Approximately 200 million years ago
Earth's land was grouped together in one large super-continent named
Pangaea. http://pubs.usgs.gov/gip/dynamic/historical.html

Pangaea began to break apart during the Jurassic Period. http://pubs.usgs.gov/gip/dynamic/historical.html

http://pubs.usgs.gov/gip/dynamic/historical.html

Scientists have found evidence that the continents are moving in fossils.
For example, fossils of tropical plants were found in Antarctica which led to the conclusion that this frozen land had previously been closer to the equator.

http://pubs.usgs.gov/gip/dynamic/continents.html

Plate Tectonics is a theory that was developed in the 1960's. This theory states that Earth's top layer called the crust is divided into plates that are moving at a speed of 1 to 10 cm per year.

http://pubs.usgs.gov/gip/dynamic/slabs.html

The breaks in the Earth’s crust where the plates meet (push together, pull apart, and slide beside each other) are called faults.

The plates move in three ways:

Divergent Plate Movement ← →

Convergent Plate Movement → ←

Transform Plate Movement or
Lateral Slipping Plate Movement ↓↑

Divergent Plate Movement ← →
Convergent Plate Movement → ←
Transform Plate Movement or
Lateral Slipping Plate Movement ↓↑

Mid-Ocean Ridges are places where the Earth's tectonic plates are gradually moving apart. As the plates move apart, magma rises up to fill the gap. This can cause undersea volcanic eruptions.
This shallow magma provides a heat source that creates many seafloor hot springs along the ridges which transport heat and chemicals into the ocean.

http://pubs.usgs.gov/gip/dynamic/baseball.html

You will be given a piece of waxed paper. On the wax paper spread a spoonful of icing about a half of a centimeter thick. The icing represents the magma that is under the Earth’s crust. Next you will be given two squares of fruit rollups. Place the two squares of fruit rollup onto the frosting right next to each other. These represent oceanic plates. Press down slowly on the fruit rollups because oceanic plates are dense and will sink a bit. Slowly push the
“plates” apart about half a centimeter. Notice how the frosting is exposed and pushed up where the plates are separated. This is how magma comes to the surface where real plates are moving apart at divergent plate boundaries .

The black dots represent volcanic activity.
Notice that most of these are along the edges of the plates.
These are called fault lines.
http://pubs.usgs.gov/gip/dynamic/zones.html

On divergent plate boundaries new crust is being formed as magma escapes from the Earth's core. In other locations under the ocean are deep trenches.

Scientist believe these deep trenches are where the old crust is being lowered back into the Earth’s core.
This cycle is known as the Wilson
Cycle, named after John Tuzo
Wilson.

The cycle, beginning with new crust being formed to the old crust returning to the Earth's core, takes approximately 100 million years.

Continents do not fall back into the
Earth's core, so land masses are much older than the Earth’s crust beneath the ocean.

When plates move apart, volcanoes may form.

2 Oceanic Plates

Submarine mountain ranges



Volcanic activity
Earthquakes
New seafloor
2 Continental Plates

Rift valley


Earthquake activity
Volcanic activity sometimes occurs

Convergent boundaries are where plates are moving toward each other.
The outcome of convergent plate movement varies depending on the types of plates that meet:
– Oceanic–Oceanic
– Oceanic-Continental (Land)
– Continental-Continental or (Land-Land)




Lithosphere - outermost layer of the earth's crust & is broken up into large solid plates called tectonic plates - about
40 - 250 miles deep
Asthenosphere - part of the earth just below the lithosphere - not liquid but there is a small amount of melted rock there - 250 - 435 miles thick
Subduction - the process that takes place at convergent boundaries → ← in which one tectonic plate moves under the other

When two oceanic plates collide, one is pushed under the other and magma from the mantle rises, forming volcanoes. http://pubs.usgs.gov/gip/dynamic/understanding.html

In some cases the magma calmly erupts. The lava emerges and cools forming new crust.

In other cases the volcano erupts violently sending dust and ash into the sky.

In other cases the area above the volcano has cracks which release gases usually during a volcanic eruption. As the gases release the emptied chamber can no longer support the weight of the earth above it. The earth falls into this void leaving deep craters called calderas.

One famous example of a caldera is North
America’s Crater Lake located in Southern
Oregon. Around 7,000 years ago a massive volcanic peak reaching 12,000 feet in elevation set in this location. A powerful eruption blasted the top 4,000 feet away, leaving a deep bowl shaped caldera. Since that time, a new dome has begun to form in the center of the caldera.

The factor that determines the type of eruption is the amount of silica that is in the lava. Silica is a very common mineral composed of silicon and oxygen (SiO2). Silicates makes up about 95% of the Earth’s crust.
Volcanoes rarely change the type of eruption that occurs.

Slide the two pieces of fruit rollups together.
Notice that one piece slides under the other. A hump forms where the two pieces hit.
Volcanic mountain building occurs when two oceanic plates meet.

When an oceanic plate pushes against a continental plate, the oceanic plate moves under the continental plate.
http://pubs.usgs.gov/gip/dynamic/understanding.html

Remove one of the fruit rollups from the frosting. (You may eat it.) Place one of the graham cracker halves lightly onto the frosting next to the remaining fruit rollup piece.
The graham cracker represents the continental crust, which is thicker and less dense than oceanic crust (fruit rollup). It floats high on the asthenosphere (upper mantle of the Earth) so don't push it down. Gently push the continent (graham cracker) towards the ocean plate (fruit rollup) until the two overlap, and the graham cracker is on top. The oceanic plate is subducted below the continental one.

When an oceanic plate meets a continental (land) plate usually an oceanic trench will develop on the ocean side and a mountain range on the continental side.

When two continental plates collide, mountain ranges are created as the crust is pushed upwards. You can feel this movement through earthquakes.
http://pubs.usgs.gov/gip/dynamic/understanding.html

Remove both the cracker and fruit roll up from the frosting asthenosphere.
Place one edge of both crackers into the glass of water for just a few seconds. Place the crackers onto the frosting with wet edges next to each other. Slowly push the graham crackers towards each other.
Notice how the wet edges crumple. This is how mountains are made at convergent plate boundaries ! When continents move towards each other there is nowhere for the rock to go but up!

When two plates move towards each other mountains form.

Oceanic and Continental Plates

Earthquake activity



Sometimes ocean trench
Volcanoes
Destruction of oceanic lithosphere
2 Oceanic Plates

Earthquakes


Oceanic trench
Destruction of oceanic lithosphere
2 Continental Plates

Folding and faulting



Mountain ranges
Earthquake activity
Shortening of plates

When two plates move sideways against each other, there is a tremendous amount of friction which makes the movement jerky. This is known as an earthquake.



Pick the two crackers up off the frosting and turn them around so that two dry edges are next to each other.
Push one cracker past the other to simulate a transform plate boundary like the San Andreas fault!

When two plates move past each other earthquakes are likely to happen.


Recurring earthquake activity

Volcanic activity


Divergent Plate Movement ← →

Convergent Plate Movement → ←

Transform Plate Movement or Lateral
Slipping Plate Movement ↓↑

http://pubs.usgs.gov/gip/dynamic/Vigil.html

http://pubs.usgs.gov/gip/dynamic/world_map.html

The Ring of Fire is a chain of volcanoes and oceanic trenches. Seventy-five percent of
Earth’s volcanoes are in the Ring of Fire. http://pubs.usgs.gov/gip/dynamic/fire.html

http://pubs.usgs.gov/gip/dynamic/Farallon.html