Susan Wells

Earth Science


Plate Tectonics


Continental Drift
Around 1912, a German scientist named Alfred Wegener theorized that all of the Earth's continents were once joined together in a single, large landmass. He further proposed that the continents have separated and collided as they have moved around over the last few million years. He called this theory continental drift. He provided several pieces of evidence to support his theory:

1) Continent Shapes- The continents appear to be shaped in such a way that they would fit together nicely, like a jigsaw puzzle.

2) Rock Formations- There are rock formations on different continents that match up beautifully when the continents are put back together.

3) Fossils- There are fossils found on different continents that would also match up nicely if the continents were all once together.

People of the time mostly thought Wegener was crazy!

New Evidence
In the 1950's, scientists discovered some surprising evidence in support of Wegener's theory. While mapping the ocean floor, scientists discovered two important, and unexpected things:

First, the age of the rocks that make up the ocean floor gets older as you move away from the ridges at the center. This meant that the youngest rocks were found near the ridges, and the oldest rocks near the continents.

Below is a graph of the rock ages for the map on top.

Second, there are stripes of alternating magnetic polarity on each side of the ridge. When the molten rock hardens, the magnetic minerals in the rock align themselves with the Earth's magnetic field. Scientists discovered that the Earth's magnetic field has reversed itself many times, at intervals of roughly every 100,000 years. The pattern they observed makes sense if the ocean floor is being formed at the ridge and gradually pushed outward in both directions.

These discoveries gave rise to the now respectable science of Plate Tectonics. This is the theory that the Earth's seemingly solid crust is actually made up of several pieces, or plates, that move around independently.

Plate Boundaries
The places where the different plates meet, called plate boundaries, are where the tectonic action really is. There are three basic types: convergent, divergent, and transform boundaries.

Convergent Boundaries: This a when two plates are moving toward each other, as shown above.

If the two plates are of relatively low, and similar densities, the plates will form a Collision Boundary.

In this scenario, the crust is forced upward by the collision, resulting in mountain building. The diagram above shows how this type of collision between India and China forced the formation of the Himalayan Mountains

If one of the plates is more dense than the other, as happens when oceanic and continental crust meet, then the more dense plate will be forced under the less dense plate. This forms a trench, or deep valley, where the plates meet. This is called subduction, and is shown in the diagram above. This often results in a chain of volcanoes running parallel to the trench.

Divergent Boundaries: As you might expect, this is essentially the opposite of a convergent boundary. This occurs when two plates are moving away from one another, as shown above. This is seen at mid-ocean ridges and rifts.

Transform Boundaries: This type of boundary forms when two plates are sliding past one another. The diagram above illustrates this motion. The most popular example of this is the San Andreas Fault in California .

All of the different boundaries and their locations are found on  page 5 of the Earth Science Reference Tables, shown below. Notice the key that shows the different boundaries and their symbols.

Tectonic Forces
The movement of the plates is driven by convection currents in the mantle. These currents cause the solid plates to float along on top of the semi-molten mantle material.

Sometimes, there is an opening in the middle of a plate that allows the molten material to flow through it. This is called a hot spot, and usually results in a chain of volcanic islands that form as the plate moves over the hot spot. The Hawaiian Islands are a great example of this.


Plate Tectonics

As you studied volcanoes, igneous, metamorphic and sedimentary rocks, and earthquakes, you learned how these topics are related to plate tectonics. In this chapter we take a closer look at plates and plate motion. We will pay particular attention to plate boundaries and the possible driving mechanisms for plate motion.

The history of the concept of plate tectonics is a good example of how scientists think and work and how a hypothesis can be proposed, discarded, modified, and then reborn. In the first part of this chapter we trace the evolution of an idea - how the earlier hypothesis of moving continents (continental drift) and a moving sea floor (sea-floor spreading) were combined to form a theory of plate tectonics

1. Continental drift was proposed by Alfred Wegner in the early 1900s based on the apparent fit of continental coastlines, similar fossil plants and animals on widely separated continents, distribution of Paleozoic glaciations and paleoclimatology, and apparent polar wandering.

2. Wegner proposed that all continents had once been connected in a supercontinent called Pangaea, that broke apart to form the present continents. Wegner thought the continents moved across stationary oceanic crust.  His ideas received little support when proposed because he could provide no mechanism that allowed continents to plow through ocean crust.

