LOS AGENTES EXTERNOS DE LA FORMACIÓN DEL RELIEVE.

Además de los agentes internos de la formación del relieve, existen una serie de agentes externos que lo modifican y tienden a igualarlo, son: el agua, el viento y la vegetación, además de la acción humana.

El agua.
El agua modifica el relieve de varias maneras:

• El agua de lluvia forma ríos, que desgastan las montañas; abren barrancos y valles, arrancando piedras y transportándolas; y finalmente deposita estos materiales, formando llanuras y deltas.
• La nieve de las montañas, forma glaciares que desplazan las piedras, abriendo valles glaciares.

• El hielo. El agua de la lluvia se mete en las grietas de la rocas y al helarse, aumenta su volumen, rompiendo las rocas.

• Disolución química. El agua reacciona con algunos compuestos de las rocas, provocando su disolución. Como consecuencia aparecen cuevas y ríos subterráneos (aquíferos).
• El agua marina. Las mareas, las olas y corrientes, desgastan las rocas, al mismo tiempo que acumulan arena y piedras en las playas,

El viento.

Desgasta las rocas y transporta pequeñas partículas que contribuyen a erosionarlas al impactar con ellas. Su fuerza erosiva depende de: la sequedad y la ausencia de la vegetación.



La vegetación.

Las raíces de los arboles penetran en el terreno, abriendo grietas en las rocas y erosionándolas.
Al mismo tiempo, la vegetación fija el suelo, impidiendo que sea arrastrado por el viento o el agua.

El Ser Humano.

El hombre ha transformado y transforma el relieve, con mayor intensidad en el último siglo, debido a los avances tecnológicos, de dos formas principalmente:

• La construcción de infraestructuras.
• La explotación de recursos mineros.

THE SEASONS: Solstices and equinoxes.

The Earth's axis is gently inclined (about 23º) but always points in the same direction (in the direction of the Polar Star, because of that the Axis orientation do not change even though the Earth is rotantig around the Sun but its position in relation to the Sun does.

Inclination of the Earth's Axis.

The seasons of the year.
As a consequence, during different moments of the year the quantity of light and heat that the different parts of the Earth receive from the Sun is going to change during the year:

During summer and winter, the rays of Sun falls directly on one hemisphere while the others reveives them with a strong inclination, making it winter. Nevertheless, during spring and autumn, the rays fall on both hemispheres wiht the same inclination.

When the rays of the Sun are inclined, the same amount of heat is projected onto a larger area, and there is less concentration of heat.

Equinox and solstice.

Equinox: is the time of the year when the Sun's rays falls perpendicular to the Equator. The Sun iluminates the North Pole and the South Pole for the same number of hours. There are two equinox:

1. The spring equinox (March 21), it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
2. The autumn equinox (September 21). it is autum in the Southern Hemisphere and spring in the Southern Hemisphere.

Solstice: is the time of the year when the Sun's rays fall perpendicular to one of the tropics.

There are two solstices:

1. The Summer solstice (June 21): the Sun is facing the Tropic of Cancer. It is summer in the Northern Hemisphere and winter in the Southern Hemisphere.
2. The Winter solstice (december 21): the Sun is facing the Tropic of Capricorn. It is summer in the Southern Hemisphere and winter in the Northern Hemisphere.

THE EARTH'S MOVEMENTS: Rotation and Revolution.

The Earth is not still in space, it is moving around itself (rotation) and and around the Sun (revolution or translation). This two movements have important consequences for life in Earth.

The rotation of the Earth.
Definition: The rotation of the Earth is the turning of the Earth around its imaginary axis, from west to east.
The length of this movement is about 24 hours.
This movement has important consequences:

• Alternation of day and night. As a consequence of this movement the Sun gives light and warmth to one side of the Earth, while the other side of the Earth remains in darkness and cools down.

• The apparent movement of the Sun. As a consequence of the rotation of the Earth from West to East, the Sun "appears" from the East and "disappears from the West. The name of the apparition of the Sun over the horizon is dawn, whereas the name of its dessaparition is dusk. Remember: it is the Earth that it is moving not the Sun.
• Geographic coordinates. The Earth's rotation allow us to identify the cardinal points and locate the position of any point in Earth.
• The division of the day in 24 hous and the establishment of time zones:

1. The Earth takes 24 hours to rotate completely, because of that we can divide the 360º of the Earth's sphere into 24 time zones that have a separation of 15º. Every place that is situated in the same time zone has the same solar time. 
2. In order to calculate the hour in a time zone we take as a reference the Meridian 0 or Greenwich meridian. We put the clock forward one hour for every time zone we move toward the East and we put the clock back one hour for every time zone we move towards the west from the Meridian 0.

