Plate Boundaries & Movement
At this type of boundary, two tectonic plates move apart from one another
What is a divergent boundary?
This is the thinner, denser type of crust found at the bottom of the oceans, usually composed of basalt.
What is oceanic crust?
In 1915, Alfred Wegener proposed this theory, suggesting that all continents were once joined together and have since drifted apart.
What is Continental Drift?
This is a complete flip of Earth's magnetic field, where magnetic North and South swap places.
What is a magnetic reversal?
This is a break or fracture in Earth's crust along which movement occurs.
What is a fault?
This famous 800-mile-long California fault is a classic example of a transform boundary where plates slide past each other.
What is the San Andreas Fault?
This layer of the Earth, consisting of the upper mantle and crust, is broken up into the tectonic plates.
What is the lithosphere (or lithospheric plates)?
Wegener proposed that 200 million years ago, all continents were joined together in this giant supercontinent.
What is Pangaea?
This is the name of the scientific instrument used to "read" the direction and strength of the magnetic field locked in rock layers.
What is a magnetometer?
This point on Earth's surface sits directly above the "focus," which is where the earthquake actually originates underground.
What is the epicenter?
At a convergent boundary, this process occurs when a denser oceanic plate is forced downward beneath a less dense continental plate.
What is subduction?
This partially molten, plastic-like layer of the mantle underlies the lithospheric plates and allows them to move.
What is the asthenosphere?
Wegener used this specific fossil reptile, found only in eastern South America and western Africa, as evidence that the continents were once connected.
What is the Mesosaurus?
When new magma cools at a mid-ocean ridge, this specific magnetic mineral aligns itself with Earth's magnetic north pole, preserving a record of Earth's polarity.
What is iron (or basalt/magnetite)?
These are the fastest seismic waves, which compress and stretch rocks back-and-forth and can travel through solids, liquids, and gases.
What are Primary waves (P-waves)?
This incredibly tall mountain range—home to Mt. Everest—was formed by the collision (convergence) of two continental plates, meaning it has no volcanoes.
What are the Himalayas? (Bonus detail: Formed by the collision of the Indian and Eurasian plates.)
This is the primary heat-transfer mechanism in the asthenosphere that drives plate motion, where hot rock rises, cools, and sinks.
What are convection currents?
This process, developed by Harry Hess, occurs at divergent boundaries where new oceanic crust is continuously created.
What is sea-floor spreading? (Bonus detail: This is where we find the youngest rocks in the middle and the oldest rocks furthest out.)
This term refers to a long, tube-shaped piece of rock and sediment drilled from the ocean floor that acts as a magnetic timeline for scientists.
What is an ocean core sample?
Unlike P-waves, these slower seismic waves move at right angles (up and down/side-to-side) and can only travel through solids, which is how we know Earth's outer core is liquid.
What are Secondary waves (S-waves)?
When comparing old and young oceanic plates in a subduction zone, this is how the angle of subduction differs
What is steeper for old (colder, denser) plates and shallower for young plates?
Compared to oceanic crust, continental crust differs in these three key physical variables.
What are thickness (it is thicker), density (it is less dense), and composition (it is more granitic/silica-rich rather than basaltic/iron-rich)?
This is the primary reason the scientific community initially rejected Wegener's theory of Continental Drift.
What is he could not explain what caused the plates/continents to move?
Explain why a scientist could not prove the entire theory of seafloor spreading using only a single ocean core sample.
What is because a single core only shows a vertical timeline of one spot; they need to drill multiple cores across the ocean floor to show the symmetrical "striped" pattern of magnetic reversals on both sides of the ridge?
While the Richter Scale measures an earthquake's energy (magnitude), this scale assesses an earthquake's damage based on actual observations.
What is the Mercalli Intensity Scale? (Bonus detail: It uses Roman numerals from I to XII.)