Two Plates Diverged in a Yellow Wood EarthCache
Two Plates Diverged in a Yellow Wood
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Plate tectonic theory arose out of the hypothesis of continental drift proposed by Alfred Wegener in 1912 and expanded in his 1915 book The Origin of Continents and Oceans. He suggested that the present continents once formed a single land mass that drifted apart, thus releasing the continents from the Earth's core and likening them to "icebergs" of low density granite floating on a sea of denser basalt.
The Earth is made up of eight major and many minor plates. These plates are part of the lithosphere, the upper-most part of the interior of the Earth, and are in motion. These plates as is all of the lithosphere ride on the asthenosphere. Although solid, the asthenosphere has relatively low viscosity and shear strength and can flow like a liquid on geological time scales.
The location where two plates meet is called a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, andoceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being most active and most widely known
Three types of plate boundaries exist, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:
Transform boundaries occur where plates slide or, perhaps more accurately, grind past each other along transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). The San Andreas Fault in California is one example.
Convergent boundaries occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or acontinental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. The subducting plate melts as it moves deeper into the crust, producing volcanism. Examples of this are the Andes mountain range in South America and the Japanese island arc.
Divergent boundaries occur where two plates slide apart from each other. Mid-ocean ridges and active zones of rifting.
Here in York County where you are standing once roughly 500 million years ago was a divergent plate boundary. In this long outcrop is found metabasalt. The metabasalt is more dense than the surrounding schist of the Octoraro formation and did not weather as quickly as the surrounding schist.
Upon close examination, you may see small cavities in the rock. These are known as vesicles and were formed when gas bubbles in the lava broke, leaving a cavity.
To gain credit for this earth cache, please do the following:
1. Describe the rocks and the rock formation that you are viewing.
2. Look around the general area here and describe some of the other interesting rock outcroppings in the area.
3. Imagine you were looking at this plate boundary 500 million years ago, by using the clues of the outcroppings in the area seen today tell me what you think the plate boundary looked like in relation to these clues?
4. What are other examples of divergent boundaries in the world?
5. Take a photo of yourself with your GPSr in hand in front of the outcroppings in the area and post it with your log.
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