PART 7: PLATE TECTONICS, EARTHQUAKES & VOLCANOES 1. Theory of Plate Tectonics: a theory born in 1968 that there are massive rock plates under the surface of the Earth called tectonic plates that are in continuous motion due to convection currents which push and pull continents. Important People in History o F. B. Taylor: was the first to propose the idea of Continental Drift, although is idea was not accepted because he provided no evidence. o Alfred Wegner: was the first to propose Continental Drift with a lot of supporting evidence. His idea was not widely accepted because he could not explain why the continents moved (more later). o Arthur Holmes: he was the first to suggest an explanation for the movement of continents. He proposed that convection currents in the athenosphere caused the movement. o Alexander Du Toit: mapped out that the continents fit together almost perfectly when you include the less eroded continental shelves under the eroded landmasses. o Harry Hess & Robert Deitz: were the first to suggest the theory of Seafloor Spreading and said that no rocks on the seafloor can be older than 200 million years old. o Frederick Vine & Drummond Mathews: found magnetic reversals on the ocean floor, which led to magnetic polarity reversal theory. o Tuzo Wilson: a Canadian geologist who compiled all other theories mentioned above to create the Theory of Plate Tectonics. Evidence Supporting Plate Tectonics o Continental Drift: the theory that all of the continents have moved from their original position and continue to move throughout time. - Wegner s Hypothesis: he hypothesized that, hundreds of millions of year ago, all of the continents formed a super-continent called Pangaea. He said that Pangaea split into it s landmasses approximately 200 million years ago. Below are Wegner s main points of evidence. Evidence from Rock Formations: he found similar rock formations along opposite coastlines. Evidence from Fossils: he found similar fossils on opposite continents. Page 1 of 12
Evidence from Ancient Climates: he found similar glacial deposits, tropical fauna, etc. on opposite continents. Why was it rejected?: Wegner s hypothesis was rejected partially because it was widely believed that landmasses were permanent and could not change. He also could not explain what made them move in the first place his hypothesis that the continents ploughed through the ocean was not widely accepted. - Main Points of Continental Drift: Supercontinent called Pangaea existed 225 million years ago Pangaea split into two super continents called Laurasia and Gondwanaland about 200 million years ago Laurasia consisted of most of Asia, Europe and North America Gondwanaland consisted of Africa, Australia, Antarctica, South America and India These supercontinents split into their current positions about 150 million years ago - Summary of All Evidence Supporting Continental Drift Fit of the Continents: as proposed my Alexander Du Toit, the continents fit almost perfectly together with their continental shelves. Fossil Correlation: Wegner and other scientists found very similar fossils on opposite continents. Rock Types and Structures: almost identical rocks were found on opposite continents by Wegner and other scientists. Ancient Climates: glacial deposits were found in South America, Africa, India and Australia, all of which have had warm climates for millions of years. This suggests that they all surrounded Antarctica where they were once covered by glaciers. o Convection Currents: convection currents are circular currents that run in liquids that are heated. It is theorized that this type of heating of the asthenosphere is what causes the movement of tectonic plates. Page 2 of 12
- Process Within Earth: The heated material (heated by radioactive decay and residual heat) within the asthenosphere becomes less dense, rises, cools as it nears the surface, becomes more dense, sinks back down, heats again, and the cycle continues. These motions push and pull tectonic plates, causing all of the movement we observe from the Earth s surface. o Seafloor Spreading & Magnetic Reversals: seafloor spreading is the motion of the ocean floor spreading apart as a result of convection currents (studied by Hess and Deitz). Magnetic reversals refer to the striped change in magnetism along the ocean floor due to the movement on the ocean floor (studied by Vine and Mathews). - Sonar: system for the detection of objects under water and for measuring the water's depth by emitting sound pulses - Earth s Magnetism: the difference in polarity from the North to the South of the Earth. Magnetic Reversal: a complete reversal of the polarity of the North and South. - Isochron Map: a map of the ocean floor that connects rocks of the same age. Page 3 of 12
o Earthquakes and Volcanoes: earthquakes and volcanoes tend to occur in long rounded belts (along the edge of tectonic plates). As a plate dives under another plate and melts, heat that rises can form a volcano around a coastline. When two plates separate, the resulting hole in the crust can allow magma to escape. All of this movement can also cause seismic activity (earthquakes). o Fossil Magnetism: The permanent magnetism in rocks which indicate the direction of the magnetic field when the minerals became magnetized. o Ocean Drilling: further supported the theory of seafloor spreading and magnetic reversals. o Hot Spots: smaller volcanic chains (ie. Hawaii) that are not connected to larger ones (ie. The Ring of Fire) are evidence of hot spots. There is a weaker area of the mantle in the middle of a tectonic plate where magma can burn through and form volcanoes and eventually islands. As the plate moves, the hole in the mantle does too, allowing new magma to be burned through a new area of crust, creating another island. This process repeats until an Island Chain is formed. Page 4 of 12
