Evidence for Plate Tectonics Shape of Continents Rock Types Fossils Glaciers Mountain Chains Location of Earthquakes Mid-ocean ridges and ocean floor rock ages Magnetic Polarity Reversals Go over what will be covered in this ENTIRE UNIT (as today is the first day). All subjects covered in this unit will link back to how they show evidence for plate tectonics.

Shape of the Continents The edges of the continents appear to fit together like the pieces of a puzzle The shape of the continents themselves are evidence as they appear as though they could fit together, somewhat like puzzle pieces. And in fact, they used to be fit together, according to evidence in rock, sediment, and fossil records.

Alfred Wegener - Continental Drift Noticed that continents appear to fit together Hypothesized that continents were once together and had drifted apart Idea was called “Continental Drift” The first person to notice and document this trait was Alfred Wegener, and he called the idea/theory “Continental Drift”

Continental Drift When all the pieces (continents) are pushed together like a puzzle, the form a single continent. We refer to this continent, which appears to have once existed long ago, as “Pangea”.

Permian 225 m.y.a. Triassic 200 m.y.a. Jurassic 150 m.y.a. Cretaceous Pangea existed about 225 million years ago in the Permian. Over time (many millions of years), the continents separated and were pushed apart as a result of plate tectonics. Fossils that we find from certain time periods (for example, dinosaurs in the Jurassic) show that certain continents were still attached. We see animals that could never have swam or flown on the edges of continents in the form of fossils from the same time period. Today, the continents are fully separated, but this will change over time as well. You may be wondering “What evidence do we have to support this? How do we know this happened?” Today we will be exploring a variety of evidence supporting this theory, and the mechanisms for how the continents moved. Present Day

Rock Type Evidence Same sequence of rocks of same age can be found all over the world Indicates that when these rocks formed all these places were connected One source of evidence is found in geology, the study of rocks. We see similar rock types on the edges of continents that were once next to each other.

Fossil Evidence Same fossils found on many different continents Fossils of organisms that could not fly or swim between continents Continents were together when these animals lived, so they could walk from one continent to another Fossils also help tell the story, not only of continental drift, but of migrations. When looking at the fossil record, we can look at where they are found and how old they are. We see similar animals existing in similar time periods along the edges of continents that were once connected. This shows us migration trails as well.

Glacier Evidence Glaciers leave marks on rocks called striations that show which direction they move Glacial movement can leave striations that indicate the direction of movement. These striations can help indicate where continents were once joined as well.

Glacier Evidence Striations have been found in places too warm to have glaciers now Striations point in different directions If all the continents were together at the south pole, striations point in the same direction In lab today, glacial striations will appear in the form of arrows.

Mountain Chains Mountain ranges around the world appear similar to each other in composition and age. If we reconstruct the past arrangement of the continents, these ranges formed as one mountain range and were later split Similarities across mountain ranges, in the form of the age of the rocks, type of rocks and sediments, formation types and erosion, can indicate which ranges used to be connected but are now found on completely different continents.

Mountain Chains This is true of mountain ranges all over the world In this image we see older mountain ranges in purple and younger in orange.

Location of Earthquakes Do earthquakes appear randomly on this map? NO! Earthquakes and volcanoes can be classified as occuring near the surface, deep under the surface, or at intermediate levels. We can see that where two plates collide we get formation of earthquakes and volcanoes, particularly when an oceanic plate (which is more dense) collides with a continental plate (which is less dense). The oceanic plate collides with the continental, and being more dense, the oceanic plate sinks and is pushed down. This is called subduction, or a subduction zone. As this happens, we first see shallow, then intermediate, then deep earthquakes and volcanoes. This pattern is consistent across the entire globe.

Location of Earthquakes Earthquakes form a definite pattern- we know now that these are the edges of “plates” of rock that fit together to form the earth’s crust Other types of plate boundaries that we will study include more on subduction zones, as well as mountainous regions created by convergence zones, and faults (such as our local San Andreas fault) created by transform plate boundaries.

Mid-Ocean Ridges First discovered when scientists started to map the bathymetry of the oceanfloor Long chain of underwater mountains that stretches around the entire earth Ridges are formed in regions where oceanic plates are actually moving AWAY from each other. We can see a ridge forming where new lava is erupting from below the plates, and we can track that this pushes the plates (and therefore the continents) further away from one another.

Ocean Floor Rock Ages Rocks all over the ocean floor were dated using absolute dating Oceanic plates can be age-dated as they were formed from cooling lava on the ocean floor as it spewed out of the ridge zones. The younger rocks are found directly along the ridge system, while the oldest are seen along the edges of continents. In this image, blue represents older rock, whereas the dark red is newly-forming young rock.

Ocean Floor Rock Ages Rocks closest to the mid-ocean ridges were youngest Rocks farthest from the mid-ocean ridges were oldest Pattern of ages is the same on both sides of the ridges Indicates that new rocks are forming at the mid-ocean ridges! We can see that plate tectonics are continuing to drive continental drift, and that the process is occurring literally while we speak, although the results are only viewable over many millions of years.

The Theory of Plate Tectonics Explains all evidence Similar to continental drift, but more complex Earth’s crust is composed of “plates” that make up the crust under the ocean and on the continents Continents DO NOT float on the oceans Plate boundaries do not always occur at the edges of continents Earthquakes and volcanoes occur where two plates meet Scientists no longer use the term “Law” to describe something that has been successfully supported by the scientific community; instead, we now use the term “Theory” in its place. The theory of plate tectonics combines and summarizes all of the evidence found thus far into a singular explanation of the driving forces behind plate movement, as well as the results.

Plate Tectonics Today Today we can observe plate tectonics in a number of ways, mainly in formations found along specific plate boundaries. We will study these boundary zones, and how they cause things like volcanoes and earthquakes to form, in the upcoming chapter. Today, however, we will be looking at one of the original forms of evidence found all across the world: FOSSILS 