LESSON 2.2 - EXPLORING THE EARTH'S PAST.pptx
- 1. EXPLORING THE PAST: Theories about the History of earth’s Landscape and the Geologic Time Scale
- 3. CONTINENTAL DRIFT In 1912, geophysicist Alfred Wegener (1880–1930) developed the concept of and hypothesized the continental drift theory. He claimed that Earth used to have only one supergiant land mass where all the continents came from.
- 4. CONTINENTAL DRIFT He called this massive land mass PANGAEA. According to the hypothesis, Pangaea broke apart and each land mass “drifted” away from each other in different locations.
- 5. CONTINENTAL DRIFT Initially, two giant continents were formed: - LAURASIA which comprised the northern continents today - GONDWANALAND which comprised the continents in the present Southern Hemisphere.
- 8. CONTINENTAL DRIFT The separation continued until the present-day continents were formed. Wegener supported his theory with references from past authors with similar ideas.
- 11. WEGENER’S EVIDENCES TO SUPPORT THE CONTINENTAL DRIFT THEORY 1. Similarity of fossils found in different continents. 2. Presence of tillites in areas whose present climates do not suggest glacial formation
- 12. WEGENER’S EVIDENCES TO SUPPORT THE CONTINENTAL DRIFT THEORY 3. Presence of coal seams in polar regions. 4. Continuity of rock layers found in different continents. 5. Similarity of rock types in different continents.
- 15. PLATE TECTONICS As early as 1929, Arthur Holmes (1890–1965), a British geologist, suggested the idea of THERMAL CONVECTION as the driving force for the movement of the continents.
- 16. PLATE TECTONICS The concept of thermal convection, as Holmes put it, is based on the fact that as a “substance is heated, its density decreases and rises to the surface until it is cooled and then sinks again.”
- 17. PLATE TECTONICS The repeating process of heating and cooling may produce a current that is strong enough to make continents move.
- 18. PLATE TECTONICS Holmes further suggested that thermal convection works like a “conveyor belt” where the pressure that goes up could break a continent apart. The broken pieces can be carried by the same currents to opposite directions.
- 19. PLATE TECTONICS While the basis for the movement of continents progressed, geologists started to use a more precise term to refer to the moving piece of crust as “plate” because it is believed that continents are not the only ones moving.
- 20. PLATE TECTONICS The boundaries of tectonic plates were accidentally discovered and eventually studied during the magnetic surveys of the ocean floor and the seismic studies for nuclear testing
- 23. SEA FLOOR SPREADING The concept of seafloor spreading was developed by geologists Harry Hammond Hess (1906–1969) and Robert Dietz (1914–1995). They coined the term seafoor spreading and showed how it aligned with the unaccepted ideas of continental drift by then.
- 24. Seafloor spreading is a continuous process where tensional forces on both sides of the plates cause them to constantly move apart. Magma rises to the surface from the mantle. In time, the magma is cooled by seawater and forms the oceanic crust. New rocks then form in this area. SEA FLOOR SPREADING
- 26. - Since the newer rocks are younger in geologic features, scientists are able to determine areas of seafloor spreading because of the types and compositions of the new rocks in those areas. - Rocks from the separated plates would have similar rock types and compositions while the rocks at the center of the ridge would be mostly igneous in type. SEA FLOOR SPREADING
- 27. - Usually, seafloor spreading happens along mid-ocean ridges. This is exhibited in the actual site of seafloor spreading occurring at the Mid-Atlantic Ridge. - The new oceanic crust is found in the middle of the Atlantic Ocean whose geological composition is different from those of the North American plate and the Eurasian plate, and also those of the South American plate and African plate. SEA FLOOR SPREADING
- 29. - Ocean basins are formed from a series of processes beginning with the separation of two diverging plates where molten rock materials well up from the underlying mantle into the ridge or the gap between the diverging plates, solidifying into an oceanic crust. Ocean Basin Formation
- 30. - To explain how ocean basins develop from breaking up, drifting, and colliding of continents, John Tuzo Wilson proposed the cycle of ocean basin evolution, now known as the WILSON CYCLE. Here are the stages of the Wilson cycle: Ocean Basin Formation
- 32. - In this stage, a continent starts to break apart due to the movement of the asthenosphere under it. The movement is primarily caused by the convection current in the mantle. As the continent breaks apart, the rift valley starts to form. Embryonic Stage
- 33. - In this stage, the rifting continues to the point that the water starts to fill the gap between the previously connected continents. A gulf may be formed in this stage Juvenile Stage
- 34. - The continuous rifting widens the gap, which leads to the formation of new oceanic crust. It will be covered by an ocean. Mid-oceanic ridges form under this stage Mature Stage
- 35. - This stage is reached when the subduction prevails more than the spreading of the plates. Gravity will pull the plates downward, which will shorten the gap between the continents. Trenches are common in this stage of the ocean basin evolution Declining Stage
- 36. - At this stage, continents move toward each other but will not meet just yet. The ocean that used to separate them is narrowing. Terminal Stage
- 37. - This is the last stage of the Wilson cycle. In this stage, continents have now collided with each other. Their collision will cause the formation of mountains. Saturing Stage
- 38. DATING METHODS: Establishing the Timeline of the Life on Earth
- 39. The capability of sedimentary rocks to hold valuable information of Earth's past is attributed to the way it forms. Sedimentary rocks form by arranging in layers in a process called STRATIFICATION.
