Marine Science I Honors - Plate Tectonics and Earthquakes

Tectonic Plates

• Tectonic plates are pieces of Earth's crust and uppermost mantle, collectively known as the lithosphere.
• These plates are approximately 100 km (62 mi) thick.
• Composed of oceanic crust and continental crust.
• Geologists believe tectonic plates have been moving for about 3.4 billion years.
• This concept is known as continental drift, developed in the early 20th century.

Earth's Structure

• Lithosphere: The rigid outermost shell of the planet, including the crust and upper mantle.
• Asthenosphere: The mantle layer below the lithosphere, close to its melting temperature, allowing it to flow slowly.

Plate Boundaries

• Plates meet at faults, where their relative motion determines the type of boundary.
• Types of plate boundaries: convergent, divergent, and transform.
• Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these boundaries.
• Plate movement ranges from zero to 10 cm annually.

Crust vs. Plates

• Plates are more accurately termed "lithospheric plates."
• The upper part of these plates is Earth’s crust.
• The deeper part is the mantle, sometimes called "lithospheric mantle."
• Lithospheric plates are about 100 km thick.
• They are colder, more rigid, and more brittle than deeper layers.

Asthenosphere

• The mantle below the plates is the asthenosphere.
• It is not liquid but can flow slowly due to being near its melting temperature.
• Silly putty is a useful analog for its mechanical behavior.

Tectonic Plates (Review)

• The approximately 100 km thick surface of the Earth.
• It contains crust and part of the upper mantle.
• Fractures in the plates cause earthquakes.

Asthenosphere (Review)

• The hotter upper mantle below the lithospheric plate.
• It can flow like silly putty because it's a viscoelastic solid, NOT liquid.

Plate Movement

• Driven by cooling of Earth through convection.
• Gravity provides additional force.
• Convection is like a boiling pot: heated material rises, cools, sinks, and reheats.

Heat Energy

• Earth cools by giving off heat.
• Heat from Earth’s interior melts oceanic plates and moves them.
• Convection moves hot material to the surface; gravity draws dense material back into the interior.

Source of Heat

• The heat in Earth’s interior is about 50% from its formation and 50% from radioactive decay.
• Radioactive decay involves the loss of particles from an isotope's nucleus, releasing energy as heat (e.g., uranium, thorium, potassium).
• Residual heat is gravitational energy from Earth's formation 4.6 billion years ago.

Plate Boundaries and Motion

• There are a dozen large and smaller lithospheric plates.
• Some have continents; some don’t.
• All plates are in motion due to convection.
• Below 700 km, descending slabs soften and flow.
• Collisions create interesting structures.

Divergent Boundaries

• Exist between plates moving away from each other.
• Within continents, they create rifts that become rift valleys.
• Most active ones are between oceanic plates, forming mid-oceanic ridges (e.g., Mid-Atlantic Ridge).
• New crustal material fills the space, sourced from molten magma.
• Their origin may be linked to hotspots.

Hotspots

• Volcanic locales fed by anomalously hot mantle.
• Examples: Hawaii, Iceland, and Yellowstone.
• Their position is independent of plate boundaries.
• They can create chains of volcanoes as plates move above them.

Divergent Movement

• Complex convection allows material to rise beneath divergent plate boundaries.
• This supplies heat and reduces pressure, melting rock from the asthenosphere.
• This forms large flood basalt or lava flows.
• Eruptions fill the opening gap as plates move apart.

Convergent Boundaries

• Areas where two or more lithospheric plates collide.
• One plate slides beneath the other in subduction.
• Subduction zones are defined by earthquake-prone planes.
• These collisions occur over millions of years, leading to volcanism, earthquakes, mountain formation (orogenesis), lithosphere destruction, and deformation.

Convergent Movement

• Convection cells bring hot mantle material to the surface, creating new crust.
• As crust moves from the spreading center, it cools, thins, and becomes denser.
• Subduction begins when dense crust converges with less dense crust.

Continental Collision

• Occurs at convergent boundaries.
• The subduction zone is destroyed, mountains are produced, and continents are sutured together.

Mountain Formation

• Continental crust is subducted with difficulty to depths of 90-150 km or more.
• Normal subduction continues until the continent enters the trench.
• The lithosphere is less dense than the asthenosphere, disrupting subduction.
• The volcanic arc is extinguished.
• The crust buckles, raising mountains where the trench was.

Transform Boundaries

• Occur when two tectonic plates move past one another.
• Shear stress operates at these boundaries, involving sliding motion.
• No lithosphere is destroyed or created, and mountain chains are not built.
• They accommodate lateral offset between segments of divergent boundaries, forming a zigzag pattern.
• Smaller numbers are found on land, like the San Andreas Fault.

Transform Movement

• Faults are focused areas of deformation or strain.
• Transform faults accommodate lateral strain by transferring displacement between mid-ocean ridges or subduction zones.
• They act as planes of weakness, which may result in splitting in rift zones.
• As plates rub against each other, stresses cause rocks to break, producing earthquakes.

Earthquakes

• Over a million earthquakes of Magnitude 2 and lower annually.
• 1,500 Magnitude 5 earthquakes annually.
• Strong earthquakes happen more than once per month.
• Magnitude 2 earthquakes occur several hundred times a day.
• The deepest earthquakes occur in subduction zones.

Earthquake Patterns

• Earthquakes coincide with plate boundaries.
• Deepest quakes (blue) are in subduction zones.
• Thousands more earthquakes occur every year than shown on maps.
• Many are in the middle of plates due to adjustments on a round globe.

