Earth's Interior and Plate Tectonics Notes
Earth's Interior and Plate Tectonics
Discoveries and Boundaries
Andrija Mohorovičić (1909): Yugoslavian seismologist discovered that the velocity of seismic waves changes and increases at a depth of about 50 km below the Earth's surface.
Mohorovičić Discontinuity (Moho): Boundary between the Earth's crust and the mantle, named in honor of Mohorovičić.
Shadow Zone: Area on Earth's surface where seismographs cannot detect direct P or S waves due to liquid layers within the Earth.
Beno Gutenberg: German seismologist who explained the existence of the shadow zone by proposing that the Earth contains a core composed of material different from the mantle, causing the bending of P-waves.
Gutenberg Discontinuity: Transition zone between the mantle and the core.
Inge Lehmann (1936): Danish seismologist who predicted the innermost layer of the Earth and discovered a new region of seismic reflection within the core.
Thickness of Earth's Layers
Crust: 40 km
Mantle: 2900 km
Outer Core: 2200 km
Inner Core: 1278 km
Composition of Earth's Interior
The Earth is composed of three major layers: the crust, mantle, and core (subdivided into outer and inner core).
Crust
Thinnest and outermost layer, extending from the surface to about 32 km below.
Thickness can extend to 72 km underneath some mountains.
Subdivided into two regions:
Continental Crust: Mainly made up of silicon, oxygen, aluminum, calcium, sodium, and potassium.
Composition Insights
Iron and nickel are both dense and magnetic.
Overall density of the Earth is much higher than the density of rocks in the crust.
Meteorite Analysis:
Most common type is chondrite, containing iron, silicon, magnesium, and oxygen; some contain nickel.
The whole earth and the meteorite roughly have the same density, thus the Earth's mantle rock and a meteorite minus its iron, have the same density.
Continental Drift
Alfred Wegener (1912): German meteorologist proposed the theory that about 200 million years ago, the continents were once one large landmass called Pangaea (Greek for "All Earth").
Pangaea started to break into two smaller supercontinents, Laurasia and Gondwanaland, during the Jurassic Period. These smaller supercontinents broke into the continents, which have separated and drifted apart since then.
Evidence for Continental Drift
Continental Jigsaw Puzzle:
The shape of the continents suggests they were once connected.
The edges of continents surprisingly match each other.
Fossils:
Preserved remains or traces of organisms from the remote past.
Fossilized leaves of the extinct plant Glossopteris (250 million years old) were found in Southern Africa, Australia, India, and Antarctica.
Mesosaurus and Lystosaurus: Freshwater reptiles whose fossils were discovered in South America and Africa. It would have been impossible for these reptiles to swim across vast oceans.
Rocks:
Rock formations in Africa line up with those in South America, resembling a long mountain range.
Coal Deposits
Coal beds were formed from the compaction and decomposition of swamp plants that lived million years ago.
These were discovered in South America, Africa, Indian subcontinent, Southeast Asia, and even in Antarctica.The Seafloor Spreading
According to this theory, hot, less dense material from below the earth's crust rises towards the surface at the mid-ocean ridge. This material flows sideways carrying the seafloor away from the ridge, and creates a crack in the crust. The magma flows out of the crack, cools down and becomes the new seafloor.
The process of seafloor spreading allowed the creation of new bodies of water.
Seafloor spreading is also pulling the continents of Australia, South America, and Antarctica away from each other in the East Pacific Rise.
East Pacific Rise-one of the most active sites of seafloor spreading, with more than 14 centimeters every year.
Findings that support Seafloor Spreading Theory:
Rocks are younger at the mid-ocean ridge.
Rocks far from the mid-ocean ridge are older.
Sediments are thinner at the ridge.
Rocks at the ocean floor are younger than those at the continents.
Magnetic Reversal-called magnetic 'flip' of the Earth. It happens when the North Pole is transformed into a South Pole and the South Pole becomes the North Pole. This is due to the change in the direction of flow in the outer core.
Plate Tectonic Theory -provided an explanation about the movement of the lithospheric plates.
his theory evolved from the two former theories and was developed during the first decades of the 20th century.
Plate Tectonics
Earth's Lithosphere
Outermost layer of the Earth, made up of the crust and the upper part of the mantle.
Crust:
Outer portion of the Earth.
Average density of 2.8 g/cm^3
Thickness ranges from 5 to 50 km.
Made of various solid rocks:
Sedimentary (forms from sediments compaction)
Metamorphic (forms by transformation of other rock)
Igneous (forms from magma or lava solidification)
Two kinds of crust:
Continental Crust:
Thick part of the Earth's crust, not located under the oceans.
