Day 15- Plate movement

1. How are plated defined and identified?2. How do convection cells drive the plates

around Earth’s surface? 3. Define and describe the three types of plate

boundaries. 4. What causes volcanic activity unassociated

with a plate boundary?

How are plated defined and identified?

What is a Plate?

• During the 1950s and early 1960s, scientists set up seismograph networks to see if enemy nations were testing atomic bombs. These seismographs also recorded all of the earthquakes around the planet. The seismic records were used to locate an earthquake’s epicenter , the point on Earth’s surface directly above the place where the earthquake occurs.

• The lithosphere is divided into a dozen major and several minor plates.

• A single plate can be made of all oceanic lithosphere or all continental lithosphere, but nearly all plates are made of a combination of both.

• The movement of the plates over Earth's surface is termed plate tectonics . Plates move at a rate of a few centimeters a year, about the same rate fingernails grow.

How do convection cells drive the plates around Earth’s surface?

In a convection cell, material deep beneath the surface is heated so that its density is lowered and it rises. Near the surface it becomes cooler and denser, so it sinks.

How Plates Move

Steps of Mantle convection1. Hot mantle from the two adjacent cells rises at the ridge

axis, creating new ocean crust.2. The top limb of the convection cell moves horizontally

away from the ridge crest, as does the new seafloor.3. The outer limbs of the convection cells plunge down into

the deeper mantle, dragging oceanic crust as well. This takes place at the deep sea trenches.

4. The material sinks to the core and moves horizontally.5. The material heats up and reaches the zone where it rises

again.

Define and describe the three types of plate boundaries.

Plate Boundaries

• Plate boundaries are the edges where two plates meet. How can two plates move relative to each other? Most geologic activities, including volcanoes, earthquakes, and mountain building, take place at plate boundaries. The features found at these plate boundaries are the mid-ocean ridges, trenches, and large transform faults.

• Divergent plate boundaries : the two plates move away from each other.

• Convergent plate boundaries : the two plates move towards each other.

• Transform plate boundaries : the two plates slip past each other.

• The type of plate boundary and the type of crust found on each side of the boundary determines what sort of geologic activity will be found there. We can visit each of these types of plate boundaries on land or at sea.

What causes volcanic activity unassociated with a plate boundary?

Intraplate Activity

A small amount of geologic activity, known as intraplate activity, does not take place at plate boundaries but within a plate instead. Mantle plumes are pipes of hot rock that rise through the mantle from near the core. The release of pressure causes melting near the surface to form a hotspot. Eruptions at the hotspot create a volcano. Hotspot volcanoes are found in a line

Summary

• The plate in plate tectonics is a large chunk of lithosphere that can carry continental crust, oceanic crust, or some of each. Plates can be identified by the locations of earthquake epicenters. At the boundaries of plates are mid-ocean ridges, trenches, and large faults. Plates move by mantle convection. Plates meet at plate boundaries. The three types are divergent, convergent, and transform.

PHET simulation

Day 16 – topographic features of

plate boundaries

Is it easier to get to the bottom of the ocean or out into

space?

How many people have been to “space”?

537

How many people have been to the deepest point on

earth?3

Today’s Agenda:1. Opening

2. Notes3. Sea floor spreading activity

4. Finish Plate tectonics simulation5. Independent research project

6. Call on students to review findings from Phet activity.

7. Turn in PHET activity

1. How does the topography of the seafloor give evidence for plate tectonics?

2. How does the pattern of magnetic stripes give evidence for seafloor spreading?

3. How does the age and thickness of seafloor rocks and sediment support the idea of plate tectonics?

How does the topography of the seafloor give evidence for plate tectonics?

The people who first mapped the seafloor were aboard military vessels during World War II. These vessels used sound waves to search for submarines, but also produced a map of seafloor depths. Depth sounding continued in earnest after the war. Scientists pieced together the ocean depths to produce bathymetric maps of the seafloor.

Features of the Seafloor• Although scientist expected an expanse of flat, featureless

plains, they were shocked to find tremendous features like mountain ranges, rifts, and trenches. This work continues on oceanographic research vessels as they sail across the seas today.

• mid-ocean ridges: these features rise up high above the deep seafloor as a long chain of mountains

• rift zones: in the middle of the mid-ocean ridges is a rift zone that is lower in elevation than the mountains surrounding it.

• deep sea trenches: these features are found at the edges of continents or in the sea near chains of active volcanoes

• abyssal plains: these features are flat areas, although many are dotted with volcanic mountains

A modern map of the southeastern Pacific and Atlantic Oceans made with data from several decades.

