The tectonic plates moved away from each other and into each other causing the continental crust on top to be ripped apart or crash into other continental crust. This is how Earth arrived at seven continents separated by five oceans, but if we would have been around 240 million years ago, Earth would have looked very different to us.

 

Around 240 million years ago, between the Palaeozoic and Mesozoic Eras, almost all of Earth’s land masses joined to make one giant supercontinent. The supercontinent was named Pangaea and it was home to the dinosaurs. The name Pangaea comes from the Greek words ‘pan’ which means all, and ‘Gaia’ which, like ‘ge’ means Earth. So, Pangaea means all of Earth. But the same processes that created the Pangaea, the tectonic plate movements, also caused the Pangaea to break apart.

 

Some 150 million years ago, towards the end of the Jurassic Period, the movement of the tectonic plates split the Pangaea into two continents, Laurasia and Gondwana. Laurasia in the north and Gondwana in the south.

 

About 100 million years ago, the two continents continued to break up into recognisable continents we know today. Laurasia became North America and Eurasia, which consists of Europe and Asia. Gondwana became South America, Africa, Antarctica and Australasia, or Oceania. About 140 million years ago, part of east Africa broke away and was carried north across the Indian Ocean before it collided with Asia and became India. However, when the first species of human appeared at the beginning of the Quaternary Period 2.6 million years ago, the continents were still not as they are today. For instance, the United Kingdom was connected to France by a land bridge. About 25 million years ago, the Africa started splitting in two, at the Great Rift Valley in east Africa.

Human evolution

 

About 25-30 million years ago, a magma plume rose up beneath north-eastern Africa. The land here began to swell upwards like a huge spot stretching the continental crust until it eventually ripped open, and water flooded in to create the Red Sea. The Red Sea separates what is now the Middle East and the Horn of Africa. The two land masses continue to move apart as more magma rises up to form new crust beneath the Red Sea.

 

Around 4 million years ago, the East Africa Rift Valley took its current form, lined on both sides with mountains. These mountains formed a barrier stopping moisture (air containing water vapour) from the Indian Ocean passing over east Africa. This made east Africa hotter and drier leading to the formation of the world’s largest desert, the Sahara.

 

Over the same period, our species, hominins began to evolve and search further for food. About 4 million years ago, the ‘southern ape’ shared lots of similarities with us humans. They had slender (thin) bodies and the ability to walk on two feet. We know this from fossils that have been discovered. One such fossil is the skeleton of a female that lived 3.2 million years ago where Ethiopia is today, that came to be known as Lucy (named after the Beatles song that was playing at the time of excavation in 1974).

 

By 2.6 million years ago, at the beginning of the Quaternary Period Lucy’s species had fallen extinct, and our own ancestors had emerged. At the same time, Earth’s orbit around the Sun had changed and the planet fell into an ice age. The icesheets did not reach Africa, so hominins continued to evolve. The first was homo habilis. This was followed by homo erectus which had a much bigger body and brain, as well as a different lifestyle. It is believed that homo erectus lived as hunter gatherers and were the first to control fire. It is also thought that the homo erectus built rafts to travel over large bodies of water, leaving Africa and dispersing through Eurasia, and back to Africa again.

 

Our species of hominin, homo sapiens appeared in East Africa around 200,000 years ago and started migrating from Africa between 60,000 and 70,000 years ago. We do not know exactly where homo sapiens went first, or when they arrived there because we do not have that many fossil records. Instead, we use DNA to work out how long-ago different populations emerged from each other. One thing this has taught us, is that despite differences in hair and skin colour, or skull shape, genetically, the 7.7 billion of us living on Earth today are not that different. We are all connected to Africa.

 

Homo sapiens first arrived in India, then further into Asia, then Australasia some 50,000 years ago. Humans travelled back north and entered Europe around 40,000 years ago, but they met with an icesheet extending south from the north pole. With most continents now populated, all that was left was the Americas. With lots of water locked up in icesheets and glaciers, sea levels fell and homo sapiens were able to get across a land bridge between Siberia and Alaska to populate the Americas some 15,000 years ago.

 

In 2018, researchers extracted DNA from Britain’s oldest skeleton, Cheddar Man. It is believed that Cheddar Man was part of a wave of migrants that walked across a land bridge called doggerland. Back then, doggerland connected Britain to mainland Europe. Cheddar Man lived in Britain around 10,000 years ago; his skeleton was excavated from a cave in Somerset in 1903. From the DNA, researchers show that early Europeans had dark skin. From this, we believe that when humans first migrated from Africa, they all had dark skin. As homo sapiens migrated north from the Equator, the sunlight was less intense. Our skin changed in response, over thousands of years. So now we have diverse skin colours.