Thicker but less dense.
Oceanic Crust:
Thin part of the Earth's crust, located under the oceans.
Thinner but dense.
Plate Tectonics Theory
The Earth's crust is broken into segments (plates) that move slowly but constantly (tectonics).
Plate Tectonics: A theory which suggests that Earth's crust is made up of plates that interact in various ways, producing earthquakes, mountains, volcanoes, and other geologic features.
Major Plates
North American Plate
South American Plate
Pacific Plate (largest plate)
African Plate
Eurasian Plate
Antarctic Plate
Australian Plate
Minor Plates
Indian Plate
Arabian Plate
Scotia Plate
Cocos Plate
Philippine Plate
Caribbean Plate
Juan De Fuca Plate
Nazca Plate
Effects of Plate Movement
Earthquakes
Formation of Volcanoes
Formation of Mountains/Mountain Ranges
Earthquake
Vibration of Earth due to the rapid release of energy.
Releases three types of seismic waves.
Formula for computing the distance of the epicenter from the station:
d = \frac{T_d}{8 \text{ seconds}} \times 100 \text{ km}
Where:
d = distance (km)
T_d = time difference in the arrival time of P-wave and S-wave (seconds)
Seismic Waves
Primary (P) wave: First type of seismic wave to be recorded in a seismic station.
Secondary (S) wave: Second type of earthquake wave to be recorded in a seismic station.
Long (L) surface wave: Can be detected using a seismograph; the record is called a seismogram.
Fault: A break in a rock along which movement has occurred.
Fracture: Any break in a rock in which no significant movement has taken place.
Epicenter: The point on the ground directly above the focus.
Types of Plate Boundaries
A. Divergent Plate Boundary
Formed when plates move apart (moving away from each other).
Creating a zone of tension.
Along these boundaries, earthquakes are common, and magma (molten rock) rises from the Earth's mantle to the surface, solidifying to create new oceanic crust.
The Mid-Atlantic Ridge is an example of divergent plate boundaries.
Geologic Events/Events Present:
Rift valleys (down faulted valleys)
Oceanic ridges (underwater mountain ranges)
Earthquakes
Formation of rift valleys and oceanic ridges are indications that the crust is spreading or splitting apart.
B. Convergent Plate Boundary
Plates move towards each other (come together).
Present when two plates collide (one plate dives [subducts] beneath the other).
The colliding plate can cause the edges of one or both plates to buckle up into mountain ranges, or one of the plates may bend down into a deep seafloor trench.
The Pacific Ring of Fire is an example of a convergent plate boundary.
Geologic Events/Events Present: Trenches Earthquakes
Mountains
Volcanoes
Three Types of Convergent
Boundaries
(oceanic-oceanic)- the denser plate will end up sinking below the less dense plate, leading to the formation of an oceanic subduction zone.
(oceanic-continental)-the oceanic crust will always subduct under the continental crust; this is because oceanic crust is naturally denser.
(continental-continental)
Collision zone is formed.
Colliding plates eventually collide and end up producing mountains; this was how the Himalayan Mountains were created. Neither continental crust will subduct underneath one another because of their similar densities.
C. Transform Fault Boundary
Plates that are sliding past each other (without diverging or converging).
Unlike the other two types of plate boundaries in which new seafloor is created at divergent boundaries and where old seafloor is subducted at convergent boundaries, transform plate boundaries neither create nor destroy the seafloor.
One example would be the San Andreas fault.
Geologic Events/Events Present:
Earthquakes
Hot Spots
Concentration of heat in the mantle capable of creating magma.
Not simply a shallow reservoir or a pipe from the liquid outer core.
Rising heat causes convection in the mantle; hot material rises and cooler material sinks.
Hawaiian Island-style hotspot beneath a moving lithospheric plate.
Seismic Waves Detailed
Energy radiates in all directions from the focus in the form of waves, which are recorded in seismographs.
Two main types of seismic waves:
Body waves
Surface waves
Surface Waves
Can only travel through the surface of the Earth.
Arrive after the main P and S waves and are confined to the outer layers of the Earth.
Two types of surface waves:
Love waves:
Named after A.E.H. Love.
Faster than Rayleigh waves.
Move the ground in a side-to-side horizontal motion, causing the ground to twist and causing the most damage to structures during an earthquake.
Rayleigh waves:
Named after Lord Rayleigh.
Roll along the ground like a wave across a lake or ocean.
Move the ground either up and down or side-to-side similar to the direction of the wave's movement.
Most of the shaking felt from an earthquake is due to this wave.
Body Waves
Can travel through the Earth's inner layers and are used by scientists to study the Earth's interior.