Plate Boundaries: the areas where two tectonic plates interact. o Divergent Boundaries: when two plates are moving apart from each other. - In Oceans: magma can push the thinner crust upward and sometimes break through as a volcano. This upward motion creates a sub-ocean mountain range called an ocean ridge. - On Continents: although this is uncommon, the separation of these plates leaves a gaping hole in the crust called a rift valley. Volcanoes may form within the valley itself. o Convergent Boundaries: when two plates are moving towards each other. The denser plate will be pushed below the lighter plate this is called subduction. The denser plate is then destroyed (it melts), allowing heat to rise, creating volcanoes along the collision plane. This motion creates valleys or trenches. - Ocean + Ocean: when two ocean crusts collide, the denser plate will subduct, creating an ocean trench. Page 5 of 12
- Ocean + Continent: when a continent collides with an ocean plate, the ocean plate will sink (it is denser thinner but still denser), creating an ocean trench. - Continent + Continent: when a continent collides with another continent, the two plates will push and rise, forming a mountain range. o Transform Boundaries: when two plates run side-by-side in opposite directions. There is no creation of mountains or trenches, however there is a lot of seismic activity recorded (earthquakes). Newfoundland and Plate Tectonics: the island of Newfoundland is made up of three distinct regions involving two different tectonic plates. o Geological Formation - Iapetus Ocean: the ocean that originally separated the North American and African plates. Appalachian Mountains: mountains in Central Newfoundland that were formed by the squeezing out of the Iapetus Ocean. This mountain range runs down through the United States and all the way to Great Britain. They are not as apparent anymore as they have been eroded flat. - Tablelands: the table lands are areas on the West Coast that resemble a desert they have a lot of brown and red rock and very little plant growth. This was caused by a very rare occurrence called an Ophiolite complex the rocks are from the mantle of the Earth. Somehow, that section of the Earth s crust flipped over on itself. o Geological Layout: Newfoundland is separated into three zones. - Humber Zone (Western) - Mobile Belt Zone (Central) - Avalon Zone (Eastern) 2. Faults and Folds: two features seen in collision mountains that occur when stresses are greater than the strength of the rock and the rock deforms as a result. All rocks have some level of elasticity (the ability to bend without breaking and to snap back to the original shape) when the resistance of the elastic capability is reached, the rock breaks. Deep Earth: deep in the Earth, folding is more likely to occur (bending). Surface Earth: within the Crust or at the surface itself, a fault is more likely to occur (cracking). Fault: the cracking of rock as a result of extreme pressure within the Earth. o Parts: the parts of a fault are: - Hanging Wall: the top part of the rock above the fault plane. - Foot Wall: the bottom part of the rock below the fault plane. Page 6 of 12
- Fault Plane: the surface that separated in to two different pieces of rock. - Horst: an uplifted block of crust caused by faulting. - Graben: a valley formed by a downward faulting motion. o Types: the two types of faults are: - Dip Slip: faults with vertical movement. Normal Fault: when a hanging wall drops lower than the foot wall. This is caused by tension (pulled apart). Reverse Fault: when the foot wall drops lower than the hanging wall. This is caused by compression. Thrust Fault: when the hanging wall moves up and over the foot wall. - Strike Slip: faults with no vertical movement. Transform Fault: when the fault forces horizontal movement side-by-side. Fold: the bending of rocks that don t break. o Parts: the parts of a fold are: - Anticline: the upward hill within the rock ( Anti-gravity ). - Syncline: the downward valley within the rock ( You sin, you go down to hell ). - Limbs: the sides of anticlines and synclines. Page 7 of 12
- Fold Axis: the point where the limbs change the angle of direction. - Strike: the direction of the fold. - Dip: the angle of the limb against a horizontal axis. 3. Earthquakes: the vibration of Earth produced by the sudden, rapid release of energy. Seismology: The study of how seismic waves behave within Earth. Focus: The exact location within Earth where seismic waves are generated by sudden release of stored elastic energy. Most often located on a preexisting fault. Epicenter: The point on the surface of Earth directly above the focus. Causes: the causes of Earthquakes are: o Bombs: massive bomb blasts can cause earthquakes that can be sensed across the world. o Volcanic Eruptions: volcanic eruptions can cause enough vibrations to register around the world. o Slipping Faults: the movement and breaking of faults can cause major earthquakes. - Elastic Rebound: whenever a fault cracks, it is rebounding from its elastic capabilities. This sudden movement shakes the Earth in the form of an earthquake. Contribution to Science: the study of earthquakes has revealed the following to modern science: o Earth s interior in layered o Density increases with depth Page 8 of 12
