ASTR-301 Lessons

Opening Remarks

 

Good evening class!  Today we’ll be delving into the internal structure of Earth and the movement of continents.

 

Hell on Earth

 

If you remember from last week, we discussed some of the phenomena that led to the birth of our home planet.  Early Earth was shaped by the heat and pressure generated by the mass of accreted rock and dust along with additional energy gained from heavy asteroid impacts.  As a result, instead of solid ground, the surface of the Earth was a comparatively thin, cooled sheet of rock broken everywhere by volcanoes.  This major volcanic activity belched noxious elements such as carbon dioxide and hydrogen into the atmosphere.  While there was plenty of water vapour, this atmosphere, too, resembled nothing we see today.  This period in Earth’s history is known as the Hadean Eon, named after Hades, the Greek god of the Underworld.  It is an appropriate name, as there were no signs of life.

 

       

Artistic conception of early Earth

Source: here

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Earth is like an ogre; It has layers

 

The lack of life on the baby planet does not mean that nothing was moving, growing, or changing.  The interior of the newly formed Earth was hot and molten, allowing elements and magic to move freely within the planet.  In the magma, heavier elements, mostly iron with some nickel, were pulled into the centre of the planet by gravity, while lighter elements like carbon and oxygen remained closer to the surface. This shifting of elements is a process called differentiation, which is how the different layers of the Earth were created.  The high pressure at the centre of the Earth means that even though the iron core is hot enough to be a liquid, it is compressed into a solid instead.  This solid mass is the innermost layer of the Earth, known as the inner core.

                                                      

Layers of Earth

Source: here

The second layer is also made up of the same heavy elements.  In fact, about a third of Earth’s mass and all but one percent of the total iron in the planet are contained within the first two layers.  However the outer core, this second layer, is not subjected to the same pressure as the inner core, so this layer is liquid, rather than solid.  Many believe that the innermost layers contain vast amounts of untapped magical energy, though no one has been able to create an instrument that can survive long enough to prove this.

 

The largest layer by far is the mantle.  This third layer makes up more than eighty percent of the planet and contains about half its mass.  Even though this layer is mostly solid, it can move and change over very long periods of time.  This is also the only interior layer that we can currently see.  Material from this layer is the molten rock that comes to the surface through volcanoes.  While the rocky material in this layer moves incredibly slowly, energy is still able to move through it, bringing heat and magic from the core closer to the surface of the Earth.  In places where the surface layer of the Earth is thinner, people have begun to harvest this heat as geothermal energy.  High volcanic activity is also the cause of greater concentrations of more “wild” magic.  Here there is a richer abundance of raw power coming from the centre of the earth, as opposed to the magic from the Sun, which is filtered by the atmosphere.

 

The final, fourth, layer of the Earth is smallest, but also the most important from a human perspective.  This layer, the thin shell of the planet, is the crust.  Here is where we live, work, and play, and also what keeps the hot inner layers from instantly vaporising the seas.  Although there are cracks, places where the mantle can come through, the crust protects us all from the other hotter and more unpredictable layers.  It is not one solid piece, but rather many pieces interacting with each other and floating on the mantle beneath them.

 

Set adrift

 

The map of Earth we know today is far different from the Earth that came into being so many millions of years ago.  In fact, the continental plates that exist today are far younger than Earth itself. Known as tectonic plates, these pieces of Earth’s lithosphere were subject to many different things over time such as tectonic movement (the motion of crust plates floating on the mantle), meteor impacts, and extreme volcanic activity that characterised early Earth.  These factors eventually gave rise to supercontinents, the term used to refer to early, massive continental structures that have come and gone throughout the history of Earth.  Our current continents are fragments of the most recent supercontinent: Pangea.

 

One of the first supercontinents known to exist on Earth was Vaalbara. Vaalbara takes its name from the two cratons (stable pieces of the continental crust) that it sat upon: the Kaapvaal Craton and the Pilbara Craton.  While most of Vaalbara has been completely destroyed by geological processes, the two cratons it consisted of exist to the modern day. Evidence of Vaalbara’s existence has been found in Africa and Australia, where the Kaapvaal and Pilbara Cratons are located, respectively.  Discoveries in these locations have revealed some of the earliest known fossils, dating back to about three and a half billion years ago, as well as early evidence of photosynthesis, the process by which plants produce oxygen.

                                                                         

 

Cratons

Source:   here

Kenorland is another supercontinent that formed about 2.7 billion years ago. This continent is thought to have formed by accretion, which is different from the astrophysical accretion that led to the formation of Earth. Instead of dust and gas, it was cratons that came together on Earth’s surface to create a very large land mass.  Intense volcanic activity across this continent caused it to break up into the cratons Laurentia, Baltica, Kalahari, and Western Australia.

 

Several other supercontinents came together and fell apart over the millennia including Columbia, Rodinia, and Pannotia.  Some of these lasted longer than others.  However, many of these continents were made up of cratons that form the basis of continents today.  Parts of Africa, Antarctica, and Russia have existed for far longer than the continents of which they are apart.

Continental Drift

Source: here

Siberia, an ancient craton located in the Russian heartland today, is actually about two and a half billion years old.  For some time it was a continent all by itself, though it also became part of several other continents through accretion.  It formed a major part of the supercontinent Rodinia, a name that comes from the Russian word родина (“rodina”), which means Motherland.  Though it is mainly tundra and forest today, geologists predict that in another 250 million years, Siberia may have a subtropical climate and be part of a new supercontinent altogether.

