Science 20 Unit C
Home.
It is a nice place to live. Sure beats Mars. Or Uranus.
Here is our home within a home. Canada. Mountainous on
the left, flat in the middle, mostly rocky on the right and
frozen at the top.
The world used to look like
this a long time ago.
The land mass was all
squished up into a gigantic
“supercontinent” called
Pangaea.
How long ago
are we talking?
POS Checklist:
•
Describe the challenges in investigating the changes that
take place over hundreds of millions of years to Earth's
crustal plates, to past climates and to life forms.
Most of Canada was formed with the rest of the continents, around 4.5 billion years ago (bya). This time is called the Precambrian Era.
The rocks formed during the Precabrian era are the oldest on the earth.
There are many of these rocks found in the Canadian Shield in Ontario and Quebec.
The North American Craton (brown) comprised a portion of a series of
supercontinents, and has remained stable for about 600 million years.
This cratonic region is precambrian rock, exposed on the Canadian shield, and
The oldest rocks here in Alberta are only about 2 billion years old. By
comparison to the rocks in Eastern Canada they are quite young. Why is that?
The Earth is like an onion. It has layers.
Let's take a look at these layers:
As we get closer to the centre of the Earth, the layers become more dense.
Layers in the Earth include:
One interesting feature of the Earth’s crust had
been noticed by many early cartographers and
explorers. The outlines of many coastlines (like
South America and Africa) look like they would
fit together like pieces of a jigsaw puzzle.
• In addition, fossils of the late paleozoic (300 Ma) reptile, Mesosaurus, are found only in South America and Africa.
• If Mesosaurs could swim across the Altlantic ocean, then they could have
crossed other oceans as well, and would be more widely distributed across the globe. The fact that we don’t find theses fossils anywhere else suggests that South America and Africa were once joined.
Plate Tectonics: The main features of plate tectonics are:
1. The Earth's surface is covered by a series of crustal plates.
2. The ocean floors are continually moving, spreading from the
center, sinking at the edges, and being regenerated.
3. Convection currents beneath the plates move the crustal plates
in different directions.
4. The source of heat driving the convection currents is
radioactivity deep in the Earths mantle.
In 1915 Alfred Wegener proposed his theory of "continental drift"
in which continents plowed through crust of ocean basins, like
ice-breakers cutting through ice. Wegener’s theory faced a lot of
Plate tectonic theory proposes that continental and oceanic lithosphere both move as rigid or nearly rigid caps on top of the circulating (convecting) mantle. A
single tectonic plate can consist of only oceanic lithosphere or both oceanic and continental lithosphere, depending on where the plate's boundaries are located. The theory is supported by a huge body of geologic evidence – for instance
Convection currents in the mantle provide the motive force for tectonics.
Cold, denser materials convect downward and hotter, lighter material rises. The rising intrusion of magma into a spreading ridge provides a "ridge push" and the sinking of cold, denser oceanic slabs in subduction zones provides a "slab pull“ – forcing the tectonic plates to move relative to each other.
Convection cannot take place without a source of heat - within the Earth this comes from two main sources: radioactive decay and residual heat.
Radioactive decay involves the loss of particles from the nucleus of a “parent “ isotope to form an isotope of a new “daughter” element. The radioactive decay of the elements uranium, thorium, and potassium releases energy in the form of
heat, which slowly migrates toward the Earth's surface. Residual heat is gravitational energy
left over from the formation of the Earth -- 4.6 billion years ago -- by the "falling together" and compression of cosmic debris.
