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Chapter 2

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Environmental Science
Lisa Tutty

EESA06 READING #2 CHAPTER 2 – PLATE TECTONICS What are plate tectonics? Tectonics – the study of the origin and arrangement of the broad structural features of the earth’s surface including folds, faults, mountain belts, continents, and earthquake belts Plate Tectonics – the Earth’s surface is divided into a few large, thick plates that move slowly and change in size; dominates geologic thought because it can explain so many features Plate boundaries – plates move away from one another, past one another, or towards one another; intense geologic activity occurs here Eight large plates/smaller plates – makes up the outer shell of the Earth (the crust and upper part of the mantle) Plate tectonic theory developed by: Continental drift – continents move freely over the Earth’s surface, changing their positions relative to one another Sea floor spreading – A hypothesis that the sea floor forms at the crest of mid-oceanic ridges, then moves horizontally away from the ridge crest toward an oceanic trench. – Two sides of the ridge are moving in opposite directions How did plate tectonics theory evolve? The Early Case for Continental Drift Alfred Wegener • Continental drift • Examined continents that have identical late Paleozoic rocks and fossils Pangea – continents form a giant supercontinent  Laurasia –northern supercontinent (North America)  Gondwanaland – southern supercontinent Paleoclimatology – study of ancient climate – Distribution of Late Paleozoic glaciation strongly supports the idea of Pangea – Examined ancient sedimentary rocks and discovered that paleoclimatic reconstructions suggested polar positions very different to those at present Skepticism about Continental Drift – Wegener believes that the less dense continents drifted through oceanic crust, crumpling up mountain ranges on their leading edges as they pushed against the oceanic crust – Calculations of these forces showed them to be too small to move continents, so Wegener’s ideas received little support Renewed Interest in Continental Drift – Study of the Sea Floor o Oceans cover more than 70% of the Earth’s surface and make the study of sea-floor rocks a difficult and challenging task -> new methods of investigating sediment + topography have provided most of the information that led to the concept of plate tectonics o Samples of rocks and sediments can be taken from the sea floor by: 1. Rock Dredge – open steel container dragged over the ocean bottom at the end of a cable 2. Corer – a weighted steel pipe dropped vertically into the mud and sand of the ocean floor 3. Sea-floor drilling – drilling a hole in the deep sea floor o Submersibles – small research submarines for geologists; used to observe, photograph, and sample rock + sediment o Single-beam echo sounder – measures water depth and draws profiles of submarine topography 1. A sound is sent downward from the ship and bounces off the sea floor and returns to the ship 2. Water depth = determined from the time it takes the sound to make the round trip 3. Multibeam sonar uses a variety of sound sources to produce detailed shaded relief images of the sea-floor topography o Sidescan sonar – measures the intensity of sound reflected back to the tow vehicle from the ocean floor and provides detailed images of the sea floor and information about sediments and bedforms o Seismic reflection profiler – works on the same principles as the echo sounders but uses a louder noise at a lower frequency; sound penetrates the sea floor and reflects from layers within the underlying sediment and rock -> records water depth and reveals the internal structure of the rocks + sediments o Magnetic, gravity, and seismic refraction surveys can also be made at sea, where deep-sea cameras can be lowered to the bottom to photograph the rock and sediment o Using data collected by these methods, detailed maps of the ocean floors were dreated in the early 1960s that showed extensive mountain chains, trenches + fracture and fault zones – Geophysical Research o Polar wandering  Magnetic poles are located close to the geographic poles  Magnetic poles move from year to year o Magnetism of old rocks can be measured to determine the direction and strength of the magnetic field in the past o Paleomagnetism – study of ancient magnetic fields o Looking at the polar wandering paths for North America and Europe don’t make sense UNLESS if North America and Europe were joined at one point Recent Evidence for Continental Drift • Refined rock matches between now-separated continents (EX. South America and Africa) • Rocks are similar in type, structure, fossils, age, etc. • Glacial striations also prove the linkage between Africa and South America What is sea-floor spreading? Harry Hess ♦ The Sea floor might be moving too like continental drift ♦ Sea