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# Structure of the Earth – Plate Tectonics

Jun 3, 2023

## Introduction

Do you ever wonder why Earth looks the way it does when you look at a globe? Why are large bands of land separated by oceans? Why is there, not a single large land mass? Why are there different continents?

You will look for answers to these questions in this book. You will learn about the structure of the Earth and how it has changed over time. You’ll also get a glimpse of how it might look in the future.

One thing you will undoubtedly discover is that the Earth is constantly changing.

## Structure of the Earth

The diameter of the Earth, approximately 12,249 kilometers (7,650 miles), has been known since the time of the ancient Greeks. However, it wasn’t until the early 1900s that scientists realized the Earth was divided into three layers: the crust, the mantle, and the core.

### Crust

In comparison to the other two layers, the crust, or outer layer, is very rigid and thin. Its thickness varies depending on where you are on the planet. The crust beneath the oceans is very thin, only about 4 kilometers (3 miles) thick.

However, beneath a mountain range, such as the Swiss Alps or the Cascades in the northwest United States, the crust can be up to 100 kilometers (62 miles) thick. The Earth’s crust is brittle and can break under extreme pressure.

### Mantle

The mantle is located beneath the crust. The mantle is a thick layer of semi-solid rock that is approximately 2,800 kilometers (1,750 miles) thick.

Because the mantle is much hotter than the crust, it is semi-solid. The pressure at the Earth’s core is much higher than at the surface. Because of the pressure, the mantle becomes hotter and denser at its deepest point.

### The Core

The core is located beneath the mantle and in the center of the Earth. Because the pressure is higher in the core, it is nearly twice as dense as the mantle. Unlike the mantle, the core is mostly made of metal.

The Earth’s core is divided into two layers. The outer liquid core is about 2,080 kilometers (1,300 miles) thick. There is also a solid inner core, which serves as the true center of the Earth. The inner core is almost entirely made of iron. The liquid outer core spins as the Earth rotates. The Earth’s magnetic field is created by this spinning action.

So there’s the thin outer crust, which is stiff and brittle. The mantle is the middle layer, which is much hotter and less solid. The core is in the center, with a solid center and a liquid-like outer layer. Some people compare the planet to a hard-boiled egg.

The crust represents the shell, the mantle represents the white, and the core represents the yolk. You could use this analogy to help you remember the Earth’s layers. But keep in mind that, unlike an egg yolk, the Earth’s core has two layers.

### Earth’s Fragile Crust

When you consider the mantle, keep in mind that it is extremely hot. When rock or metal is heated to high temperatures, it softens and becomes liquid.

When a blacksmith makes a horseshoe, for example, he heats the metal until it turns red. When it gets hot enough, it softens, and he can hammer it to fit the horse’s hoof. Because the mantle is always very hot, it is not completely solid. It’s similar to Play-doh

It may feel solid, but pushing on it will cause it to change shape. This is how the mantle functions.

The mantle is solid, but not completely. It is trapped between the Earth’s rigid crust and the spinning, liquid outer core. Being caught in the middle increases stress.

The mantle is constantly caught between two opposing forces. As a result, the mantle is constantly in motion.

The crust sits on top of the moving mantle. The crust is usually solid and stays in one piece. However, the stress and pressure that cause the mantle to move can also cause large slabs of the Earth’s crust to move. The pressure and motion can crack the crust. Earthquakes are known to occur along the cracks, which are known as fault lines.

Fault lines are frequently found where large slabs of the Earth’s crust collide. This movement will be discussed further in the following section.

If the pressure is high enough, enough heat is generated to transform the mantle into a liquid known as magma. The hot magma expands, putting pressure on the crust above it. The magma is forced through crustal cracks. It has the potential to get very close to the Earth’s surface over time.

The magma will break through the surface and form a volcano if the pressure is high enough. When the pressure is high enough, the magma explodes through the crust, spewing dust, ash, steam, and liquid rock known as lava into the air. Volcanoes, like earthquakes, are typically found where two or more slabs of Earth’s crust meet.

The Earth’s crust is made up of about fifteen massive slabs, or plates. Some of the plates are separating. Others are slipping past one another. The plates can be found beneath both land masses and oceans. Continental plates are those found beneath the earth’s surface.

