ONACD - Editable curriculum designed for teachers by teachers Plate Tectonics Evidence of the Process ONACD –Editable Curriculum designed for teachers by teachers

ONACD - Editable curriculum designed for teachers by teachers Evidence for Continental Drift Theory By looking at the shape of the plates (especially the continental plates) you will notice that they can fit together like a puzzle. At one point in time the continents were joined as one large mass called Pangaea. (Not to be confused with May 10 th, Pangaea day where the humans of the world celebrate being together and sharing the earth.) Look at the diagram (following slide) of Pangaea and see if you can identify the following 7 plates: North America, South America, Africa, Eurasia, Australia, India, and Antarctica.

ONACD - Editable curriculum designed for teachers by teachers Pangaea A – Eurasia B – North America C – South America D – Africa E – Antarctica F – India G – Australia Some of the smaller pieces make up some of the smaller plates (Ex Pilipino plate).

ONACD - Editable curriculum designed for teachers by teachers Pellegrini’s illustration The observation that the continents fit together began in the 1500’s with map makers. By the 1800’s many others made the same observations. Look at Antonio Snider- Pellegrini's illustration from 1858:

ONACD - Editable curriculum designed for teachers by teachers Fossil Evidence It wasn’t until the early 1900’s that evidence started to be found that could help prove this theory. Alfred Wegener ( ) pursued the theory of “continental drift” and noticed that matching fossils could be found in distinct locations that corresponded to where the continents joined.

ONACD - Editable curriculum designed for teachers by teachers This was not enough evidence to convince many scientists and the theory did not become accepted (in fact it was opposed) until the 1950’s when more evidence on plate tectonics agreed with the theory of continental drift. These other evidences included mountain belts and paleoglaciation. We have already looked at why mountain belts form along the edge of some continents when we looked at plate boundaries.

ONACD - Editable curriculum designed for teachers by teachers Evidence of Paleoglaciation Fossils weren’t the only thing that left remains when the continents were joined as Pangaea. When glaciers cover the land and then retreat they leave scars, large random rocks and a definite boundary. A boundary can be observed along the following locations:  Antarctica, the east coast of South America, southern portion of Africa, west coast of India, west coast of Australia Before the theory of continental drift these glaciations appeared to be random and did not make sense as many of these regions are in the tropics.

ONACD - Editable curriculum designed for teachers by teachers However, if you take into account that these regions formed a polar area during Pangaea it is understood. The “random” paleoglaciation marks form one boundary when the continents are joined as Pangaea.

ONACD - Editable curriculum designed for teachers by teachers Spreading ridges and Magnetic Reversals The discovery of spreading sea floors convinced many scientists that the theory of continental drift was true. This revealed that the ocean plates weren’t as rigid as they were once suspected. Another observation was made within the spreading seafloor. Think back once again to the Mid-Atlantic ridge. As new material comes out of the ridge it is pushed both to the east and to the west. As the molten rock cools, the compounds in the rock solidify as crystals with magnetic fields. These magnetic fields align with the North-South poles on earth. For some reason the Earth’s magnetic field reverses (the north pole becomes a south pole). This happens sometimes within years and sometimes it takes millions of years. These were first observed in the 1920’s from cooled lava around Japanese volcanoes.

ONACD - Editable curriculum designed for teachers by teachers In the 1950’s oceanographers observed long bands running parallel to the Mid-Atlantic ridge and the pattern was symmetrical on each side of the ridge. This gave strong evidence that the ocean floor was moving!

ONACD - Editable curriculum designed for teachers by teachers Ring of Fire As we have noticed earlier, earthquake and volcano activity form continuous boundaries around the plates. The combination of all the above evidences have convinced scientists that the earth’s surfaces is moving. But there is still much work to be done to better understand what is causing the movements.