Earth's climate

 

Climate is the average weather of a place over a long period of time, usually 30 years. Climate can be cold, hot, wet, dry, or windy. The climate of a place depends on what is going on in the atmosphere. Like Earth’s continents, the climate is not ‘fixed’, it experiences change.

 

We can study past climates by looking at Earth’s geology. Rocks contain minerals called zircons that allow us to work out how old they are and if liquid water was present when they formed. So, what was the climate like during the Precambrian Eon?

 

We do not know much about the climate of Earth during its first 500 million years because the records, or rocks, were destroyed by the collision between Earth and the Mars-sized planet, Theia.  

 

However, the Jack Hills is a range of hills in the near desert of western Australia which formed some 3 billion years ago. They are not the oldest rocks (those are in California), but they do contain the oldest zircons that can be dated as far back as 4.3 billion years ago during Earth’s first eon, the Hadean. The Hadean was named after Hades, the Greek God of the underworld and existed 4.5 to 4 billion years ago. From the 4.3-billion-year-old zircons, we know that liquid water was present on the Earth’s surface before the formation of the rock that makes up the Jack Hills. This means that the water vapour in Earth’s atmosphere must have cooled and condensed to form clouds that rained down to fill the oceans some time before 4 billion years ago (but we already know that – this is how we know). However, there was little or no oxygen in the atmosphere, so life could not evolve.

 

Earth’s second eon, the Archean lasted between 4 to 2.5 billion years ago. Archaen comes from the Greek word ‘arkhein’, meaning beginning. It was during the Archean that life began to form in our oceans. Like the Hadean eon, records of Earth’s climate during the Archean are patchy and poor. During the Archean, the Sun was much dimmer giving off less sunlight and less heat, so Earth should have been extremely cold. But Earth was not cold during the Archean, it was about as warm as it is today. So, what stopped Earth from freezing?

 

Earth’s early atmosphere was dominated by greenhouse gases, especially methane and carbon dioxide. Lots of this carbon dioxide was emitted along with water vapour by young volcanoes when Earth started to cool down. The greenhouse gases trapped heat in the atmosphere which kept Earth warm. As a result, stromatolites started forming on our ocean floors some 3.8 billion years ago.

 

This brings us to Earth’s third eon. The Proterozoic eon lasted between 2.5 billion and 540 million years ago. Proterozoic comes from the Greek words ‘proteros’ meaning former, and ‘zoic’ meaning life. During this eon, stromatolites absorbed lots of the carbon dioxide that was keeping Earth warm and lots of the oxygen produced as a by-product removed methane from the atmosphere. The result was ice, and lots of it. Earth fell into a ‘snowball Earth’. It is thought that most of Earth, from the poles to the Equator was covered in ice sheets and sea ice. Increased volcanic activity eventually allowed Earth to thaw (defrost/melt). But a second Snowball Earth occurred some 715 million years ago and lasted for 120 million years. Again, increased volcanic activity topped up atmospheric greenhouses gases enough to insulate (warm) Earth and melt the ice. This was the beginning of our fourth and current eon, the Phanerozoic Eon.

 

The start of the Phanerozoic Eon was marked by the Cambrian Explosion 540 million years ago. Life flourished – no one really knows why. The climate was very warm and humid (high levels of water vapour in the atmosphere). Since then, there have been around three ice ages. We are currently in an interglacial period, which is a period of warming.

The making of Great Britain

 

About 600 million years ago, towards the end of the Proterozoic Eon and before the Cambrian Explosion, the continental crust that makes up Great Britain was south of the Equator and part of the ancient continent, Gondwana. We already know that processes inside the mantle cause the tectonic plates to move and that this causes continental crust to break apart and join with other continental crust.

 

So, around 440 million years ago, the continental crust that makes up Great Britain had migrated towards the Equator. Then, by 250 million years ago, the continental crust that is to be Great Britain was fully enclosed in the middle of the Pangaea, north of the Equator.

 

Following the separation of the Pangaea 150 million years ago, between the Jurassic and Triassic Periods and as the dinosaurs were diversifying, Earth began to resemble something we might recognise today. Great Britain was part of a series of islands.

 

An opening in the Earth’s crust, now known as the Mid-Atlantic Ridge, allowed magma to rise to the Earth’s surface, solidifying and creating new oceanic crust. This formed the Atlantic Ocean and continuous seafloor spreading pushed North America away from Europe and South America away from Africa some 80 million years ago. During that time, sea levels were high and most of Great Britain was under water.

 

About 65 – 22 million years ago, during the Paleogene Period of the Cenozoic Era, seafloor spreading in the Atlantic Ocean caused volcanoes to form along the west coast of Scotland and in Northern Ireland. The islands of Aran, Mull, and Skye are remains of these volcanoes. Giants Causeway are remnants of the lava which flooded out. The North Sea also formed around this time – it is a shallow sea.