Dangers of Earthquakes

• Caused by the movement of tectonic plates that happen millions of times a year.
• Powerful earthquakes can cause landslides, tsunamis, and flooding.
• Damage and deaths occur in populated areas due to collapsing structures and fires.

Epicenter

• The spot on the surface above where an earthquake starts.
• Seismic waves travel out from the epicenter.
• This causes vibrations that people can feel.
• The distance people can feel depends on the earthquake’s magnitude.

Richter Scale

• Magnitude is based on the strength and duration of seismic waves.
• 3 to 4.9 is minor.
• 5 to 6.9 is moderate to strong.
• 7 to 7.9 is major.
• 8 or more is an extremely powerful tremor.

Seismograph

• Used to measure the magnitude of an Earthquake using wave amplitude (mm/s).

Aftershock

• Another temblor after an earthquake as the crust settles.
• Usually less powerful but can still be strong.

Japan as an Earthquake Zone

• Many plates converge below Japan's surface (Philippines Sea Plate under the Eurasia Plate).
• Located along the Ring of Fire.
• Experiences large magnitude earthquakes ranging from 7-9 on the Richter Scale.
• Increased risk for tsunamis if earthquakes occur in the ocean with a magnitude of at least 6-7.

Earthquake Locations in the US

• Common in California due to the Pacific and North American tectonic plates grinding against each other along the San Andreas Fault.
• The New Madrid Seismic Zone affects Missouri, Arkansas, Tennessee, Kentucky, and Illinois.

Safety Tips Before an Earthquake

• Talk about safe places in your home and create an emergency kit containing things like first-aid supplies, a flashlight, a cell phone charger, and a battery-operated radio.

Safety Tips During an Earthquake

• Drop: Get down on your hands and knees and crawl to your shelter.
• Cover: Underneath a sturdy table, desk, or bed, cover your head and neck with your arms. If furniture isn’t nearby, crouch down on your knees with your arms over your head and neck next to an interior wall.
• Hold on: If you’re under a piece of furniture, hold on with one hand and move with the furniture if it starts sliding. Stay where you are until the shaking stops.

Safety Tips After an Earthquake

• Check for injuries.
• Listen to the radio for warnings and instructions.
• Be prepared for aftershocks.

Volcanoes

• Occur along plate boundaries on divergent boundaries and on convergent boundaries where an oceanic plate subducts.
• They can also occur in the middle of a plate due to hotspot processes or continental rift areas.

The Ring of Fire

• A region around the rim of the Pacific Ocean with many volcanic eruptions and earthquakes.
• A result of the movement, collision, and destruction of lithospheric plates.
• The collisions have created a series of subduction zones.

Correlation

• There is a strong concentration of earthquakes and volcanoes near plate boundaries.
• Plate motions produce earthquakes and volcanoes.

Yellowstone Supervolcano

• There are no increased geologic hazards at Yellowstone, and there is no chance of a 'megaeruption' in the near future.
• Yellowstone sits on top of the Yellowstone caldera, which is a large cauldron-like hollow structure that forms shortly after the emptying of a magma chamber in a volcano eruption.

Plate Movement Speed

• Plates move 1-10 centimeters per year.
• The Nazca and Pacific Plates are the fastest.
• Rate of plate motions are typically an inch or two per year (the rate of fingernail growth).
• This movement over 100s of millions of years builds and destroys land masses.

Theory of Plate Tectonics

• Remember it takes millions of years for plate tectonics to show significant movement.

Pangea

• From about 300-200 million years ago (late Paleozoic Era until the very late Triassic), the continent we now know as North America was contiguous with Africa, South America, and Europe.
• They all existed as a single continent called Pangea that began to break apart about 200 million years ago, at the end of the Triassic and beginning of the Jurassic periods.

Evidence For Pangea

• The geography of the continents bordering the Atlantic Ocean was the first evidence suggesting the existence of Pangea.
• Fossil evidence for Pangea includes the presence of similar and identical species on continents that are now great distances apart.
• Geologists can also determine the movement of continental plates by examining the orientation of magnetic minerals in rocks.

Supercontinents Before Supercontinents

• There were actually a few supercontinents before the most famous Pangea, showing movement of continents and plates has occurred for billions of years!
• Last 4: Columbia/Nuna, Rodinia, Pannotia, and Pangea

Supercontinent Structures

• Laurasia
• Gondwana
• Tethys Ocean
• Tethys Sea

Laurasia

• Around the Early-Middle Jurassic (about 175 Ma), Pangaea began to rift from the Tethys Ocean in the east to the Pacific Ocean in the west.
• The rifting that took place between North America and Africa produced multiple failed rifts that resulted in a new ocean, the North Atlantic Ocean.

Gondwana

• The second major phase in the break-up of Pangaea began in the Early Cretaceous (150–140 Ma), when the landmass of Gondwana separated into multiple continents (Africa, South America, India, Antarctica, and Australia).
• The subduction at Tethyan Trench probably caused Africa, India and Australia to move northward, causing the opening of a South Indian Ocean.

Madagascar and India

• Madagascar and India began to separate from Antarctica and moved northward, opening up the Indian Ocean.
• Madagascar and India separated from each other 100–90 Ma in the Late Cretaceous.
• India continued to move northward toward Eurasia at 15 centimeters (6 in) a year, closing the eastern Tethys Ocean, while Madagascar stopped and became locked to the African Plate.
• New Zealand, New Caledonia and the rest of Zealandia began to separate from Australia, moving eastward toward the Pacific.

The Himalayan Mountain Range

• As Pangea breaks apart, the seas of the Early Carboniferous period were dominated by corals, brachiopods, sharks, and the first bony fish.
• Life on land was dominated by forests inhabited by insects, other arthropods, and the first tetrapods (4-legged animals)