The Earth's Crust
Scientists cannot see deep within the earth so it is not certain what the inside of the earth looks like. However through indirect evidence (earthquake waves, and satellite images), it is believed that the earth is made of four main layers. The inner core, outer core, mantle. and crust.
INNER CORE– The deepest and hottest layer of the Earth. Because of the extreme heat and pressure, scientists believe the inner core is made of solid iron and nickle.
OUTER CORE – The second deepest layer of the Earth, This layer is probably made of liquid iron and nickle because it is still extremely hot but there is less pressure than at the inner core.
MANTLE – The largest and most complex layer of the earth. Composed of the upper mantle which is solid and the lower mantle which is partially melted.The rocks in the mantle are plastic which means they can move or flow.
Crust-The thin, outer layer of the earth.This layer is solid rock, and varies from between 5 to 70 km thick. The crust is thickest under the continents and thinnest under the oceans. The crust is broken into several large areas called plates which are constantly "floating" and moving over the mantle.
* egg example
- crust- shell of the egg ( cracked)
- mantle - egg whites
- cores - egg yolk
Today, we know the earth's surface is made of huge plates that are constantly in motion. This idea that the continents are moving around the earth was first proposed by a scientist named Alfred Wegener. He called his ideas "The Theory of
Wegener suggested that at one time all the continents were joined together into a " super continent" called Pangaea.
Wegener collected evidence to prove his idea that the
continents slowly move around.
1) Paleogeographic- Wegener noticed the jig-saw fit of the continents. For example: it looks as if Africa andSouth America were once joined.
2) Biological- Wegener noticed that fossils of the same
organisms were found on different continents. For
example, the same dinosaurs fossils were found in Africa
and South America.
3) Geological- Wegener noticed that the rocks were similar on both sides of the Atlantic Ocean. For example,
the rocks in the Applachians Mountains ( Long Range
Mountains in Newfoundland) were the same as rocks in
the mountains in Britian and Norway.
4) Meteorological- Wegener realized that there was
evidence that showed the climate in certain places was
much different in the past. There were coal beds that
only form in warm tropical climates, located in places
such as Canada that now has cold climates. As well, there is evidence of glaciers in areas near the equator
that is warm.
Wegener could not explain what could cause the continents to move so most scientist did not agree with his ideas and he died trying to prove his ideas.
In the 1940's, new technology began to be developed that began to prove Wegeners ideas. There were three main technologies.
1) SONAR- This technology involves sending out sound waves and recording the time it takes to return. Scientists can use this information to map thesea floor.They discovered that there were large mountain ranges on the sea floor. One mountain range extended the whole length of the Atlantic Ocean exactly halfway
between S.A. and africa ( Mid-Atlantic Ridge).
2) Magnetometers- this is a device that can measure the earth's magnetic field. When magnetometers were carried across the Atlantic Ocean, scientists noticed a strange thing. The magnetometer usually pointed north but sometimes pointed south. This pattern was repeated on both sides of the Mid-Atlantic Ridge. This pattern was caused by a reversal of the earth's magnetic poles. Sometimes in the past the north pole is located where the south pole is today. This pattern shows the Earth's crust is spreading apart on the sea floor.
Deep Sea Drilling-In the 1960's and 1970's scientists collected samples of rock on the sea floor by using deep sea drilling techniques. They discovered that the rocks were younger closer to the Mid-Atlantic Ridge and oldest closest to the continents. This proves that the ocean floor is spreading.
All this new evidence led to the development of
the "Theory of Plate Tectonics."
-replaces the continental drift theory
- states " the Earth's crust is broken into pieces called plates that are continuously moving around the Earth
The earth is divided into about 50 plates. There
are three different types of boundaries between plates.
1) Divergent Boundaries. At some places on the earth plates are moving away from each other. ( for example at the Mid-Atlantic ridge)
2) Convergent Boundaries. Here the plates are coming together. If one plate has ocean crust and the other one continental crust it is called a subduction Zone.
3) Transform Boundaries
Sometimes two plates may slide past each other. This is a transform boundary. The San Andreas Fault in California is an example of this type of boundary.
Scientists are still not sure what causes plates to move. The most popular theory today is that convection currents in the mantle cause the plates to move. At some places hot rocks are rising up and moving horizontally, causing the plates to move apart. At other places the hot rocks are cooling down and sinking, which pulls the plates down with it.
The Plate Tectonic Theory was developed by many different scientists. Some Canadian scientists who helped to develop the theory were:
Tuzo Wilson- worked on transform boundaries.
Joseph Tyrell- discovered dinosaur fossils in Alberta that showed the climate there must have been different in the past.
Harold Williams- worked on plate activity on the east coast of canada.
The moving of the tectonic plates cause pressure and energy to build up in the rocks. When the pressure gets too big, the rocks break suddenly, releasing the energy. This energy is released as seismic waves which causes the ground to shake.
This shaking of the earth is called an Earthquake.
Earthquakes are measured with Seismographs and the strength of an earthquake is measured on the Richter Scale. Each number on the Richter Scales represents an earthquake that is 30 times stronger than the number below it. For example; an earthquake with magnitude 4 on the Richter Scale is 30 times stronger than an earthquake with magnitude 3 and 900 times stronger than an earthquake of magnitude 2.
A volcano is an opening in the earth’s crust. Most volcanoes are located at plate boundaries. Volcanoes are found:
- at divergent boundaries such as the mid-atlantic ridge, magma flows up to the surface.
- at convergent boundaries ( subduction zones) one plate is forced down where it melts and the melted rock builds up and is forced to the surface where it forms volcanoes
- at hotspots (in areas where the plate is thin), magma rises to the surface through cracks in the crust. For example the Hawaiian Islands.
There are many volcanoes encircling the Pacific Ocean which is called the Ring Of Fire. for example Mount St. Helens and Krakatau.
Mountains are large areas that have been uplifted due to plate tectonics. It takes millions of years for mountains to form.
Fold- a bend in rock layers caused by great pressure over a long period of time.
Fault- a break in rock layers also caused by great pressure.
Mountains form in three main ways.
1) Fold mountains:These mountains form where two plates converge. The rocks between the plates gets bent and folded and pushed up into a mountain chain. For example: Himalayans and Appalachians.
2) Fault block mountains: Sometimes when rocks are put under pressure they break and one side of the break moves up over the other side which may form mountains. For example the Andes
3) Volcanic mountains: these mountrains form at subduction zones. When magma from the tip of the melted plate rises it pushed up the earth into mountains and when the magma gets to the surface it builds up making the mountains bigger. These mountains are usually rounded dome-shaped mountains. For example Mount St. Helens.
Geologic processes such as volcanoes and earthquakes have been going on for billions of years. The Earth is 4.6 billion years old. Human have been present on the earth for only a very short time. If the history of the earth was compared to a 24 hour clock, then human would only be present for the last second of the day.
Geologists have divided the earth's history into time units called the Geological Time Scale. There are 4 major eras on the scale.
CENOZOICPresent 65 million years agoappearance of mammals and humans
MESOZOIC248 million years agoAppearance and excinction of the dinosaurs
PALEOZOIC590 million years agoAppearance of complex lifeforms
PRECAMBRIAN4 billion 600 million years agoFormation of the earth and later the appearance of microscopic life forms.
Scientists use fossils as clues about the history of life on earth. Fossils are important for two reasons.
First, Fossils are important for showing what life was like in the past and we can use fossils to date rocks.( tell when they were formed)
Second, fossils can be used as evidence to prove plate tectonics. For example Palm Tree fossils in Antarctica proves that Antarctica was probably nearer the equator than it is today