AP Environmental Science

## Learning Outcomes

- Explain how divergent plate boundaries create new crust
- Distinguish between **seafloor spreading** as a process and **mid-ocean ridges** or **rift valleys** as features
- Identify the differences between oceanic and continental divergent boundaries
- Understand why divergent boundaries are linked to shallow earthquakes and volcanism
- Use examples such as the **Mid-Atlantic Ridge** and the **East African Rift**
- Apply this knowledge to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand divergent boundary outcomes, with a focus on the following syllabus points:

- Know that a **divergent boundary** is where two tectonic plates move apart
- Identify **magma upwelling** as the source of new crust at spreading centers
- Explain **seafloor spreading** as the formation of new oceanic crust at oceanic divergent boundaries
- Recognise **mid-ocean ridges** as the major feature formed in oceanic divergence
- Understand that **rift valleys** form where continental crust is stretched and pulled apart
- Distinguish oceanic divergent boundaries from continental divergent boundaries
- Contrast divergent boundaries with convergent boundaries: crust creation versus crust recycling
- Identify common associated features, including shallow earthquakes, submarine volcanism, and basaltic crust

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What happens to tectonic plates at a divergent boundary?
2. How does seafloor spreading produce new oceanic crust?
3. What is the difference between a mid-ocean ridge and a rift valley?
4. Why are shallow earthquakes common at divergent boundaries?
5. Name one oceanic and one continental example of divergent boundary activity.

## Introduction

## Introduction

Plate tectonics is one of the foundation ideas in AP Environmental Science because it explains how the solid Earth changes over time. Divergent boundaries matter especially because they are one of the main places on Earth where **new crust is created**. That idea connects plate movement to ocean formation, volcanic activity, earthquakes, and long-term changes in landforms.

For the exam, students often remember only the simple phrase “plates move apart.” That is not enough. You need to understand what fills the gap, why the crust formed there is new, how the process differs in oceans and continents, and what landforms result.

> **Key Term: divergent boundary**
> A plate boundary where two tectonic plates move away from each other, allowing magma to rise and create new crust.

### How divergent boundaries create new crust

At a divergent boundary, plates separate. This creates tension in the lithosphere. As the plates move apart, material from deeper below rises toward the gap. That material is **magma**, molten rock beneath the surface. When it rises and cools, it solidifies and becomes new crust.

This is the key chain you should know:

- plates separate
- pressure is reduced in the gap
- magma rises from below
- magma cools and hardens
- new crust forms

That is why divergent boundaries are sometimes described as **constructive** boundaries. They build crust rather than destroying it.

> **Key Term: magma upwelling**
> The rise of molten rock from below the lithosphere into the gap created as tectonic plates move apart.

Students often miss an important detail here: the plates do not leave behind an empty hole for long. The gap is continuously filled by rising magma. This means divergence is not just “pulling apart.” It is also a process of crust formation.

At oceanic divergent boundaries, the new crust is usually **basaltic oceanic crust**. Basalt forms from magma that cools into dense, dark igneous rock. In underwater settings, lava may cool rapidly on contact with seawater, helping build the seafloor.

This process also explains why the youngest ocean crust is found near the center of a mid-ocean ridge, while older crust is farther away. As new crust forms in the middle, older crust is pushed outward on both sides. In diagrams, this often appears as symmetrical bands of crust spreading away from the ridge.

> **Key Term: seafloor spreading**
> The process in which magma rises at an oceanic divergent boundary, cools, and solidifies to form new oceanic crust.

A common AP error is to confuse the process with the feature. **Seafloor spreading** is what happens. A **mid-ocean ridge** is the large landform produced by that process.

> **Test Tip:** If a question asks what process creates new ocean floor, choose **seafloor spreading**, not **mid-ocean ridge**. One is the mechanism; the other is the feature.

### Oceanic divergent boundaries: mid-ocean ridges, shallow earthquakes, and volcanism

Most divergent boundaries occur under the ocean. Here, the major surface feature is the **mid-ocean ridge**, a long underwater mountain chain that marks where plates are pulling apart.

> **Key Term: mid-ocean ridge**
> A long underwater mountain chain formed at an oceanic divergent boundary where seafloor spreading creates new crust.

The ridge forms because hot material rising from below is less dense and pushes upward, so the seafloor is elevated in that zone. The result is not a trench or a flat crack, but a raised system running through the ocean basin.

Two examples named in APES materials are:

- **Mid-Atlantic Ridge**
- **East Pacific Rise**

The Mid-Atlantic Ridge is especially useful because it clearly shows the idea of two plates moving apart with new crust forming between them. If the exam asks for an oceanic example of divergence, this is a very safe answer.

Divergent oceanic boundaries are also associated with **frequent shallow earthquakes**. Why shallow? Because the fracturing happens near the surface where the lithosphere is being pulled apart. At convergent boundaries, especially subduction zones, earthquakes can occur much deeper. At divergent boundaries, the crust breaks in the upper region as tension is released.

> **Key Term: shallow earthquake**
> An earthquake that begins close to Earth’s surface, commonly caused at divergent boundaries by crust being pulled apart and fractured.

Volcanism is also common here. As magma rises and reaches or nears the seafloor, it can erupt. Because this often happens underwater, students may forget it is still volcanism. In fact, much volcanic activity on Earth happens beneath the oceans.

Some oceanic divergent boundaries also support **hydrothermal vents**. These form where seawater circulates through hot, newly formed crust, becomes heated, dissolves minerals, and then rises back to the seafloor. While vents are not always the main exam focus, they are a strong clue that the question is describing a spreading center.

