Exhaustive Guide to Divergent Boundaries, Plate Tectonics, and Seafloor Spreading

A divergent boundary is a tectonic region characterized as a line where two plates are moving away from each other.
While common intuition might suggest that a boundary where plates separate would form a massive valley, this is generally not the case for most divergent boundaries on Earth.
Contrary to the idea of a simple valley, these boundaries typically form extensive underwater mountain ranges known as ocean ridges.

Mid-ocean ridges are significantly elevated, standing between $6,000$ and $12,000\text{ feet}$ above the surrounding seafloor.
There are two primary reasons why these boundaries form ridges rather than valleys:
- Magma Infills: As plates separate, magma rises to fill the resulting gap. Approximately $70\%$ of all volcanic activity on Earth occurs on the ocean floor, with the vast majority of that percentage occurring at ocean ridges. This constant supply of magma solidifies, preventing the formation of a deep void.
- Upwelling Asthenosphere: This is the more significant reason for the elevation. Material is convected upward from the mantle. Through the process of decompression melting, some of this rising material melts to create magma chambers beneath the ridge. The upward force of the convecting asthenosphere physically bows the lithosphere upward, creating a tall mountain range feature.
On tectonic maps, the ocean floor appears consistently shallow along these plate boundaries due to this mountain-forming process.

The Earth's lithosphere has an average depth of approximately $100\,\text{km}$ . It is slightly thicker beneath continents and slightly thinner beneath the ocean floor.
Earthquake Physics: Earthquakes can only occur in brittle materials. They cannot happen in liquids or ductile solids.
- A common metaphor for this is breaking a pencil: the "snap" heard and the vibrations felt represent the energy release and waves (P-waves and S-waves) that occur when brittle material breaks.
- In the lithosphere, this breakage creates seismic waves that emanate from the site of the energy release.
Shallow Earthquakes: At divergent boundaries (and transform boundaries), earthquakes are always considered "shallow," meaning they occur within the $100\,\text{km}$ thickness of the brittle lithosphere.
Deep Earthquakes: Deep earthquakes occur in subduction zones (convergent boundaries) where the lithosphere dives deep beneath another plate.

Divergent boundaries are the primary sites where oceanic crust is created.
The intensive volcanism and melting at these sites lead to magma rising and solidifying, forming new crust as the plates move apart.
By contrast, crust is generally destroyed at convergent boundaries and remains neutral at transform boundaries.

The Rock Clock: In the context of mid-ocean ridges, the "age" of a rock begins at the moment the lava solidifies into solid rock.
The primary rock type created at mid-ocean ridges is Basalt. Basalt is a mafic rock, meaning it is dark-colored and rich in iron.
Magnetic Alignment: Because basalt contains iron, it acts as a record of the Earth's magnetic field. Iron atoms within the rock function like tiny compass needles.
These iron molecules align themselves with the Earth's magnetic North Pole at the time the rock is formed.
Throughout Earth's history, the magnetic field has reversed its direction (switching between the North and South poles). Scientists do not yet understand the mechanism behind these reversals, but the evidence is clearly preserved in the rock record.

In melted rock (magma), atoms and molecules of iron and silicates move rapidly and randomly due to high heat. This state prevents the magnetic field from aligning the iron molecules.
The Curie Point Definition: This is the specific temperature threshold where cooling magma (still technically liquid but slowing down) allows the iron molecules to align with the Earth's magnetic North.
Once the rock solidifies, these "compass needles" are locked into the crystalline lattice. Even if the Earth's magnetic field subsequently reverses, the iron atoms in that specific rock remain fixed in their original orientation.

The ocean floor provides a symmetrical record of magnetic reversals on either side of a divergent boundary.
Process of Record Formation:
1. New magma erupts at the ridge during a period of "normal" magnetism and solidifies.
2. Over millions of years, seafloor spreading pushes this older rock away from the ridge.
3. If the magnetic field reverses, new magma erupting at the ridge will record this "reversed" magnetism.
4. This creates a pattern of symmetrical magnetic stripes across the ocean floor.
Historical Data:
- The oldest oceanic crust is approximately $220,000,000\,\text{years}$ old.
- Magnetic reversals occur on average every $800,000\,\text{years}$ , though they are not periodic. Reversals have taken place at intervals ranging from $300,000$ to $1,500,000\,\text{years}$ .
- While scientists are monitoring the current magnetic field for signs of a reversal, there is no evidence that reversals correlate with mass extinctions or catastrophic biological failure, though they might disrupt animal navigation and modern travel.
Scientific Discovery: In the mid-1900s, scientists Vine and Matthews used magnetometers to map the ocean floor. Finding this symmetrical pattern of magnetic reversals was one of the leading lines of evidence for seafloor spreading and plate tectonics.

A common student misconception is that magnetic force moves the tectonic plates.
Clarification: Magnetic lines of force are physically too weak to move plates.
The forces that actually move plates include:
1. Convection in the asthenosphere.
2. Slab pull during subduction.
3. Ridge push during seafloor spreading.
Comparing magnetic force to plate movement is akin to trying to move the entire Sierra Nevada mountain range with an index finger; the force is simply insufficient.

While most divergent boundaries are underwater, some occur within continents, leading to continental rifting.
East African Rift Zone: This area shows active divergence. Satellite imagery reveals linear lakes formed as the rift valley splits and fills with precipitation. In tens of millions of years, this will likely become a new ocean basin.
Lake Baikal (Russia): Another area of divergence. Some geologists believe Asia acts as a heat trap because it is a thick, buoyant landmass, causing the continent to split.
Process: Continents are buoyant (not more dense than oceanic crust, despite a previous slip of the tongue by the speaker), but their thickness can trap heat underneath, eventually leading to rifting and the creation of new ocean basins over roughly $100,000,000\,\text{years}$ .

Question: How is a rock age dated (where does the clock begin)?

Choice A: At the time the crust melted.
Choice B: At the time the melted rock made it to the Earth's surface as lava.
Choice C: At the time the lava solidified into rock.
Answer: The correct answer is C. The age refers to when the material became the specific rock in question (e.g., basalt), which happens upon crystallization/solidification. This is when the permanent magnetic record is locked into the rock's lattice.