3. Paleomagnetism is the study of the ancient magnetic fields of the earth. Magnetized minerals preserve a record of the direction of the magnetic pole and their distance from it at the time of their formation. Paleomagnetic data revived interest in continental drift by demonstrating polar wandering and supporting the reconstruction of Pangaea.

4. Other recent evidence for continental drift includes better continental margin fits, similar rock contacts and age relations between continents when fitted together, glacial movements indicated by striations, and sources of boulders in ancient tills, and similar geologic sequences including metamorphic rocks in Brazil and Gabon.

5. The idea that the sea floor spread away from mid-oceanic ridges and was subducted beneath a continent or island arc as a result of mantle convection was proposed by Harry Hess in the early 1960s.

6. Sea-floor spreading explains processes at the mid-oceanic ridges as the result of rising mantle: the existence of the ridge itself, high heat flow, basaltic volcanism, a rift valley and shallow-focus earthquakes.

7. Sea-floor spreading explains processes at the oceanic trenches as the result of descending oceanic crust: existence of the trench itself and volcanism.

8. Sea-floor spreading explains the young age of the sea floor, loss of older oceanic crust, and increasingly older oceanic crust away from the ridge crest.

9. Plate tectonics is the theory that the earth's surface is divided into a few large, thick plates that move and change size. It combines the older ideas of continental drift and sea-floor spreading. Plates are formed by lithosphere (crust and uppermost mantle) and are carried along by the  asthenosphere to a depth of about 200 km. New lithosphere is added along the ridges at the trailing edge of the plate and lost to subduction. Plate boundaries are either diverging, converging or transform.

10. Sea floor magnetic anomalies were symmetrical with respect to the mid-oceanic ridge crests and matched the pattern of magnetic reversals discovered previously in stacked lava flows. Spreading rates are 1 to 6 cm/year. The hypothesis also allows prediction of the sea floor age based on magnetic anomalies that can be tested with samples recovered by deep-sea drilling.

11. Diverging plate boundaries experience extension that produces normal faults, shallow-focus earthquakes, rift valleys, basaltic volcanism, crust thinning, uplift, and creates new ocean basins. Whether rifting causes uplift, or vice versa is unclear.

12. Transform boundaries allow plates to slide past one another. These boundaries exhibit strike-slip motion and may connect two ridge segments, a ridge and a trench, or two trenches. The straight course of these faults resolves mechanical constraints caused by divergence along curved boundaries.

13. Ocean-ocean convergence is characterized by andesitic to basaltic island arcs and trenches

14. Ocean-continent convergence exhibits an active continental margin associated with young volcanic and some metamorphic mountain belts and trenches.

16. Continent-continent convergence passes through the stages exhibited by ocean-continent convergence, but results in a suture zone of young mountains in continental interiors marking the former subduction site, thickened continental crust, and shallow focus earthquakes. Ex. the Himalayas.  Collision zone not subduction zone -- no trench.

18. Plate tectonics explains consistently: distribution of basaltic and andesitic volcanoes, shallow-, intermediate-, and deep-focus earthquakes, young mountain belts, mid-oceanic ridges, oceanic trenches, and fracture zones.

19. Convection currents in the asthenosphere cause mantle movement.  The overlying plates are carried along with mantle movement.

20. Mantle convection may result in mantle plumes or hot spots. They are stationary with respect to moving plates and produce hot spots, such as Yellowstone, Iceland and the Hawaiian Islands. Mantle plumes may also be responsible for the initial fracturing of the lithosphere causing divergence.  (e.g. Red Sea region).

Divergent Boundary               2 plates move away from each other Mid-ocean ridges, rift valleys - basaltic magma creating new crust
Transform Boundary 2 plates slide past one another San Andreas Fault, mid-ocean ridges


3 types of convergent boundaries - destroying crust

ocean crust - ocean crust 2 plates move together
  • older, denser crust - subducts
  • trench
  • curved volcanic island arc
  • andesitic to basaltic magma
  • ex. Japanese island arc, Aleutian Islands
  • shallow, intermediate, deep earthquake focus



ocean crust - continental crust 2 plates move together
  • oceanic crust - subducts - denser
  • trench
  • andesitic magma
  • metamorphic rock
  • ex. Andes Mtns., Cascades
  • shallow, intermediate, few deep earthquake focus
continental crust - continental crust 2 plates move together
  • No subduction - no trench
  • Double crust thickness
  • Mountain range which may have marine fossils from oceanic crust that became narrow (Think India plate movement)
  • Himalayas
  • shallow earthquake focus

Earthquakes & Volcanoes

Earthquakes, volcanoes and mountain ranges tend to happen in similar areas.