Cause and effect matters here:

- divergence causes crustal tension
- crustal tension causes fractures and shallow earthquakes
- mantle-derived magma rises into fractures
- eruptions and cooling form new basaltic crust
- repeated formation builds a ridge system

Students sometimes wrongly link divergent boundaries to deep-ocean trenches. Trenches are mainly associated with **convergent** subduction zones, not spreading centers.

> **Test Tip:** If the stimulus mentions a ridge, shallow quakes, submarine volcanism, and new crust, think **divergent boundary**. If it mentions a trench and subduction, think **convergent boundary**.

### Continental divergent boundaries: rift valleys and the start of ocean basin formation

Not all divergent boundaries are underwater. Some begin within continents. In these cases, the continental crust is stretched, thinned, and broken along faults. The main feature produced is a **rift valley**.

> **Key Term: rift valley**
> A structural low area formed when crust is pulled apart and drops between faults at a divergent boundary.

A rift valley is not just “any valley near plate movement.” It forms specifically because extension pulls the crust apart. As the crust thins, blocks of it drop downward relative to surrounding land, creating a long, low valley.

The key continental example for APES is the **East African Rift** or **East African Rift Zone**. This is one of the clearest real-world examples of continental rifting.

In a continental divergent setting, the sequence is often:

- continental crust is stretched
- faults develop
- a rift valley forms
- volcanism and earthquakes occur
- if spreading continues long enough, the continent may split further
- eventually a new ocean basin can begin to form

This progression matters because it shows that some ocean basins may start as continental rifts. A rift valley can therefore be an early stage in much larger tectonic change.

Students often assume new crust at divergent boundaries is always oceanic from the start. That is not quite right. In a continental rift, the earliest stage mainly involves stretching and faulting of continental crust. If rifting continues long enough and magma keeps adding material, the area may later develop into a new ocean basin with oceanic crust.

That is why it is useful to compare the two settings side by side:

| Feature             | Oceanic divergent boundary               | Continental divergent boundary                                        |
| ------------------- | ---------------------------------------- | --------------------------------------------------------------------- |
| Plates involved     | Usually oceanic plates                   | Continental crust being pulled apart                                  |
| Main landform       | Mid-ocean ridge                          | Rift valley                                                           |
| New crust formation | Clear production of new oceanic crust    | May begin with crust stretching, later leading to new basin formation |
| Common activity     | Submarine volcanism, shallow earthquakes | Volcanism, shallow earthquakes, faulting                              |
| Example             | Mid-Atlantic Ridge                       | East African Rift                                                     |

This comparison clears up a frequent confusion: both settings are divergent, but the immediate visible outcome is different.

### How to analyse divergent boundary questions on the AP exam

AP Environmental Science questions usually test this topic through diagrams, maps, and process chains. You may be shown arrows pointing away from a central crack or ridge, and asked to identify the boundary type. Or you may be asked which boundary creates new crust, rift valleys, or seafloor spreading.

The most reliable reasoning pattern is:

1. Identify plate motion.
2. Link that motion to a geologic process.
3. Link the process to a landform or hazard.

For example:

- If plates move apart, that suggests divergence.
- Divergence allows magma upwelling.
- Magma cooling forms new crust.
- In oceans, this creates a ridge.
- On continents, this creates a rift valley.
- Fracturing near the surface causes shallow earthquakes.

This kind of multi-step reasoning is what AP exam questions reward.

A typical worked example might look like this:

A diagram shows two plates moving away from each other beneath the ocean. A raised ridge runs between them, and arrows show magma rising into the central gap.

How should you identify it?

- Plates moving apart = divergent boundary
- Magma rising = crust creation
- Under ocean = seafloor spreading
- Raised feature = mid-ocean ridge

Therefore, the best answer is: **an oceanic divergent boundary where seafloor spreading forms new oceanic crust at a mid-ocean ridge**.

Another likely question style is contrast:

Which boundary creates new crust rather than recycling it?

The answer is divergent boundary, because convergent subduction zones commonly destroy or recycle crust into the mantle.

One final distinction students must keep straight:

- **Magma** is molten rock below the surface
- **Lava** is molten rock after it erupts onto the surface

If new crust is forming in the gap before eruption or beneath the seafloor, the best word is usually **magma**, not lava.

> **Key Term: oceanic crust**
> Dense crust, commonly basaltic, that forms much of the ocean floor and is created at oceanic divergent boundaries.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- A divergent boundary is where two tectonic plates move apart
- Divergent boundaries are major sites of new crust formation
- Magma rises into the gap between separating plates and cools to form crust
- Seafloor spreading is the process that creates new oceanic crust
- A mid-ocean ridge is the major landform formed at oceanic divergent boundaries
- Rift valleys form where continental crust is stretched and pulled apart
- The Mid-Atlantic Ridge is a major oceanic example of divergent boundary activity
- The East African Rift is a major continental example of divergent boundary activity
- Divergent boundaries are commonly associated with shallow earthquakes
- Oceanic divergent boundaries are often associated with submarine volcanism
- Divergent boundaries create crust, while convergent boundaries often recycle crust
- Students must distinguish process from feature: seafloor spreading versus ridge or rift valley

## Key Terms and Concepts

- divergent boundary
- magma upwelling
- seafloor spreading
- mid-ocean ridge
- shallow earthquake
- rift valley
- oceanic crust


Plate Tectonics - Divergent boundary outcomes