 

Siberia also became an integral part of the most recent supercontinent, Pangea.  Pangea is the youngest supercontinent.  It formed only about three hundred million years ago, but is also the first supercontinent to be reconstructed by geologists because of its relative youth.  Pangea was made up of all the continental plates on Earth today, though it was located mainly in the Southern Hemisphere.  The name comes from the Greek words pan, meaning “whole,” and Gaia for “Mother Earth.” Approximately 175 million years ago, Pangea began to break up. Continental drift and volcanic activity pushed the various tectonic plates apart, shifting them over the course of millions of years into  the places they are today.  However, these plates are still moving, and the face of our Earth is changing because of them.

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Shake and Bake

The plates that make up the Earth’s crust are constantly moving, growing, and shrinking.  Heat from the core makes them drift apart and clash together, and in doing so, they have shaped the world we live in; this is plate tectonics.  

 

There are two types of plates: continental and oceanic.   Continental plates are thick, less dense plates that have continents on them.   Oceanic plates are thinner, denser plates located under the oceans.  Boundaries form where plates interact with each other.  These come in three types: converging, diverging, and transform.  

 

There are three ways in which converging boundaries interact that create their own physical features.  When two oceanic plates come together, the older, denser plate goes through subduction – a process where the heavier plate is forced down and into the mantle beneath the less dense plate.  As the subducting plate begins to melt, molten rock known as magma begins to rise.  Volcanoes form where this magma is forced to the surface. In the case of oceanic convergent boundaries, these volcanoes often form strings of volcanic islands.  Subduction also occurs when an oceanic plate and a continental plate converge.  The continental plate forces the oceanic plate downwards.  This type of plate boundary is seen best in the Ring of Fire, the large ring of volcanoes at the boundary of the Pacific Plate and lining the eastern side of Asia around to the western side of the Americas.  Finally, when two continental plates converge, instead of subduction, they push up against each other, causing the plate material to fold up into ridges, creating mountains.

                                      

Subduction Types

Source: here

Diverging boundaries are where plates move away from each other.  Heat from the mantle below presses upwards, pushing the plates apart.  Molten rock from beneath the plates is forced up into the gap, where it cools and becomes part of the new plate boundary.  When continental plates diverge, they create large rift valleys and are often filled with water, making long, skinny lakes.  When oceanic plates diverge, magma is cooled more rapidly by the water, forming a large ridge of new rock, like the Mid-Atlantic Ridge, which also resulted in the formation of Iceland - a relatively small island with a lot of volcanic activity.  

                                            

Formation of a Divergent Boundary

Source: here

Finally, transform boundaries occur when plates are moving up against and past each other.  Since tectonic plates rarely have smooth edges, they do not slide easily.  Sometimes they get locked together which stores up a tremendous amount of energy.  Heat and pressure within locked up plates also cause a buildup of magical energy stored within the crust from the Earth’s formation.  Once energy builds up past the resistance of the rock, the plates release suddenly, sliding past each other, creating shockwaves called earthquakes.

                                                

Plate Movement  

Source: here

Earthquakes generate seismic waves - waves of energy and magic that travel through the Earth’s crust from the epicentre, the point at which the earthquake was generated, outwards to the surrounding area like ripples in a pond.  This can cause massive amounts of damage to buildings, roads, and other infrastructure located on top of the shaking ground.  There are several scales for measuring the strength of earthquakes, but the most common is the Richter magnitude scale.  Created in 1935 by the Muggle physicist and seismologist Charles Francis Richter, this scale is used to demonstrate the strength of an earthquake.  The scale is measured in factors of ten: this means that an earthquake that measures at a 7 on the Richter scale is ten times as powerful as an earthquake that is measured at a 6 on the Richter scale.

 

The release of magical energy during an earthquake can also have very damaging effects on nearby objects and enchantments.  The rippling shockwaves of magic can disrupt nearby spells and charms, causing them to fail or backfire.  In 1938, Hubert Keiser, a magical physicist and inventor in Los Angeles, California, noticed that enchanted clocks often stopped during earthquakes, a common occurrence in that area.  When the shaking was more noticeable, the clocks were behind for a longer amount of time.  Keiser set many magical clocks throughout the city in an attempt to measure the strength of the disturbance, which led to the development of the Keiser disturbance magnitude scale.  The Keiser scale measures magical disturbance in factors of seven.  An earthquake that is a 5 on the Keiser scale is seven times more powerful than one that is only a 4 on the Keiser scale.  

 

The cracked lunascope you see before me was sent to me by a friend who went to Durmstrang.  He now lives in Christchurch, New Zealand, which suffered an earthquake in 2011 that measured at 6.3 on the Richter scale and at 7.5 on the Keiser scale.  As you can see, it has been broken beyond use or repair as a result of the magical shockwave that went through the city, as well as the rather heavy bookcase that fell on top of it.

 

 Christchurch Earthquake: February 22, 2011

Source: here

That will be all for this week.  Do remember to look over your notes before our next class, as you will also be taking your midterms next week.  In addition to the information we have already covered, we will also be looking at the various ways that we are protected from interstellar dangers by the Earth.

Original lesson written by Professor Gagarina.