The map above shows frequent earthquake activity as the bands of dots.

Most of the world's active volcanoes (triangles) are along the edges of tectonic plates (the lines).

Types of Plate Boundaries

Convergent- plates collide into each other.

Where an oceanic and a continental plate collide, the denser oceanic plate will be forced under (subduction) the other.


  • plates spread apart
  • caused by magma upwelling from deep in the Earth
  • usually found in the oceans along with mid-ocean ridges

Speed Links

Continental Drift

Seismic Waves

Time Math

P & S Wave Chart

EQ Strength

EQ Waves as X-Rays


Vocabulary Words

Determining Epicenter Distance

Finding Epicenters I

Finding Epicenters II

Epicenters 3

When Cars Race

*Seismic Eruption

*Seismic Waves

Time Math I

Time Math II

* Seismic Waves and Seismic Eruption are free demonstration programs available at:

Transform- two plates slide past each other.

In this aerial view of the San Andreas Fault (transform) the trees in the orchard (dots) have been offset by the slipping of the plates. The Pacific Plate is to the left and the North American to the right.

This picture shows how things like fences, roads, rivers and buildings can be offset by the sliding of the plates.

The exception to this arrangement are "Hot Spots" which are plumes of hot material (rather than belts) in the middle of plates. These spots stay stationary while the plate moves above it. The spot melts through the plate like a blow torch and produces a volcano above it. As the plate moves, the spot melts through another spot producing a chain of volcanic islands. Hawaii is an example of a hot spot island chain.

The movement of the plates is caused by convection currents deep within the Earth. The force that moves the plates around the earth are convection currents inside the mantle.

  • Hotter Mantle material rises while cooler material sinks.
  • The crust is split and diverges where the material rises and spreads out.
  • The plates converge and subduct where the material is sinking.

The different types of plate boundaries are caused by a combination of the direction of convection as well as they type of crust: continental or oceanic.

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Evidence of Continental Drift

  • Puzzle Fit of the continents to form Pangaea (see below)
  • Fossil Evidence (see below)
  • Glacial Evidence (see below)
  • Coal in Antarctica- coal is formed in tropical swamps. Coal was formed when Antarctica was closer to the equator.
  • Magnetic Stripes on the ocean floor (this one's going to take some explaining so it gets its own page)
  • Mountain Chains appear where they should if continents are colliding

Puzzle Fit- if the continents were cut out of a map, most of the landmasses will fit together to form a larger supercontinent, which is called Pangaea.

Fossil Evidence- in the picture above, fossils of many land-living have been found on opposite shores. When Pangaea is re-assembled, the fossils match up.

Glacial Evidence- when Pangaea is re-assembled, there is evidence of a single ice sheet (at least for this episode) affecting many of the southern continents. When viewed this way, this sheet leaves consistent evidence of a single glacier. When viewed on the current continents, it is inconsistent and even highly improbable. For example, India, which is north or the Equator, has glacial evidence coming from the south!


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An earthquake is an event where two pieces of crust shift against each other. The rumbling felt is from the rocks slipping, sticking and breaking. The vibrations are called seismic waves. There are different types of seismic waves that vibrate in different ways.

The focus is the spot within the earth where the earthquake began.

The epicenter is the spot on Earth's surface closest to the focus.

A fault is a crack along which the rocks slide.

Seismic Waves- during an earthquake, several types of waves are generated. The vibrations felt are actually called seismic waves that are traveling through the Earth.


  • Primary wave- travels phastest so it arrives at seismic stations phirst.
  • Push-pull wave: rock vibrates forward and backward in the same direction that the wave travels ("parallel to propagation").
  • Pass through solids and liquids (magma).