o Outer core is a liquid Waves: the movement of forces from an earthquake moves in wave motions. o Body Waves: Originate from the focus and travel in all directions through the body of the Earth. - Primary Waves: P-waves move by compressing and expanding (push-pull motion) the material as it travels. Much like sound waves. These waves are fast, vibrate in the same direction as the wave motion, and can pass through any state of material. - Secondary Waves: S-waves travel through material by shearing it or changing its shape in the direction perpendicular to the direction of travel. They only pass through solids, are almost half the speed of P waves, and move in a similar fashion to the surface of the ocean. o Surface Waves: Surface waves differ from body waves in that they do not travel through Earth, but instead travel along paths nearly parallel to the surface of Earth. Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves. - Love Wave: Surface waves that cause horizontal shearing of the ground. They move in much the same way as a snake slithering across the ground. - Rayleigh Wave: Surface waves that cause both horizontal (side-to-side) and vertical (up and down) movement within the ground. o Seismogram: An instrument used to record seismic waves and the resulting graph (seismograph) that shows the vibrations. - Inertia: the principle that states that things that are at rest tend to stay at rest. Page 9 of 12
Mapping Earthquakes: the difference in arrival times of P-waves and S- waves is used in mapping Earthquakes. o Locating the Epicentre - Seismogram Records: To locate the position of an earthquakes epicenter, we need a seismogram reading from at least three different seismograph stations and a travel - time graph which shows the speed of both P- and S-waves. - Geographic Triangulation: using the above records, the distance from each station is determined and then measured radially. Using three stations will pinpoint one location that represent the distances found at each station. That point is the epicentre. - NOTE: To determine the distance at one station, find the time difference between the arrival time of both types of waves (using the graph on the previous page). Then find where on this graph the lines are separated by the amount of time on the y axis. The x point you are above is your distance from the epicentre. Page 10 of 12
Earthquake Intensity and Magnitude: there are two ways to measure how strong an earthquake is: o Modified Mercalli Scale: this scale measures the INTENSITY of the earthquake, which is assessed by the amount of damaged caused. This scale is NOT considered to be very accurate, as the following factors can have a major effect on the amount of damage: - Distance from the epicenter - Nature of the surface materials - Building design o Richter Scale: this scale measures the MAGNITUDE of an earthquake on a ten point scale and is based on wave amplitude (height) on a seismogram. This scale is considered to be accurate since it is based off a scientific measurement. - Wave Amplitude Tenfold: each point of magnitude represents a 10 fold increase in the wave amplitude. - Energy Released Thirtyfold: the actual amount of energy released in an earthquake is a 30 fold increase for each point on the scale. Reflection and Refraction: when a wave increases in speed, it will be refracted upward. When a wave decreases in speed, it will be refracted downward. Seismic waves tend to increase in speed as they move deeper into the Earth. 4. Volcano: An opening in Earth s crust through which igneous matter (lava, ash, cinder, and gases) are erupted. Types of Volcanoes o Shield Volcano: a very wide and large volcano formed from thin lava flows that are mainly basaltic (ex. Hawaiian Islands). o Ash & Cinder Volcano: Built mainly of ejected pyroclastic materials, such as, ash and cinder. They are usually steep and relatively short. They have thick explosive lava. Page 11 of 12
o Composite (Strato) Volcano: a combination of shield and ash/cinder volcano. These volcanoes are considered to be the most dangerous, as they have a continuous flow of lava with large explosions as well (ex. Mt. Fuji). Plate Boundary Volcanism: volcanic activity that occurs along plate boundaries. They occur at the following boundaries: o Spreading Centres (Ridges) o Subduction Zones (Trenches) o Intraplate Volcanism (Hot Spots) Products of Volcanoes o Volcanic Edifice: dust and hardened lava that build up on the volcano structure itself. o Pyroclastics: volcanic fragments. - Tephra: solid volcanic fragments. - Flows: fast moving semi solid dust clouds of volcanic fragments. o Volcanic Gas: sulphur, oxygen, and water vapour are among the gases released from volcanic eruptions. o Lahars: a volcanic mudflow composed of ash and water that follows river channels. It hardens like concrete. Page 12 of 12