Canada’s Geologic History Introduction

Cross-section of Earth The Earth’s surface is a thin layer of moving plates, floating on more fluid layers of molten rock surrounding the Earth’s core.

About 300 million years ago (MYA), the plates were one large plate or supercontinent called Pangaea. Over millions of years, the plates slowly moved apart to form the present continents. This is why many of the continents such as Africa and South America look as if they could fit easily together like a jigsaw puzzle.

Continental Drift and Alfred Wegener In 1915, Alfred Wegener suggested the idea of Continental Drift to explain the movement and relative position of the Earth’s continents. Wegener based his theory on four observations: - Jigsaw puzzle outline of continents - Similar fossils of the same age on different continents - Similar rocks in mountain ranges on different continents - Remnants of an ice age in present warmer regions of Earth

Plate Tectonics Continental Drift is caused by the movement of the Earth’s plates. This theory is called Plate Tectonics. Plate Tectonics is a relatively new idea…developed between 1960 and The plates move constantly, in different directions at a very, very slow rate (about 2-10 cm/yr). Plate movement is caused by many factors including currents in the Mantle and the separation of the ocean floors.

Continental Drift - Proof The Appalachian Mountains in North America bear a striking resemblance to the Caledonian Mountains in Europe (e.g., similar age of rocks) Although thousands of kilometres apart, the two mountain ranges must have been formed by the same geologic forces

More Evidence for Continental Drift Fossil records indicate that Africa, India, Australia and South America were all covered by ice sheets approximately 250 MYA. How could ice form in areas with such warm climates? With Wegener’s theory, these land masses were, at one point in geological history, located closer to the South Pole…and its extremely cold climate

Key Principles The Earth’s surface is made up of about 20 different plates floating on a liquid layer of molten rock called the Mantle. The plates are constantly moving at a slow rate. One of the causes of plate movement is CONVECTION CURRENTS in the Mantle. At the core, the Mantle is hot, while at the crust, it is cooler. These heat differences create a current (i.e., hot rises, cools falls). These currents apply force to the plates causing them to shift and move.

These plates are constantly interacting with each other In processes called FOLDING or FAULTING. These interactions are responsible for geologic occurrences which shape the world’s geography. For example, mountain ranges such as the Rockies in British Columbia were formed by tectonic forces. As well, Canada’s fossil fuel resources were formed by the movement of plates. Gas, oil and coal were formed when Canada was located in a warmer climate.

Types of Plate Boundaries TRANSFORM BOUNDARIES – These occur when two plates grind past each other in a side to side motion (e.g., San Andreas fault in California) DIVERGENT BOUNDARIES – Two plates moving apart from each other form this type of boundary. The cooling of the molten rock when it reaches the surface causes new crust to be formed constantly (e.g., Mid-Atlantic Ridge) CONVERGENT BOUNDARIES – Two plates moving towards each other create this fault. Typically one plate is forced above the other, contributing to the formation of mountain ranges (e.g., Andes mountains in South America)

Plate Boundaries

Earthquakes and Volcanoes Why are most earthquakes and volcanoes located near plate boundaries? As different plates collide and interact, they cause a number of phenomena. Two moving plates, typically on a Transform Fault, could potentially snag on each other and build up energy. If this energy is released suddenly, it causes an earthquake. If two colliding plates build up friction, that heat can break down the crust in certain areas causing vents to form from the outer core of magma to the surface. These take the form of volcanoes.

Connections? Is there any link between the fault lines between tectonic plates and the sites of major earthquakes and volcanoes. The region encircling the Pacific Ocean is called the Ring of Fire.

TASK - ACTIVITY You may choose between the following two exercises on Plate Tectonics theory. 1) Using any materials in the classroom create a model or diagram which demonstrates the movement of plates on the Earth’s surface. Be sure to include both the surface plates AND the Earth’s mantle and core. Consult page 92 in your textbook for additional information. 2) Using any materials in the classroom create a model or diagram which explains how volcanoes or earthquakes are caused by plate tectonics. Consult page 95 in your textbook for additional information.