  • Secondary wave- arrives at a seismic station second.
  • Slow wave- not as fast as the P-wave.
  • Shake wave (shear wave)- vibrates side-to-side.
  • Solids wave- only travels through solids.

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Time Math

In order to do the calculations that help find the distance to epicenter and the time of the earthquake, you’ll need to do math with time. It is difficult to do time math in a calculator, and by hand it does not work exactly the same way as regular math. Regular math is what they call “base 10” which means that whenever you count past 9 you must move over one place to the tens column. Time is base 60. You can count 59 seconds and then you go to the minutes column.
For example, if you want to subtract 82 minus 17 in regular base ten numbers you would “borrow” a ten and start by taking 7 away from 12.


is the same as
8712 (70 plus 12)

Time math works almost the same way except instead of taking over ten from the neighboring column you’ll take one minute and convert it into 60 seconds.
3:13:25 (“3 hours, 13 minutes, and 25 seconds”)
turns into:
3:12:85 (3 hrs,12 min, 85 secs is the same as 13:25)

How to Use the P-Wave and S-Wave Travel Time Chart

P-Wave & S-Wave Chart (opens a new window so your pop-up blocker may stop it)

The P-line shows how much time it take a P-wave to travel a certain distance. So if you need to know how much time it takes the p-wave to travel 2,000km, it is just over 4 minutes (about 4:05 ). The S-wave works the same way: for 2,000km it takes 7:20 .

To find the distance to epicenter:

You are in charge of watching the seismic station tonight when the seismograph detects an earthquake. The earthquake didn’t happen where you are- you can’t even feel it. As a result, you don’t know what distance or direction the earthquake happened. The P-wave and S-wave are separated by 4:05 (4 minutes, 5 seconds). You need to find a spot on the graph where the P-line and the S-line are separated by 4:05 .

  • Take a scrap piece of paper, line it up along the left edge of the chart.
  • Put a small tick mark on your scrap paper at zero, and a small tick mark at 4:05 .
  • Slide the scrap paper up along the chart until it the two tick marks just touch the P and S lines. BE SURE THAT YOUR SCRAP PAPER IS PERFECTLY STRAIGHT UP AND DOWN (use the lines on the grid as a guide).
  • Now that you have found the right spot on the graph, drop a line straight down to the bottom of the graph to read the distance- 2,600km.

To Find The Time That The Earthquake Occurred

When a seismograph detects an earthquake that happened at some distance, (2,600km for example) you know that the earthquake happened some time in the past and it took time for the waves to reach your station. But how long ago? All you need to do is answer the question “how long does it take a P-wave to travel 2,600km?

  • Find 2,600km on the bottom of the chart.
  • Go straight up until you reach the P-line and read the time from the left of the chart: 5:00 (5 minutes).
  • Now compare times: if you detected the earthquake at 3:17:00 and it took 5:00 then the earthquake happened 5 minutes before 3:17:00 or 3:12:00 .

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Earthquake Strength

The intensity or strength of an earthquake is measured in two main ways:

  • The Richter Scale
    • measures the amount of energy that an earthquake releases
    • Each number of magnitude is 10x stronger than the number below it.
  • The Mercalli Scale
    • Measures the amount of damage from an earthquake
    • Ranges from I to XII
    • Based on common earthquake occurrences such as "noticeable by people" "damage to buildings" chimneys collapse" "fissures open in the ground”.

Seismic waves as “x-rays”

  • P-Waves travel through solid and liquid
  • S-Waves travel only through solids
  • Seismic waves travel faster through denser material.
  • Because of this, the path traveled by a seismic wave is bent towards the surface.

Properties of the material (such as density and pressure) that the waves pass through can be inferred by the speed and angle that the waves travel.
The layers of the earth are determined by the jumps in velocity and “echoes” of seismic waves.

The MOHO is a boundary between the crust and the upper mantle where the velocity of waves jumps up sharply. This sharp increase in velocity is called a discontinuity.

A shadow zone occurs on the opposite side of the earth from an earthquake because of the liquid outer core. S-Waves are stopped all together while the P-Waves are refracted (bent) to create a zone where no waves are picked up at all. This zone is between 102° and 143°around the earth from the earthquake.

Lab research and studies of meteorites suggest that the core is made of Iron and Nickel (FeNi).