How does the pattern of magnetic stripes give evidence for seafloor spreading?

• WWII navy ships towed magnetometers to search for enemy submarines. Shipboard magnetometers also revealed the magnetic polarity of the rock beneath them. When scientists plotted the points of normal and reversed polarity on a seafloor map they made an astonishing discovery: the normal and reversed magnetic polarity of seafloor basalts creates a pattern.

Observations of Patterns:• Stripes of normal polarity and reversed polarity alternate

across the ocean bottom. • Stripes form mirror images on either side of the mid-

ocean ridges• Stripes end abruptly at the edges of continents,

sometimes at a deep sea trench.

How does the age and thickness of seafloor rocks and sediment support the idea of plate tectonics?

Additional Observations

• The scientists noticed that the rocks got older with distance from the mid-ocean ridges.

• The youngest rocks were located at the ridge crest and the oldest rocks were located the farthest away, abutting continents.

• Scientists also noticed that the characteristics of the rocks and sediments changed with distance from the ridge axis Away from the ridge crest, sediment becomes older and thicker, and the seafloor becomes thicker. Heat flow, which indicates the warmth of a region, is highest at the ridge crest.

• Seafloor is youngest at the mid-ocean ridges and becomes progressively older with distance from the ridge.

Seafloor Spreading

• The features of the seafloor and the patterns of magnetic polarity symmetrically about the mid-ocean ridges were the noticed by Harry Hess.

• Hess wrote that hot magma rose up into the rift valley at the mid-ocean ridges. The lava oozed up and forced the existing seafloor away from the rift in opposite directions. Since magnetite crystals point in the direction of the magnetic north pole as the lava cools, the different stripes of magnetic polarity revealed the different ages of the seafloor. Hess called this idea seafloor spreading.

• Since new oceanic crust is created at the mid-ocean ridges, either – Earth is getting bigger (which it is not)

Or

– Oceanic crust must be destroyed somewhere.• Since the oldest oceanic crust was found at the edges of

the trenches, Hess hypothesized that the seafloor subducts into Earth’s interior at the trenches to be recycled in the mantle.

Summary

• Data from magnetometers dragged behind ships looking for enemy submarines in WWII discovered amazing magnetic patterns on the seafloor.

• Rocks of normal and reversed polarity are found in stripes symmetrically about the mid-ocean ridge axis.

• The age of seafloor rocks increases from the ridge crest to rocks the farthest from the ridges. Still, the rocks of the ocean basins are much younger than most of the rocks of the continents.

Magnetic Banding Activity

Independent research project

• Get 5 pieces of copier paper for this. You may also use your phone to research this if you’re so inclined.

• Cascade mountains• Hawaii• Appalachian mountains• Himalayan mountains• The Mariana Trench

• Describe the location of the geologic formation.• Describe the tectonic movements that caused them to be formed.• Draw and label a diagram showing how they formed.• Find at least one interesting fact about them.

Earthquakes

1. How does the elastic rebound theory explain earthquakes?

2. Describe how the center of an earthquake is located and described, including focus and epicenter.

3. Describe the scales used to measure earthquakes.

This photo shows the Mission District of San Francisco burning after the 1906 earthquake.

How does the elastic rebound theory explain earthquakes?

Earthquake!

• An earthquake is rapid ground movement caused by the sudden release of energy stored in rocks. Earthquakes happen when so much stress builds up in the rocks that the rocks rupture. The energy is transmitted by seismic waves.

Elastic rebound theory. Stresses build on both sides of a fault, causing the rocks to deform (Time 2). When the stresses become too great, the rocks break and end up in a different location (Time 3). This releases the built up energy and creates an earthquake.

Describe how the center of an earthquake is located and described, including focus and epicenter.

In an earthquake, the initial point where the rocks rupture in the crust is called the focus. The epicenter is the point on the land surface that is directly above the focus.

Earthquake Zones• About 80% of all earthquakes strike around the Pacific Ocean 

basin because it is lined with convergent and transform boundaries

• About 15% take place in the Mediterranean-Asiatic Belt, where convergence is causing the Indian Plate to run into the Eurasian Plate.

• The remaining 5% are scattered around other plate boundaries or are intraplate earthquakes.

A seismograph produces a graph-like representation of the seismic waves it receives and records them onto a seismogram. Seismograms contain information that can be used to determine how strong an earthquake was, how long it lasted, and how far away it was

Interpreting a Seismogram

A seismogram shows:• foreshocks.• the arrival of the P-waves.• the arrival of the S-waves.• the arrival of the surface waves

• P-waves (primary waves) are fastest, traveling at about 6 to 7 kilometers per second, so they arrive first at the seismometer.

