Plate Boundaries and Their Many Motions

13 05 2008

Plate BoundariesFrom our last posts we discussed the continental drift amongst the many plates all over the Earth’s crust. We now understand that these plates have motion. Thanks to a geologist named Harry Hess, we found that the seafloor may be spreading apart from one another which he used from the theories of geologist Arthur Holmes. He believed that molten rock comes from beneath the Earth’s crust along “mid-oceanic ridges.”

Hess knew that people would then ask, “well, isn’t Earth gonna get huge?” Hess would respond with, “well, no because the seafloor will sink back into the deep ocean trenches through subduction zones.” Hess gave Wageners unexplained theory validation.

Why is this important? Because plates are moving, this causes motion and earthquakes. The discovery of the mid-oceanic ridges opened doors to not only plate motion but the age of the ocean floor! Now there are different ways in which the plates move. The different types of plate boundaries that all have relatively different motion, and characteristic geologic features.Convergent Plate

The first is a Convergent boundary, where one plate is literally going over another plate. Through a subduction zone (where one plate goes under another) this causes movement and earthquakes. Thrust faults are then created because the upper plate is being pushed into a fold or broken by thrust faults. Rock layers also scrape off the ocean floor and are then stacked into piles against the upper plate creating island and mountain chains.

Another boundary is the Divergent boundary: this is where the
pDivergentlates separate and move apart. They often form a rift zone. Most are located on the oceanic floor where new seafloor is created at the separating edges. One example is the Mid-Atlantic Ridge.

Another plate boundary is called the Strike-Slip or Transform Fault where the plates are sliding against one another without spreading apart or going over or under each other. They are also found on the ocean floor. The San Andreas fault in California is a prime example of a Transform Fault on land.

Transform Fault

Click to see an animation form of a Thrust Fault, Divergent Boundary, Transform Fault.

Why are these boundaries important to understand? How do they help geologist and seismologist understand earthquakes?

Here is a quick video interpretation of plate boundaries, I hope you find it as interesting as I did! Enjoy!


Difference Between Earthquake Focus and the Epicenter?

12 05 2008

There is a lot of terminology that can confuse geographic enthusiast. Focus, Hypocenter, Epicenter? What does that exactly mean? When we spoke about seismic waves we mentioned that they originate from different areas. The point of which the wave originates is what we call the Earthquake focus (foci plural). It is also known as the Hypocenter. These foci of natural earthquakes are found at different depths below the ground surface. There are three levels, Shallow at 10-100 km below, Intermediate 70-300 km and Deep foci at 300 km or below.

Shallow earthquakes create the most damage and 3/4 of total energy released of earthquakes in the world. They unlike intermediate and deep focus earthquakes, shallow earthquakes occur at all type of plate boundaries like ocean trenches, subduction zones, etc.

So what is the epicenter of an earthquake, we always hear that. “The epicenter of the quake.” The point on the ground surface directly above the focus is known as the epicenter [Bolt].

There is also another amazing geographical phenomenon that occurs. It is a major tectonic feature related to foci and it is called the Wadati Benioff (dipping seismic) zone. This is a narrow zone, that dips under the Earth’s crust to depths of up to 700 km. It is a deep foci that generally occurs in Island arcs and deep ocean trenches.

How do foci and epicenters relate to earthquakes? Why do we study them? What are some of the biggest quakes and are they from shallow foci or deep?

Click here for some excellent, fun and fast Earthquake Facts!

This is a photo of the destruction created after a great earthquake occured in Loma Prieta, CA in October 1989.

“The epicenter was located at 37.04° N. latitude, 121.88° W. longitude near Loma Prieta peak in the Santa Cruz Mountains, approximately 14 km (9 mi) northeast of Santa Cruz and 96 km (60 mi) south-southeast of San Francisco.”


Alfred Wegener, What did he do?

12 05 2008

Alfred WegenerAlfred Wegener did something that many of us only dream of doing. He had wanted to elaborate on an opinon of Earth that was totally out there and tried to prove it! He was one of the many scientist who believed in the idea of one large continents that he imagened roughly fit together like a puzzle and wanted to go out on a limb to prove this theory. Scientest like Wagener imagened that Earth looked something like this at one point: One single landmass.

Alfreds theory

He argued that the similarity of these continents and the fossils found on the now distanced continents were evidence that these continents were drifting pieces of an ancient “supercontinent” [Bolt]. Wegener suggested that the lighter thicker coninental blocks floated above the oceanic crust millions of years ago similar to an iceburg in the sea.

The idea of “Float and Drift” is his explanation of the geographical phenomenon that occurred when oceanic crust similar to that of tar, floated to the surface and for thousands of years reached a balance and then drifted. We call this large floating continent Pangaea. However, Wegeners idea was not as easily accepted because he did not find reasoning to identify the strong forces that would propel these continents away from one another. What drove them apart? How did these plates drift through such a sea floor? Alfred would need the assistance and help of two other important figures in plate tectonics Harry Hess and Arthur Holmes to later make this plate tectonic, pangaea, and “floating and drifting” theories accepted in the science community. But they are coming up next!

How did Alfred’s theory assist with understanding Earthquakes and continents? Does this affect those theories? Why is it so important to the study of seismology?

Here is a great article about Alfred Wegener with more description and details about his theories and discoveries. Enjoy!

CLICK HERE, to see an article about Alfred Wegener in the NEW YORK POST!