Plate Tectonics Chapter 3

How Does The Earth Change With Depth Earth’s Crust – Outermost layer. Continental Crust - 35-40 km in thickness Oceanic Crust – exists beneath oceans. Average of 7 km thick. Mantle – extends from base of crust down 2,900 km. Outer Core – Molten iron and nickel. 2900 to 5150 km Inner Core – Solid iron and nickel.

Layers of the earth

Some Layers Are Stronger Than Others Earth’s layers are defined by how easily they are affected by force. Lithosphere vs. Asthenosphere Lithosphere – Uppermost portion of mantle and crust. Relatively strong layer From the Greek word, stone Asthenosphere – Lower mantle the soft, weak zone over which the lithosphere moves. From the Greek word, not strong

Some Layers Are Stronger than Others Oceanic crust Continental crust Lithosphere (stronger) Uppermost mantle Asthenosphere (weaker) Asthenosphere: hot and weak; mostly solid 01.03.b1

Why Are Some Regions Higher In Elevation than Others? Observe the relationship between the height of each block and its thickness relative to other blocks 01.03.m1 Thick blocks higher than thin blocks Dense materials are lower Relationship between crustal thickness and elevation: isostasy

The Plate Tectonic Theory Describes the formation, motion and interaction of the plates. Continents are embedded in lithospheric plates. As plates move, continents move.

Divergent Boundaries 2 plates separating. Caused the formation of the Mid-Ocean Ridge.

Initial uplift from rising mantle Continents rift apart Initial uplift from rising mantle Stretching and faulting form rift Can lead to seafloor spreading and new ocean basin Ocean widens with spreading 03.04.b1-4

Sketch a mid-ocean ridge Narrow trough, or rift Ridge high (hot rocks and thin lithosphere) Magma erupts or solidifies at depth; forms new oceanic crust Magma rises through fractures Asthenosphere rises and melts 03.04.a

Convergent Boundaries When 2 plates move toward each other 2 types Subduction – When an oceanic plate plunges beneath another plate Collision – 2 continental plates collide. Forms mountain ranges

Subduction Boundaries Ocean-Ocean subduction When 2 oceanic plates converge and one plunges below the other.. Causes formation of volcanic islands.

Subduction boundaries Ocean-Continental Form when oceanic plate collides with continental plate. Oceanic plate ALWAYS plunges because it’s more dense. Forms a trench. Due to friction. Forms volcanic mountains inland.

Transform Boundaries 2 plates sliding against each other. Causes earthquakes Transform boundaries link other types of plate boundaries such as a mid-ocean ridge and a trench.

Evidence of Plate Tectonics Location of earthquakes, volcanoes and mountain ranges. Magnetism and age of ocean floor. The change in magnetic poles is recorded in igneous rock. The shape of the continents Mesosaurus fossils in S. America and Africa. 03.03.a1

EQ, volcanism, or mtn. building = tectonic activity Compare the distribution of earthquakes (yellow), volcanoes (orange), and high elevations (brown) EQ, volcanism, or mtn. building = tectonic activity Belts of tectonic activity divide lithosphere into tectonic plates 03.03.a1

Magnetism and the age of the earth’s floor Mid-Ocean Ridge (MOR)- a long chain of volcanic mountains on the ocean floor with a deep central valley. Scientists found rock on either side of MOR were identical. The center of the ridge always showed current magnetic orientation.