- 40. - But the type of rock that bears much importance in as far as studying Earth's past is the SEDIMENTARY ROCK because it can provide lots of relevant information such as climate records and previous environmental conditions. It is also the only type of rock that CAN HOLD FOSSILS.
- 41. - Fossils are any remains of organisms or traces of their activity preserved in rocks or other materials. - Fossils form in one of two ways: when the body of an organism is immediately buried after its death or when an organism is buried alive to die eventually Fossils
- 42. - Fossils that are used as markers of the main events of Earth's past are called index fossils. - An index fossil should be easily recognizable, abundant, and widely distributed in different locations. Fossils
- 44. RELATIVE DATING
- 45. - Relative dating is a method used to determine the relative order of geologic events. This is done through stratigraphy (succession of rocks) where the order of rock formations correlates to geologic time. Relative Dating
- 46. - This method does not provide actual numerical dates for the rocks, but all are just estimates based on the profile of the strata which includes chemical composition, rock type, and presence of organisms Relative Dating
- 48. ABSOLUTE DATING
- 49. - Absolute dating methods can tell which sediments were deposited first and also the approximate age of the specimen. The most used and accepted form of absolute dating is radioactive decay dating. - Most absolute dating makes use of radiometric methods, wherein radioactive minerals are used to compute the age of rocks. Absolute Dating
- 50. - Relative dating tells how old something is in relation to other objects, but cannot provide a year or specific date of use. - In contrast, absolute dating provides a specific calendar year for the occupation of a site. - Relative dating considers how old artifacts and sites are, in comparison to other artifacts and sites. Relative and Absolute Dating
Editor's Notes
- #14: Wegener’s theory initially did not gain acceptance from the scientific community because he could not explain why the continents moved. He just asserted that continents had been pulled apart by Earth’s rotational force and some astronomical influences. Hence, his theory was given little interest that time. However in the 1940s, during an initial survey for ocean floor mapping and other geological explorations, certain findings led to the discovery of more accurate pieces of evidence that would lead to the explanation of the moving continents. Thus, the birth of the plate tectonics theory.
- #26: (usually of sedimentary type because of all the exogenous processes that these rocks were subjected to)
- #27: The plate tectonics theory has explained that despite the geologic processes that are constantly reshaping Earth, there is really no change in its overall volume and size. Hess and Dietz reasoned that if the crust is expanding along the oceanic ridges, it must be shrinking somewhere else. They did prove this by studies on oceanic crusts descending along the continental margins where oceanic trenches are forming. This whole concept of expanding (seafloor spreading) and shrinking (crustal deformation) is driven by convection currents, as asserted by Hess and Dietz.
- #38: Different types of rocks provide hints about Earth's past. Studying igneous rocks can provide records of volcanic activities and can also be used to know the specific age of a material. Metamorphic rocks give clues about the movement of plates and how the continents drifted away from one another.
- #39: As deposition, compaction, and cementation happen, sedimentary rocks form. Over time, new sediments will accumulate on this sedimentary rock to form a new rock layer.
- #45: The topmost layer suggests the most recent. In a similar manner, the oldest rocks are understood to be at the bottom.