Sec.2 slow motion on planet earthPresentation Transcript
Ch. 1, Section 2 Slow Motion on Planet Earth
Some questions we will answer today:
How is the earth always changing?
What forces inside the earth create and change
landforms on the surface?
What is the theory of plate tectonics and how
does it work?
What two theories help make up the theory of
What is continental drift and sea floor
What happens when the plates crash together,
pull apart, and slide against each other?
Most of these changes in the earth’s surface takes place so slowly that they are not immediately noticeable to the human eye.
The idea that the earth’s landmasses have broken apart, rejoined, and moved to other parts of the globe forms part of the
plate tectonic theory.
Plate Tectonic Theory About forty years ago, scientists exploring the seafloor found that it is full of tall mountains and deep trenches, a single seafloor mountain chain circles Earth and contains some of Earth’s tallest mountains. Along this mountain chain is a deep crack in the top layers of earth. Here the seafloor is pulling apart and the two parts are moving in opposite directions, carrying along the continents and oceans that rest on top of them. These pieces of Earth’s top layer are called tectonic plates . They are moving very slowly, but constantly. (Most plates are moving about as fast as your fingernails are growing -- not very fast!) Currently Earth’s surface layers are divided into nine very large plates and several smaller ones.
According to the theory of plate tectonics, the earth’s outer shell is not one solid piece of rock. Instead the earth’s crust is broken into a number of moving plates. The plates vary in size and thickness.
How the mountains or volcanoes were formed or how earthquakes happen?
As mentioned earlier, those tectonic plates are always moving. They are always moving:
pulling away from each other
or sliding past each other.
These areas are likely to have a rift valley, earthquake, and volcanic action.
For example: Here, the San Andreas Fault lies on the boundary between two tectonic plates, the north American Plate and the Pacific Plate. The two plates are sliding past each other at a rate of 5 to 6 centimeters each year. This fault frequently plagues California with earthquakes.
Suppose you study the motion of a plate. You find that the plate moved a distance of 5 centimeters in one year. So, the speed of the plate is 5 cm/yr. You can use this speed to predict how far the plate will move in 1000 years. Distance = Speed X Time Distance = 5 cm X 1000 yr = 5000 cm 1 yr