Higher frequency than surface waves.
Two types of body waves:
P-waves (primary waves):
Travel quickly through the Earth and through liquids, faster than S-waves.
Reach a detector first after an earthquake.
Also called compressional waves, they travel by particles vibrating parallel to the direction the wave travels.
Travel through solids, liquids, and gases.
S-waves (secondary waves):
Travel slower than P-waves through Earth and solids.
Move as shear or transverse waves, forcing the ground to sway from side to side in rolling motion.
Cannot travel through liquid medium, leading seismologists to conclude that the outer core is liquid.
Since P-waves travel faster than S-waves, they're always detected first. The farther away from the epicenter means the longer time interval between the arrival of P and S waves.
Composition of the Crust Detailed
Continental Crust
Mostly 35-40 kilometers thick.
Found under land masses, is made of less dense rocks such as granite.
Oceanic crust Around 7-10 kilometers thick which its average thickness is 8 kilometers.
It is found under the ocean floor and is made of dense rocks such as basalt.
heavier than the continental crust.
Crust consists of two layers. The upper layer is composed of granite and is only found in the continental crust.
Oceanic Crust
Around 7-10 kilometers thick, with an average thickness of 8 kilometers.
Found under the ocean floor and is made of dense rocks such as basalt.
Heavier than the continental crust.
Elements in the Earth's Crust
Oxygen: 46.60%
Silicon: 27.72%
Aluminum: 8.13%
Iron: 5.00%
Calcium: 3.63%
Sodium: 2.83%
Potassium: 2.59%
Magnesium: 2.09%
Titanium: 0.40%
Hydrogen: 0.14%
Mantle
Beneath the crust, extending to about 2900 kilometers from the Earth's surface.
Makes up most of the Earth's volume (about 80% Total Volume) and mass (about 68% Total Mass).
Made up of silicate rocks and is solid, since both S-waves and P-waves pass through it.
Mostly made of the elements silicon, oxygen, iron, and magnesium.
The crust and the uppermost part of the mantle form a relatively cool, outermost rigid shell called the Lithosphere.
Lithospheric Plates
Lithosphere is subdivided into portions that move relative to each other.
Asthenosphere
Beneath the lithosphere lies the soft, weak layer.
Made of hot molten material.
Temperature is about 300-800°C.
Plate Tectonic Theory
Three types of plate movements
Divergent-separation of two plates.
Convergent-collision of two s plates.
Transform-sliding past each other.
What facilitates the movement of the plates? Heat is produced in the core that produces convection in the mantle. This convection causes the plate to move around.
Core
Subdivided into two layers:
Outer Core
2900 kilometers below the Earth's surface.
2250 kilometers thick and is made up of iron and nickel.
Temperature in the outer core reaches up to 2000°C; nickels melt.
Inner Core
Made up of solid iron and nickel.
Has a radius of 1300 kilometers.
Temperature reaches as high as 5000°C.
Extreme temperature could have molten the iron and nickel, but it is believed to have solidified as a result of pressure freezing.
Additional Concepts
Convection Current: As a substance like water is heated, the less dense particles rise while denser particles sink. Once the hot less dense particles cool down, they sink, and the other less dense particles rise. This is exactly what happens in the Earth's mantle.
rotate very slowly, as they move and drag the plates along.Ridge push: This process occurs as the older seafloor sinks, the weight of the uplifted ridge pushes the oceanic crust toward the trench at the subduction zone.
Slab pull: Other possible process involved in the tectonic plate movement. The weight of the subducting plate pulls the trailing slab into the subduction zone just like a tablecloth slipping off the table and pulling items with it.
Glossary of Terms
Asthenosphere-soft, weak upper portion of the mantle where the lithospheric plates float and move around
Continental Drift Theory- states that all the continents were once one large landmass that broke apart, and where the pieces moved slowly to their current locations
Convection current-current in the mantle because of the heat from the inner layers of the Earth, and is the force that drives the plates to move around
Lithosphere-the topmost, solid part of the Earth that is composed of several plates
Lithospheric Plates-the moving, irregularly-shaped slabs that fit together to form the surface of the Earth
Mid-ocean ridge-area in the middle of the ocean where a new ocean floor is formed when lava erupts through the cracks in the Earth's crust
Mohorovičić Discontinuity (Moho)-the boundary that separates the crust and the mantle
Plasticity-the ability of solid to flow
Seafloor spreading- the process by which new ocean floor is formed near the mid-ocean ridge and moves outward
Subduction-the process in which the crust plunges back into the Earth
Tectonics-branch of geology that deals with the movements that shape the Earth's crust