For example, we want to know what time is it in the point B, if in the point A (located near the Greenwich Meridian) is the 12 o'clock. The point B is located at the east of the Greenwich Meridian and there is a difference of five time zones, because of this we have to add five hours and in the point B the time is going to be the 17:00.
In the same way, the point C is located at the west of the Greenwich Meridian and there is a difference of five time zones, because of that we have to extract seven hours and in the point C, the hour is going to be the 5:00.

The translation of the Earth (orbit).

Definition: The translation is the movement of the Earth around the Sun, from West to East.
Video: Earth orbit.
The length of this movement is about 365 days, 6 hours and 9 minutes, because of that every four years we have a year of 366 days (leap year).
This movement also has important consequences:

• The aparition of the Seasons of the year.
• The different duration of days and nights. During the summer the Norhtern Hemisphere is facing the Sun. As a consequence the illuminated area is bigger than the dark area and days are longer than nights. On the other hand, during the winter the Northern Hemisphere is no facing the Sun, as a consequence the illumanted area is smoller than the dark are an nights are longer than days.

 

• The apparition of different climate zones. Depending on the latitude, the Sun's rays fall with a different inclination. As a consequence Earth is divided in several climate zones:

1. One warm zone.
2. Two temperate zones.
3. Two cold zones.

Activity: 

Try to calculate the time in the points marked in the map, if in the time zone of the Greenwich meridian is the 12'oclock.

THE EARTH AND THE UNIVERSE.

The universe.

The universe appeared 13. 700 million years ago with the “Big Bang”, the explosión of a small mass which contained all the existing matter and energy.

The universe is made of:

•       Stars a celestial body which produces its own light and heat.

•       celestial bodies:

Ø  planets, a celestial body which rotates around a star and does not produces its own light and heat.

Ø  satellites, a celestial body which rotates around a planet.

•       interestelar matter (dust and gas).

The Milky Way.

In the universe, there are millions of groups of stars called galaxies. Each one of them contains thousands of million of stars and planetary systems. One of them is the Milky Way, in which the Solar System is located.

The Milky Way seen from Earth.

The Solar System.

The Solar System is the planetary system to which the Earth Belongs. It was formed about 5.000 million years ago. It consists of eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) and another celestial bodies (asteroids, satellites and comets) which move around a star of average size, the Sun.

Location of the Solar System in the Milky Way.

The Solar System.

Star size comparision HD

Why the Earth is a living planet?

The Earth is the only planet in the Solar System, and the only known planet, where there is life. Life appeared in Earth about 2.000 million years ago. There are three characteristics that explain this:

1. Abundant supply of liquid water, in the form of oceans and seas. Because of this, it is called the Blue Planet.

The Earth, the Blue Planet.

2. Moderate temperature., it is not too hot (like Venus) or cold (like Mars). It is located about the right distance from the Sun (around 150 million kilometres).

3. The atmosphere, because:

-Contains gases which are essential for life, such as oxygen.

-The atmosphere protects the Earth from dangerous solar radiation.

The Atmosphere as a protective layer.

The Earth: characteristics.

It was formed 4.600 million years ago. Its main characteristics are:

Its an imperfect sphere (geoid), wider at the Equator than at the Poles.

Its surface is 510 million km2, a thousand times the area of Spain. 

Most of its surface is covered by oceans and seas.

Can be divided in two equal parts across the Equator.

THE GEOGRAPHIC GRID: LATITUDE AND LONGITUDE.

The geographic grid.

The Geographic grid is the system of coordinates that we use in order to locate any point in the Earth’s surface.

Both extremes of the Earth´s axis are connected to two fixed points: the poles

From these points we trace imaginary lines that cross with others to form a grid.

Parallels and meridians.

This system is based in two types of imaginary lines: parallels and meridians:

• Parallels: Imaginary lines that encircle the Earth, perpendicular to its axes.

The main line of latitude is the Equator which divides the Earth into two halves. Another important parallels are the Tropic of Cancer, located in the Northern Hemisphere and the Tropic of Capricorn, located in the Southern Hemisphere.

• Meridians: Imaginary semicircles traced from pole to pole.

The most important meridian is the meridian 0º or Greenwich meridian, which runs through Greenwich, near London.

Latitude and longitude.
We use this imaginary lines in order to establish the latitude and the longitude of a point located in the Earth’s surface.


Latitude: The distance, measured in degrees, from any given point to parallel oº or  the equator

The maximum  latitude is 90º and can be either north or south.

Longitude: The distance, measured in degrees, from any given point to meridian 0º or Greenwich

The maximum longitude is 180º and can be either  east or west.

Remember: every degree (º) can be divided into 60 minutes (') and every minute can be divided into 60 seconds (''), the same way than an a hour. For example, the geographic coordinantes of the IES Cervantes High School are: 40º 24' and 12" N and 3º 42' 8" W.

