The Earth's rigid outer layer, the lithosphere (which consists of the crust and the solid upper part of the mantle), is not one solid piece. It is broken into about a dozen major tectonic plates and many minor ones. These plates "float" on the semi-molten, flowing layer beneath it called the asthenosphere. The plates are in constant, slow motion, moving a few centimeters per year (about the speed your fingernails grow).
Plate Tectonics is the grand, unifying theory of geology. It explains that the Earth's plates move and interact with each other at their boundaries, and that these interactions are responsible for most of the Earth's major geological features, such as earthquakes, volcanoes, and mountain ranges.
This is the driving force behind plate tectonics. The mantle acts like a pot of thick, boiling soup:
This slow, circular flow is called a convection cell. The lithospheric plates are "dragged" along by the movement of these cells, like a conveyor belt.
Some of the major plates include the Pacific, North American, Eurasian, African, South American, Antarctic, and Indian-Australian plates. All major geological "action" happens where these plates meet.
There are three types of plate boundaries:
| Boundary Type | Relative Motion | Geological Features | Real-World Example |
|---|---|---|---|
| Divergent | Plates move apart (Constructive) |
|
Mid-Atlantic Ridge; East African Rift Valley |
| Convergent | Plates move together (Destructive) |
|
Andes Mountains (Ocean-Continent); Japan (Ocean-Ocean); Himalayas (Continent-Continent) |
| Transform | Plates slide past each other (Conservative) |
|
San Andreas Fault, California |
The Earth acts like a giant bar magnet, with a magnetic field surrounding it (the magnetosphere). This field is what makes a compass point north.
The field is "dipolar," meaning it has two poles: a North Magnetic Pole and a South Magnetic Pole. These magnetic poles are located near (but not exactly at) the geographic poles (the axis on which Earth spins).
A key feature of the field is that it "flips" its polarity at irregular intervals (ranging from thousands to millions of years). During a magnetic reversal, the North Magnetic Pole becomes the South Magnetic Pole, and vice versa. A compass would point south instead of north. We know this has happened many times in Earth's history because of the magnetic record preserved in rocks (see Paleomagnetism).
The Dynamo Theory: The Earth's magnetic field is generated by the movement of the liquid iron-nickel alloy in the Outer Core.
This process, known as the geodynamo, works like this:
In short: The flow of liquid metal in the outer core acts as a self-sustaining electrical generator (a dynamo), producing the Earth's magnetic field.
Continental Drift was the hypothesis, proposed by Alfred Wegener in 1912, that all the continents were once joined together in a single supercontinent called Pangaea ("all lands") and have since "drifted" apart to their current positions.
Wegener's evidence was strong:
Proposed by Harry Hess in the 1960s, Seafloor Spreading was the missing mechanism for continental drift. It provided the "how" and led directly to the theory of Plate Tectonics.
The process is simple:
This explains why oceanic crust is very young at the ridges and gets progressively older as you move away from them.
Paleomagnetism ("ancient magnetism") is the study of the Earth's magnetic field as it is recorded in rocks. It provided the final, undeniable proof for seafloor spreading.
In the 1960s, scientists towing magnetometers across the oceans found a striking pattern. The seafloor was composed of "magnetic stripes" of normal and reversed polarity.
Crucially, this pattern of stripes was perfectly symmetrical on both sides of the mid-ocean ridges. This was the "smoking gun" that proved seafloor spreading was real. New crust was forming at the ridge, recording the Earth's magnetic polarity at the time, and then spreading out in both directions.