• P-waves move in a compression/expansion type motion, squeezing and unsqueezing Earth materials as they travel.

• S-waves (secondary waves) are about half as fast as P-waves, traveling at about 3.5 km per second, and arrive second at seismographs.

• S-waves move in an up and down motion perpendicular to the direction of wave travel.

• P-waves bend slightly when they travel from one layer into another. Seismic waves move faster through denser or more rigid material. As P-waves encounter the liquid outer core, which is less rigid than the mantle, they slow down. This makes the P-waves arrive later and further away than would be expected. The result is a P-wave shadow zone. No P-waves are picked up at seismographs 104o to 140o from the earthquakes focus.

• S-waves are only able to propagate through solids. S-waves cannot travel through liquid.

Describe the scales used to measure earthquakes.

Richter Magnitude Scale

• Developed in 1935 by Charles Richter, this scale uses a seismometer to measure the magnitude of the largest jolt of energy released by an earthquake.

• The Richter scale and the moment magnitude scale are logarithmic scales.

• The amplitude of the largest wave increases ten times from one integer to the next.

• An increase in one integer means that thirty times more energy was released.

Mercalli Intensity Scale

• Earthquakes are described in terms of what nearby residents felt and the damage that was done to nearby structures.

• Quantified by post-earthquake observations• What factors would go into determining the

damage that was done and what the residents felt in a region?

Japanese earthquake videos

Volcanoes

• Compare magma and lava. Locate volcanoes and relate back to plate boundaries. Explain volcanic effects on the lithosphere and relate back to plate boundaries (convergent, divergent, transform) including lahar (mud) flows and ash in the atmosphere.

1. Where do most volcanoes occur? Why? 2. How does the composition of magma

affect the behavior of the volcano?3. How can we predict volcanic activity?

Volcanoes

Magma chamber• A magma chamber is a large underground pool of

liquid rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma.

Volcanoes• A volcano is a vent from which the material from a

magma chamber escapes. Volcanic eruptions can come from peaky volcanic cones, fractured domes, a vent in the ground, or many other types of structures.

Where They Are• Volcanoes are common along convergent and

divergent plate boundaries. • Volcanoes are also found within lithospheric plates

away from plate boundaries. Wherever mantle is able to melt, volcanoes may be the result.

Convergent Plate Boundaries• Converging plates can be oceanic, continental, or

one of each. If both are continental they will smash together and form a mountain range. If at least one is oceanic, it will subduct. A subducting plate creates volcanoes.

Pacific Rim• Volcanoes at convergent plate boundaries are found

all along the Pacific Ocean basin. Trenches mark subduction zones.

• The Cascade Range is formed by volcanoes created from subduction of oceanic crust beneath the North American continent.

• The volcanoes are located just above where the subducting plate is at the right depth in the mantle for there to be melting.

Creating Magma• Volcanoes erupt because mantle rock melts forming

magma. This is the first stage in creating a volcano.• Mantle may melt if temperature rises, pressure

lowers, or water is added.• The type of material melted impacts the type of

eruption.

• Factors Affecting Eruptions–Viscosity• Def: A substances

resistance to flow• More viscous = More 

violent eruptions• Higher temperature =

Less viscous• More silica (a mineral) =

More viscous

• Factors Affecting Eruptions–Dissolved Gases• Trapped gases provide

force to eject magma from a vent– Vent = opening to the

surface

• More viscous = Harder for gases to escape = More violent eruption

TEPHRA• Tephra is a general term

for fragments of volcanic rock and lava that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains.

• Magma is called lava once it reaches the surface

Pyroclastic Material

• Scorching hot tephra, ash, and gas may speed down the volcano’s slopes at 700 km/h (450 mph) as a pyroclastic flow. Pyroclastic means fire rock

Person from the city of Pompeiiburied in a pyroclastic flow from Vesuvius’ eruption

• Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,000°C (1,800°F).

Trees knocked down by mount St. Helens’ explosion.

Predicting Volcanic Eruptions• A volcano’s history —

how long since its last eruption and the time span between its previous eruptions is a good first step to predicting eruptions

Predicting Volcanic Eruptions

• Earthquakes- Moving magma shakes the ground, so the number and size of earthquakes increases before an eruption.

• Slope Deformation• Magma and gas can push

the volcano’s slope upward. Most ground deformation is subtle and hard to detect.

• Gas Emissions• Gases may be able to escape a volcano before

magma reaches the surface. Scientists measure gas emissions in vents on or around the volcano to help predict eruptions.