1. Unit 5: Plate Tectonics
Part 2: Tectonic Plates

2. Motion of Earth’s Surface
Faults are breaks in rock along which the rock can slide.
Evidence at the locations of faults show that segments of Earth’s surface shift over time.
To investigate the motion of the lithosphere on a global scale, scientists use Global Positioning Systems (GPS) to make measurements.
The map below shows the motion of the lithosphere as velocity vectors-arrows that show speed and direction.
The longer the arrow, the faster the lithosphere is moving.

3. Patterns in Earthquakes and Volcanoes
An earthquake occurs when energy is released as masses of rock break or move.
Scientists measure the shaking to determine the precise location of an earthquake’s epicenter-the point on Earth’s surface above the earthquake’s starting point, or focus.
Earthquakes are measured on the Richter Scale, which assigns earthquakes a magnitude. This is a measure of the earthquake’s intensity.

5. The Richter Scale

7. Finding Earthquake Epicenters
Scientists use a technique called triangulation to determine the epicenter of an earthquake.
They use data from three different seismogram stations, difference in arrival time between P-waves and S-waves. This is then converted to a distance using a standard chart.
Using a scale factor, the distance is scaled down to fit on a map. A compass is then used to show all possible locations from that seismogram station that the earthquake could have occurred at.
This is done two additional times. The intersection of all three circles represents the epicenter.

9. Earthquake Distribution
The distribution of earthquakes and volcanoes on Earth’s surface helps scientists analyze the movement of Earth’s surface.
A map of epicenters reveals they are not distributed evenly or randomly over earth’s surface. There are patterns.

10. Volcanic Activity
Volcanic eruptions occur when magma-a mixture of liquid rock, solid crystals, and gas- moves up from the mantle and crust and out onto the surface.
A map of volcanic activity shows that, like earthquakes, volcanoes do not occur randomly or evenly across Earth’s surface.

11. What’s the Trend?
In most places where earthquakes are common, there are volcanoes nearby, and vice-versa.
Volcanoes and earthquakes are sometimes linked, but are often independent events.
When a volcano erupts, the rising magma can trigger earthquake events.
At volcanic centers around the world, earthquakes may trigger volcanic eruptions.
Cracks can form that may act as pipelines for magma to rise as the crust moves due to a major earthquake.

12. What does this Mean?
The map below shows that patterns between volcanic and earthquake activity are nearly identical.

13. What does this Mean?
GPS measurements show that large areas of Earth’s solid surface are moving.
Earthquakes and volcanoes are common at locations where pieces of Earth’s solid surfaces move toward or away from each other-along two rigid regions that are moving at different speeds or directions.
Mountains and valleys can occur at the edges of different plates of lithosphere.
These types of landforms can form in a variety of locations on Earth.
Measurements and data gathered have generated the tectonic plate model of Earth’s surface.
In this model, Earth’s outer layer, its lithosphere, is divided into distinct pieces called tectonic plates.

14. Tectonic Plates
Tectonic plates are large pieces of Earth’s lithosphere that move about on Earth’s asthenosphere below.
In general, all points on a particular plate move together.

15. Tectonic Plates
A map of tectonic plates-like the one above-is a model, or simplification of what is actually happening.
It is based on our current understanding of the motion of a solid surface on a large scale.
It does not show how sharp the boundaries are, or the details of the motion of the plates at the boundaries.
These models are revised as new information is obtained.

17. Earth’s Lithospheric Plates
Scientists examine the composition of earth’s plates to observe whether the composition is consistent throughout.
They examine the layers of rock at different plate locations.
They study the sequences of rock that are exposed at Earth’s surface, rock found underground in mines, and land and seafloor rock core samples to gather evidence about the composition and structure of plates.
Gravitational, seismic, and magnetic data also contribute to the picture.

18. Earth’s Lithospheric Plates

19. Earth’s Lithospheric Plates
There are two types of crust: continental and oceanic.
Continental crust is thicker and made of a much wider variety of rock types than oceanic crust.
Crust may not be classified by whether or not it is underwater.
There is a type of crust called ‘transitional crust’, which has characteristics of both oceanic and continental crust.

20. Characteristics of Earth’s Plates
The diagram above provides an overview of the composition and structure of the lithosphere.
Ocean crust is much thinner than continental crust.
Both types of crust float above the asthenosphere.
Because the oceanic crust is denser, it sinks deeper than continental lithosphere.
Oceans are found above oceanic crust, because oceanic crust has a lower elevation than most continental crust, and gravity pulls water into lower areas.

21. Earth’s Lithospheric Plates
The transition between where continental crust and oceanic crust begins is called the continental margin, which includes the continental shelf and nearby sediment.
This transition can vary depending on the area.