ACTIVITIES:

1. Indicate the latitude and the longitude of the letters that appear in the following map.

2. Write down the concepts that correspond to the definitions in the table.

UNIT 1. THE EATRH AND ITS REPRESENTATION

With an A	The envelope of gases surrounding the Earth.
With a C	A system of projection that translates the meridians and parallels onto a cone. It is the most appropriate way to represent the mod-latitudes, between the tropics and the polar circles.
With a C	A projection system that translate the parallels and meridians onto a cylinder. It is the best method to represent the low latitudes between the equator and the tropics.
With a D	The time each morning at which daylight first begins. Sunrise.
With an E	A line notionally drawn on the earth equidistant from the poles, dividing the earth into northern and southern hemispheres and constituting the parallel of latitude 0.
Contains an F	The different shapes that the surface of the Earth has adopted over millions of year, as a result of internal forces.
With a G	A coordinate system that allows us to find every location on the Earth by its latitude and longitude.
With a G	A scale that use a graduated ruler to indicate the equivalent to ground distance.
With a H	Half of the earth, usually as divided into northern and southern halves by the equator, or into western and eastern halves by an imaginary line passing through the poles.
Contains an I	The time of year when the Sun´s rays fall perpendicular to one of tropics. A 24-hour night takes place in one of the poles while the other has a 24-hours day.
Contains  a K	The darker stage of twilight, especially in the evening. To become or make dark or dusky.
With an L	The distance, measured in degrees, from any given point to parallel 0º or the equator.
With an L	A year, occurring once every four years, which has 366 days including 29 February as an intercalary day. It is due to the orbit movement.
With a M	A representation of the spherical surface of the Earth, or a part of it, on a flat surface.
With an M	Imaginary semicircles traced from pole to pole. We use them to measure the longitude.
With an N	A scale that use a fraction to indicate the equivalent to ground distance. The numerator designates the distance on the map, and the denominator tells us its equivalent on the ground.
With an O	The movement of the Earth around the sun.
With a P	A celestial body moving in an elliptical orbit around a star.
With a P	A projection system that translates the meridians and parallels onto a plane that touches the Earth on one of the poles. It is the most appropriate method to represent the polar areas.
With a P	Imaginary lines that encircle the Earth, perpendicular to its axis. We use them to measure the latitude.
With an R	The turning of the Earth around its imaginary axis.
With an S	The number or graphic that indicates the relation between a distance drawn on the map and the same distance on the ground.
With a T	Any of the 24 longitudinal divisions of the earth's surface in which a standard time is kept.
With a T	A type of map that represents in detail the physical and human aspects of the territory: relief, water, flora, communication routes, land use etc.
With a T	A type of map that represents a specific geographical aspect such as climate, relief, population, etc.
Contains a  U	A distance, measured in degrees, from any given point to meridian 0º or Greenwich.
Contains an X	The time of year when the Sun´s rays fall perpendicular to the equator. The Sun illuminates the North Pole and the South Pole for the same number of hours.
Contains a Y	A sun with planets, asteroids, comets, etc. in orbit around it.

You can find the solution here.

el relieve

The Earth’s relief is the combination of landforms which appear in Earth’s surface.

We can distinguish two phases in the evolution of the relief: formation and evolution. The formation and the evolution of the relief is the consequence of the action of a variety of internal and external forces:
-internal forces which explain the formation of the relief: orogenesis, earthquakes, volcanoes.
-external forces which explain the evolution of the relief: atmospheric elements, water and living beings.

Relief formation and its agents.

The relief is form by the earth’s internal forces which raise, sink or move blocks of terrain. The main internal forces are:

•       Orogenesis is the process of forming mountains. It occurs in the area where two tectonic plates come into contact.

Formation of the Himalayas

•       Earthquakes, sudden movements of the Earth’s crust caused by the clash of two tectonic plates.

Earthquakes for kids

•       Volcanoes are ruptures in Earth’s crust through which magma are released onto Earth’s Surface. Are located in áreas where two tectonic plates make contact.

Relief modification and its agents.

The modification of the relief is caused by the Earth’s external forces which produces:

Erosion, the wearing down of types of relief.

Movement of the eroded materials.

Sedimentation, the acumulation of eroded materials in specific areas.

The main Earth’s external forces are:

• Atmospherics elements, such as:

-Temperature, the rapid changes of temperature make the rocks expand and contract.

-Rainfall penetrates the cracks in the rocks and expands when freezes, breaking the rocks. Also transport the eroded materials.


-Wind erodes the rocks, breaking them down. Also transport the eroded materials.

Wind erosion

• Water, disolves certains components of the rocks, excavating valleys and ravines. Also transport the eroded materials.

Cliff falls into the sea due water erosion

•  Living beings. Vegetations and animals change the relief breaking the rocks.

• 

  Root wedging
  
  

  Human beings also modifies the relief by mining or building motorways

  

  The big Hole, Kimberley (Sout Africa):