Unit 2: Plate Tectonics
The evidence that convinced geologists the continents move, and the three boundary types where moving plates build every mountain range and volcanic arc.
9 topics · 29 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Continental Drift: Wegener's Evidence
- How it worksHow did matching fossils and rock layers on separate continents convince Wegener they were once joined?
- Common mistakeWhy wasn't Wegener's continental drift idea accepted right away, even with all that evidence?
- Going furtherWhat piece was still missing from Wegener's idea before scientists could accept a real mechanism for drift?
Your turn1 of 2
Wegener pointed to matching fossils of the reptile Mesosaurus found only in South America and Africa, plus matching rock layers on both continents' coasts. How did matching fossils and rock layers on separate continents convince Wegener they were once joined?Seafloor Spreading and Magnetic Striping
- How it worksHow does new crust forming at a mid-ocean ridge push older crust outward on both sides?
- How it worksWalk me through how reversals in Earth's magnetic field get frozen into stripes of seafloor rock.
- Worked exampleFind the spreading rate of a ridge if matching magnetic stripes 30 km from the ridge formed 1 million years ago.
- Common mistakeWhy isn't the seafloor just sitting unchanged since the ocean basins first formed?
Your turn1 of 2
Matching magnetic stripes 48 km from a mid-ocean ridge formed 1.47 million years ago. What is the ridge's spreading rate, in kilometers per million years?The Theory of Plate Tectonics
- How it worksHow do plate boundaries, not the middle of plates, line up with almost every earthquake and volcano on Earth?
- Common mistakeWhy isn't a tectonic plate the same thing as a continent?
- Ties togetherHow does plate tectonics tie continental drift, seafloor spreading, and mantle convection into one theory?
Your turn1 of 2
About 90 percent of the world's earthquakes occur along plate boundaries. A global survey records 1545 earthquakes in a year. About how many of them would be expected to occur away from plate boundaries?Divergent Boundaries
- How it worksHow does upwelling magma pull two plates apart and fill the gap with brand new crust?
- How it worksWalk me through how a continental rift like the East African Rift could eventually split a continent and open an ocean.
- Going furtherWhat landform does a divergent boundary build on the seafloor versus in the middle of a continent?
Your turn1 of 2
Iceland sits on the Mid-Atlantic Ridge, where two plates diverge at about 2 centimeters per year. Over 147 years, about how many centimeters wider does the gap between the plates become?Convergent Boundaries: Ocean Meets Continent
- How it worksHow does a subducting oceanic plate melt and feed the volcanoes of a chain like the Andes?
- How it worksWalk me through the deep ocean trench, the subduction zone, and the volcanic arc that form together.
- Common mistakeWhy doesn't a subducting plate just crumple up at the surface instead of sliding underneath?
Your turn1 of 2
An oceanic plate subducts beneath a continental plate at a rate of 6 centimeters per year. Over 15 million years, about how many kilometers of oceanic plate have been consumed at the trench?Convergent Boundaries: Continent Meets Continent
- How it worksHow does a collision between two equally buoyant continents crumple rock upward into mountains instead of subducting?
- Going furtherWhat happens to the crust in the middle of a continent-continent collision zone as it keeps thickening?
- Ties togetherHow do continent-continent collisions build mountains differently than ocean-continent collisions do?
Your turn1 of 2
The Himalayas continue rising at roughly 6 millimeters per year as India keeps pushing into Asia. Over 36 thousand years, about how many meters higher would this rate alone add to the mountains?Transform Boundaries
- How it worksHow do two plates grinding sideways past each other at a transform boundary store up energy between quakes?
- Common mistakeWhy isn't a transform boundary a place where crust gets created or destroyed like the other two boundary types?
- Ties togetherHow do transform faults connect segments of mid-ocean ridge to keep seafloor spreading working on a curved planet?
Your turn1 of 2
Two plates grind past each other at a transform boundary at 4 centimeters per year but lock together with friction for 147 years between major earthquakes. About how many centimeters of accumulated slip get released when the fault finally ruptures?What Drives Plate Motion: Mantle Convection
- How it worksHow does hot mantle rock rising and cool mantle rock sinking drag the plates above it along for the ride?
- How it worksWalk me through a full convection cell, from hot rising mantle to cool sinking mantle and back.
- Common mistakeWhy isn't mantle convection the same kind of slow-motion process as an ocean current?
- Going furtherWhat role does a cold, dense subducting slab play in pulling its own plate forward?
Your turn1 of 2
A convection cell in the mantle carries hot rock rising over 38 million years, then moving sideways and cooling, before sinking back down over another 36 million years to complete the cycle. How many million years does one full convection cycle take?Hotspots and Intraplate Volcanism
- How it worksHow does a fixed plume of rising magma punch through a plate that keeps sliding over it?
- Worked exampleFind the plate's speed if a hotspot chain has volcanoes 500 km apart that are 5 million years apart in age.
- Common mistakeWhy doesn't the hotspot itself move to create a chain of islands like Hawaii?
Your turn1 of 2
A hotspot chain has volcanoes 618 km apart that differ in age by 6 million years. What is the plate's speed, in kilometers per million years?