Knowt
plate tectonics lecture 2 and readings
the lithosphere (crust + upper mantle) is broken into tectonic plates that move over the softer asthenosphere
what is asthenosphere: The asthenosphere is a semi-fluid layer of the upper mantle located beneath the lithosphere, characterized by its ability to flow slowly and allow the tectonic plates above it to move. This movement is driven by convection currents within the mantle, which are caused by the heat from the Earth's core.
the lithosphere is divided into 20 plates, that shape the earths surface and are responsible for various geological phenomena such as earthquakes, volcanic activity, and the formation of mountain ranges.
they move at speeds of 1-10cm per year
major plates include: pacific plate, North American plate, Eurasian plate, African plate, South American plate, Antarctic plate, and Indo-Australian plate. Each of these plates interacts with others at their boundaries, leading to different types of plate boundaries: divergent, convergent, and transform.
what are the key characteristics of each type of plate boundary and how do they contribute to geological activity?
divergent:
Divergent: At divergent boundaries, tectonic plates move apart from each other, allowing magma to rise from below the Earth's surface, creating new crust. This process often leads to the formation of mid-ocean ridges and volcanic activity, such as the Mid-Atlantic Ridge.
convergent:
At convergent boundaries, tectonic plates move toward each other, often resulting in one plate being forced beneath the other in a process known as subduction. This can lead to the formation of mountain ranges, deep ocean trenches, and intense seismic activity, exemplified by the Himalayas and the Pacific Ring of Fire
oceanic-continental:
Denser oceanic plate subducts beneath the continental plate
The oceanic plate is thinner but denser (made of basalt), while the continental plate is thicker but less dense (made of granite).
The subducting oceanic plate melts in the mantle, forming magma that rises and creates volcanoes on the continent.
🗻 Example:
Andes Mountains & Peru-Chile Trench (Nazca Plate subducts under the South American Plate).
🌋 Key Features:
✅ Deep ocean trench (where the oceanic plate bends down).
✅ Volcanic mountain chains (on the continent).
✅ Strong earthquakes (as plates grind against each other).
oceanic-oceanic:
One oceanic plate (usually the older, colder, and denser one) subducts beneath the other
Instead of a continental plate, both plates are oceanic.
The subducting plate melts in the mantle, and magma rises to form volcanic island arcs.
🏝 Example:
Japan, the Philippines, Aleutian Islands (Alaska) (Pacific Plate subducts under smaller plates).
🌊 Key Features:
✅ Deep ocean trenches (e.g., Mariana Trench, the deepest place on Earth).
✅ Volcanic island arcs (chains of volcanic islands).
✅ Tsunamis (caused by sudden plate movement underwater).
continental-continental
How It Works:
Unlike oceanic plates, continental plates are too light to sink into the mantle.
Instead of subduction, they crumple and fold, causing the crust to rise and create huge mountain ranges.
This process leads to severe earthquakes but little to no volcanic activity (since there’s no melting of a subducting plate).
🏔 Example:
Himalayas (India colliding with Eurasia)
Alps (Europe colliding with Africa)
🌍 Key Features:
✅ Massive folded mountain ranges (Himalayas, Alps).
✅ Frequent, powerful earthquakes due to immense pressure buildup.
✅ No major volcanic activity because there’s no subducting plate to create magma.
Transform
At transform boundaries, tectonic plates slide past one another horizontally. This lateral movement can cause significant friction, leading to earthquakes along faults such as the San Andreas Fault in California.
This type of boundary is characterized by the absence of volcanic activity, as there is no creation or destruction of crust; instead, the focus is on the stress and strain accumulated in the Earth's crust.
mechanisms for plate motion - what drives the plate movements
ridge push
new crust forms at mid ocean ridges
gravity pushes away from ridges, like a sled sliding downhill
slab pull
old, dense oceanic crust sinks into the mantle at subduction zones
the sinking plate pulls the rest of the plate behind it, acting as the strongest driving force
mantle convection
heat from earth’s core causes mantle material to rise and fall, like boiling water
this movement drags plates along, though it may play a smaller role than slab pull
evidence of plate movement
seafloor spreading: the process by which new oceanic crust is formed at mid-ocean ridges as magma rises to the surface, creating new seafloor that pushes older crust away from the ridge.
continental drift: the theory that continents are in constant motion on the surface of the Earth, which explains how they were once joined together in a supercontinent and have since drifted apart over millions of years.
evidence:
Fossil distribution: Similar fossils found on widely separated continents support the idea that these continents were once connected.
Geological formations: Mountain ranges and rock types that match across continents indicate a shared geological history.
Paleomagnetism: The study of magnetic minerals in rocks reveals patterns of magnetic orientation that align with the movement of tectonic plates over time.
why was Alfred Wegener’s theory wrong or unfinished
Despite providing substantial evidence, Alfred Wegener's theory of continental drift lacked a convincing mechanism for how continents could move, leading to skepticism among contemporary scientists. His inability to explain the forces behind plate movement, such as the lack of knowledge about mantle convection, contributed to the initial rejection of his ideas.