 

During ice ages, like the one that occurred at the beginning of the Quaternary Period 2.6 million years ago, large amounts of water from the seas and oceans is locked in ice, so sea levels fall so much that land is exposed. This is how early hominins arrived in Great Britain. In 2013, a storm uncovered human footprints on the beach at Happisburgh, Norfolk. Scientist date them to be around 950,000 years old so sea levels were so low that an early species of hominin (not ours) must have walked across a land bridge joining continental Europe to Great Britain.

 

At about 450,000 years ago, the harshest glaciation means that Great Britain is inhabitable to humans – they migrate back across the land bridge to mainland Europe. A huge glacial lake builds up to the east. Eventually, the lake started to gush onto the land bridge creating huge waterfalls, and their force eroded (wears away) significant parts of the land bridge, forming the beginning of the English Channel.

 

Some 400,000 years ago, temperatures have risen, and the ice has thawed but Great Britain is still connected to mainland Europe by a narrow land bridge. This allows humans and animals to return to Britain. Over the next 350,000 years humans come and go as the climate changes.

 

125,000 years ago, the sea level has risen, and the land bridge has submerged (now beneath the water). Great Britain is an island. Hippos lie about in the Thames and lions roam where Trafalgar Square now stands.

 

60,000 years ago, sea levels fall significantly as water freezes into ice caps and glaciers (rivers of ice), creating a vast covered dry plain between England and continental Europe. Some 20,000 years ago, ice covers most of Britain.

 

Thousands of years later, as the climate warms homo sapiens including Cheddar Man start to return to Great Britain. The last ice age ended 11,700 years ago. As the ice melted, Ireland was separated from Great Britain by rising sea levels which also flooded the land between Great Britain and continental Europe making Great Britain an island once more. Today, Great Britain is the larger of the two islands that constitute the British Isles, along with over 5000 smaller ones.

Earth's natural resources

 

Events in Earth’s history have given rise to human evolution as physical processes have created natural resources. A natural resource is a material or substance found in nature that has a purpose or economic value to humans. Examples of natural resources include water, fertile soil, food, timber, rocks, minerals, oil, gas, and coal.

 

In 1943, American Psychologist, Abraham Maslow proposed a theory of human motivation – what makes us do what we do? Maslow proposed a hierarchy of needs and argued that we must satisfy each stage before moving on to the next. From this then, humans need air, water, food, energy (for warmth), shelter, and clothing. When those basic needs are satisfied, humans can move on to have good health, secure employment, and feel safe. We satisfy our basic needs by using Earth’s natural resources. Our lithosphere, atmosphere, hydrosphere, and biosphere work together to create and replenish (restock) Earth’s natural resources over time. Natural resources are distributed (spread out) across Earth, but not evenly. This means that some people have better access to natural resources than others.

 

About 71% of Earth’s land mass is oceans. Our oceans are where just over 97% of Earth’s water is stored, but it is salty. About 2.7% of Earth’s water is fresh but 77% of this water is locked away in ice sheets and glaciers. Most of the water we can use is below the ground, as groundwater. It is thought that early hominins survived the arid (dry) spells in East Africa by using springs, a point a which groundwater flows to the Earth’s surface. During dry seasons, our ancestors would have stayed close to springs, but during the wet season, groundwater levels would have risen and formed smaller rivers. At that point, our ancestors would have wondered across the landscape without concerns of water. We can go for three days without water. Initially, hominins would have used water for drinking and bathing in. Now, humans use it for cooking, cleaning our homes and our cars, agriculture (farming), manufacturing and even to produce energy.

 

Timber (wood) is one of the oldest construction materials. Many cultures around the world, especially nomadic peoples (those who move from one place to another), have constructed temporary structures such as the wigwam, tipi, and yurt out of branches, bark, reeds, or animal hides. As technology improved, timber was used to make mechanical components such as cartwheels and masts for ships. Today, timber has lots of uses including furniture, paper, and cardboard.  

 

Humanity’s earliest tools were constructed of stone – by chipping away at chert, or flint. Flint can be knapped (shaped by striking it) into sharp edges, or points. These sharp tools were great for butchering a kill, skinning, and scraping animal hides to prepare into clothing, or shelter. They were also good for shaping wood and spear points and arrowheads made great weapons. Today, we still use stone. We use rocks to construct buildings and bridges, lay roads, make glass and we mine the minerals in rock to make metals.

 

Natural resources can be renewable or non-renewable. Renewable resources are replaced within the average person’s lifetime. Non-renewables take thousands to millions of years to form.