Evolution of South America Middle Mesozoic (140 m.y ago) Observe the evolution of South America, beginning with continental rifting away from South America Late Mesozoic (100 m.y ago) Present

Shape of the Continents/ Mesosaurus

How Do Plates Move? Movement requires a driving force overpowering a resisting force. Question: What forces drive tectonic plates? Ridge Push Slab Pull

Ridge Push/Slab Pull Ridge push Slab pull Other forces, such as convection in mantle

Ridge Push/Slab Pull Ridge Push – MOR is higher than the ocean floor because the lithosphere near the ridge is thinner and hotter and gravity causes the plate to slide away and outward. Slab Pull – Subducting oceanic lithosphere is denser than the surrounding asthenosphere. Gravity pull the plate downwards into the asthenosphere. **Strongest driving force**

Lines of islands and seamounts 03.08.c Plate moves over a hot spot Volcano forms over hot spot Plate subsides as cools, so islands become seamounts Volcanism shuts off as area moves away

Observe the features around South America Envision a cross section from west of South America to the Mid-Atlantic Ridge 03.09.a1

Compare this cross section with the one you envisioned Andes over subduction zone; trench offshore Spreading along mid-ocean ridge 03.09.b1 Subduction beneath western edge Eastern edge of continent not a plate boundary

Earthquakes An earthquake occurs when mechanical energy is released and transmitted through rock as vibrations called seismic waves. Hypocenter (focus) – point where earthquake is generated. Usually depths less than 100 km.

Earthquakes continued Epicenter – point on Earth’s surface directly above hypocenter. (p. 332) Epicenter Hypocenter

Causes of Earthquakes Caused by movement along faults. Three types (p. 332) Normal Faults – Rocks above fault (hanging wall) move down against the lower rocks (foot wall). Occurs mostly at divergent plate boundaries.

Causes of Earthquakes Reverse and Thrust Faults – The hanging wall moves up against the foot wall. Happens at convergent boundaries.

Causes of Earthquakes Strike-Slip Faults – The two sides slip horizontally past each other. Occurs at transform boundaries.

Build Up and Release of Stress A build up of stress caused by friction created by sliding plates is released when the stress equals the fault strength. (P. 335) The stress is immediately released and decreases to the original level.

Earthquake Waves Earthquakes travel in seismic waves. Seismic is the Greek word for earthquake. There are 2 types. Body waves – travel inside the earth. Surface waves – travel on the surface of the earth. Body waves become surface waves.

Wave Shape Crest – the top of a wave Trough – the bottom of the wave

Surface Waves Vertical Surface Wave – Travel up and down. Horizontal Surface Waves – Travel perpendicular to the direction of travel.

Body Waves Two types Primary (p-waves) – Travel in the same direction in which it starts. (like a slinky) Travels through liquids and solids because the particles are close enough. Faster!

Body Waves Secondary (s-waves) – Travel up and down, they sheer the rock. (like a snake) Cannot travel through liquids because they’re not rigid. Only through solids.

Vertical surface wave compresses materials Surface waves Horizontal surface wave shears material Primary body wave (P-wave) compresses material; fastest Body waves Secondary body wave (S-wave) shears material 12.04.a

Recording Seismic Waves Seismometers are used to record ground motion during earthquakes They also calculate the hypocenter and magnitude.

Reading a Seismograph Seismogram plots vibrations vs. time. Time is marked at regular intervals to determine arrival of first p and s waves. (p. 339) Waves on seismogram Seismogram

Amplitude vs. Period Seismic waves are characterized by wave amplitude – How much the ground moves And…the time it takes for the wave to pass by (the period)

Determining Location Seismograph stations in different locations record the P-S interval. The difference in P-S interval shows the distance of the earthquake from the seismograph station. The longer the interval, the farther the earthquake.

p. 340 in your book

Estimating Station Distance The P-S interval is mathematically related to the distance from the epicenter using a time-travel curve. (p. 340) Takes into account the materials the wave passes through.

Triangulating the Epicenter Distance from each station can be compared graphically to find the epicenter. Circle is drawn around each station with a radius equal to the distance calculated from the time-travel curve Intersection of circles is the epicenter

Measuring Magnitude Richter Scale – Based off distance and amplitude.

Mercalli Scale The modified Mercalli intensity scale (MMI) Describes the effects of an earthquake. On a scale from I to XII.

Charleston, 1886 Hebgen Lake, 1959 Alaska, 1964 San Francisco, 1906 Mexico City, 1985 Northridge, 1994