Oceanic plates are those found beneath the oceans. One plate may disappear, and another may appear over time, causing the number of plates to change. However, the figure will not change during your lifetime. These changes take millions of years to occur.

## Plate Tectonics

Plate tectonics is the formation and movement of plates. The term tectonic is derived from a Greek word that means “to construct.”

Plate tectonics refers to the fact that the Earth is made up of rock plates. These plates are very likely to have formed very early in Earth’s five-billion-year history.

All of the continental plates were once joined as one large plate a long time ago. At the time, there was only one continent. That continent is known as Pangaea, which means “all lands” in Greek.

### Pangaea

The below-given images depict how the seven continents were once one massive mass of land. Forces within the Earth have caused the continental plates to move and spread apart over hundreds of millions of years. The spreading plates have shifted the continents to their current locations.

Take note of how the continents resemble puzzle pieces as you move from one image to the next. You can see how South America used to fit right into Africa’s coast. Who knows what the continents will look like in hundreds of millions of years?

The continents separated and moved to their current locations over 225 million years.

Some scientists believe the continents will re-join as a single mass someday. Continental drift refers to the movement of continents over time.

What forces on Earth could be strong enough to cause massive plates of the Earth’s crust to move?

### Ocean Ridges and Trenches

Scientists discovered a strange chain of mountains in the middle of the Atlantic Ocean. They discovered that this mountain chain exists between all of the continents and winds its way around the Earth over time. This mountain range is known as the global mid-ocean ridge.

Hot magma seeps to the surface through fault lines in the crust along this ridge. The magma cools as it spreads along the ocean floor. This magma buildup causes the oceanic plates to shift and grow. These plates extend outwards toward the other plates.

Underwater volcanic eruptions occur in some places along the ridge. Some of these eruptions have pushed enough magma to the surface to form oceanic islands. This is how the island of Iceland formed.

A series of deep trenches surround the Pacific Ocean. As new crust is formed, some of the older crust is pushed into these trenches. These trenches are formed when one plate is pushed beneath another. There are numerous earthquakes and volcanoes wherever this occurs.

As a result, the land rim that surrounds the Pacific Ocean is known as the Ring of Fire. Mountain ranges formed along the western coasts of North and South America and the eastern coasts of Asia as oceanic plates pushed beneath continental plates. The colliding plates pushed the crust upward, forming these mountain ranges with numerous volcanoes.

### Importance of Plate Tectonics

• Plate tectonics research tells us a lot about how the Earth has changed over time and how it will continue to change.
• Plate tectonics is a relatively new science. It will provide many more insights into how the Earth is formed over time.
• Several major earthquakes and a few serious volcanic eruptions will occur during your lifetime. Each of these events will reveal details about the Earth’s construction.
• Plate tectonics teaches us a lot about Earth’s history. It can also predict Earth’s possible future.

### Important Terms:

Continental drift: It is the slow movement of the Earth’s continents caused by plate tectonics.

Continental plates: These are the plates in the Earth’s crust that hold continents together.

Core: The Earth’s innermost layer, consisting of a liquid outer core and a solid inner core.

Crust: The crust is the Earth’s solid outer surface.

Fault: A fault is a crack in the Earth’s crust that allows movement.

Global mid-ocean: A global mid-ocean mountain range that zigzags between all continents.

Lava: It is a liquid rock that rises to the surface.

Magma: It is a molten rock that rises from the mantle and lies beneath the Earth’s surface.

Mantle: It is a semi-solid layer of the Earth present between the outer crust and the outer core.

Oceanic plates: These are the submerged crustal plates of the oceans.

Pangaea: Long ago, there was a single continent that contained all of the Earth’s land mass.

Plate tectonics: It is the process of movement of crustal plates.

Plates: It is the large rock slabs that form the Earth’s crust.

Ring of fire: The ring of volcanoes that runs along the edges of the Pacific Ocean-bound continents.

Trenches: It is the extreme depths where two plates collide and one slide under the other.

Volcano: It is a hole in the crust of a planet from which molten or hot rock and steam flow; also, a hill or mountain formed by the material flowing from the hole.

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