floor spreading – the sea floor moves away from the mid- oceanic ridge as a result of mantle convection; developed in 1962 by Hess Subduction – the sliding of the sea floor beneath a continent or island arc Convection – is a circulation pattern driven by the rising of hot material and the sinking of cold material ♦ Hot materials have a lower density so it rises ♦ Cold materials have a higher density so it sinks ♦ If convection drives sea-floor spreading, then hot mantle rock is rising under mid-oceanic ridges (high heat flow is caused by the rise of this hot mantle rock) ♦ The basalt eruptions on ridge crests are also related to this rising rock ♦ As hot rock continues to rise, the forming mantle rock moves away from ridge crests on either side of the ridge ♦ This movement creates tension at the ridge crest, cracking open the oceanic crust to form rift valleys and associated shallow-focus earthquakes ♦ As the mantle rock moves away, it carries the sea floor with it and cools (becomes denser) ♦ It is so dense that it sinks back into the mantle, and this downward plunge of cold rock accounts for the existence of oceanic trenches and their low heat flow values Andesitic volcanism – it is produced by the sea floor moving downward into the mantle along a subduction zone. This interaction between the moving sea-floor rock and the stationary rock causes these volcanoes and even earthquakes. • Formed on the edge of a continent or an island arc How old is the sea floor? • Scientists have found that the rocks + sediments of the sea floor proved to be younger than 200 million years old (true for materials from the deep sea floor, not the ones around the continental margins) • Most of the deep sea floor rocks were formed during the Mesozoic and Cenozoic eras, but not earlier • Differs from continents that have some 3-4 billion year old rocks • Young age of the sea floor is explained by sea-floor spreading • Young sea floor is being formed by basalt eruptions at the ridge crest • Basalt is then carried sideways by convection and is subducted into the mantle at an oceanic trench • Old sea floor is being destroyed (at trenches) and new sea floor is being formed (at ridge crests) • Age of the sea floor becoming progressively older toward a trench What are plates and how do they move? Continental Drift and Sea floor spreading Formed Plate tectonic theory Plate – is a large, mobile slab of rock that is part of Earth’s surface • Made up of sea floor, continental or oceanic rock Lithosphere – rigid outer shell of earth that includes rocks from the crust and the uppermost mantle • Continental lithosphere is thicker • Increases in age + thickness with distance from the crest of the MOR Asthenosphere – below the lithosphere, which is a zone of low-seismic- wave velocity that behaves plastically because of increased temperature and pressure • Allows the plates to move • Below this is mantle rock Plates made up of sea floor can be subducted down into the mantle forming oceanic trenches Plate leading edge made up of continental rock won’t subduct because continental rock is less dense than oceanic rock Three types of plate boundaries: 1. Divergent plate boundary – plate boundary moving APART 2. Convergent plate boundary – plates moving TOWARDS each other 3. Transform plate boundary – move HORIZONTALLY past each other How do we know that plates move? Paleomagnetic Evidence Magnetic reversals – changes in the polarity of the magnetic field, when the north and south magnetic pole exchange places ♦ Normal polarity – magnetic lines flow from the South Pole to the North Pole and our compass needles point to the north ♦ Reversed polarity – magnetic lines run from North Pole to South Pole and our compass needles point to the South Paleomagnetism – study of magnetic fields recorded in rocks helps us understand the magnetic fields in the past ♦ Many rocks behave like compasses and record both the strength and direction of the Earth’s magnetic field at the time they are formed Magnetic polarity time scale – this is constructed by using stacked continental lava flow to record the pattern of magnetic reversals over time ♦ Magnetometer – an instrument that measures the strength of the Earth’s magnetic field. It is taken over land surface or sea. ♦ Anomaly: any deviation of magnetic strength from average readings Marine Magnetic Anomalies • Magnetic anomalies on the sea floor are arranged in bands that lie parallel to the rift valley of the mid-oceanic ridge • Positive and negative anomalies form a stripelike pattern parallel to the ridge crest The Morley-Wine-Matthews Hypothesis • The pattern of magnetic anomalies at sea matches the pattern of magnetic reversals of lava flows on the continents 1. Opening of cracks within the rift valley on the mid-oceanic ridge crest 2. They are filled by basaltic magma 3. Cools and forms dikes (records Earth’s magnetism at th
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