Types and Distribution of Plate Boundaries

However, modern geologists recognise slab pull as the most significant driving force for plate movement. When a dense oceanic plate sinks into the mantle at a subduction zone, gravity pulls the leading edge down, dragging the rest of the plate behind it. At divergent boundaries, ridge push (or gravitational sliding) occurs when newly formed, elevated crust cools, densens, and slides away from the ridge under gravity.

Global Distribution of Tectonic Plates

You might think the ground beneath you is one solid shell, but the Earth's surface is actually cracked into moving pieces like a giant jigsaw puzzle. The lithosphere is divided into approximately seven major tectonic plates (such as the Pacific, North American, and Eurasian plates) and several minor plates (including the Nazca, Philippine, and Cocos plates).

These plates consist of two types of crust. Oceanic crust is thin ($6$ to $10\,\text{km}$), dense, basaltic, and relatively young. In contrast, continental crust is thick ($25$ to $70\,\text{km}$), less dense, granitic, and very old.

By interpreting global maps, you can identify clear spatial patterns at the plate boundaries. Map symbols are used to show movement: arrows indicate plate direction, triangles or "teeth" represent subduction zones, and double lines represent divergent margins. Key global patterns include:

• Pacific Ring of Fire: A $40,000\,\text{km}$ horseshoe-shaped belt around the Pacific Ocean containing $75\%$ of active volcanoes and $90\%$ of earthquakes.
• Mid-Atlantic Ridge: A north-south divergent belt in the middle of the Atlantic Ocean.
• Alpine-Himalayan Belt: A prominent collision zone stretching from the Mediterranean to the Himalayas.

When analysing maps, you can calculate the concentration of features using a simple formula:

$$\text{Percentage} = (\text{Number of volcanoes in region} \div \text{Total volcanoes}) \times 100$$

Divergent (Constructive) Plate Boundaries

How are entirely new oceans born? They start at divergent boundaries, where tectonic plates move apart at an average speed of $2$ to $5\,\text{cm per year}$.

As the plates separate, magma rises to fill the gap, cooling to form new oceanic crust in a process called seafloor spreading. This produces basaltic lava, which has low silica, low viscosity (it is runny), and low gas content, resulting in gentle, effusive eruptions.

This process creates distinct landforms. Underwater, it forms a mid-ocean ridge featuring a central rift valley. Above ground, broad, gently sloping shield volcanoes are formed, such as those found on Iceland.

In continental areas like the East African Rift, the crust fractures into fault lines. This creates steep-sided continental rift valleys with sunken central blocks called a graben and high standing sides called a horst.

Worked Example: Calculating Spreading Rate

If a section of seafloor is $50\,\text{km}$ wide and the rocks at the edge are $2,000,000$ years old, what is the spreading rate in cm/year?

Step 1: Write down the formula and values.

• $\text{Speed} = \text{Distance} \div \text{Time}$
• $\text{Distance} = 50\,\text{km} = 5,000,000\,\text{cm}$
• $\text{Time} = 2,000,000\,\text{years}$

Step 2: Substitute into the equation.

• $\text{Speed} = 5,000,000 \div 2,000,000$

Step 3: Calculate the final answer.

• $\text{Speed} = 2.5\,\text{cm/year}$

The evidence for this spreading comes from magnetic striping (or paleomagnetism), where symmetrical patterns of alternating magnetic polarity are found in the rocks on either side of the ridge.

Convergent (Destructive) Plate Boundaries

Understanding how plates crash together explains why some regions experience the world's most explosive volcanic eruptions and devastating tsunamis. When plates move towards each other, the resulting landforms depend on the types of crust involved.

When a dense oceanic plate meets a less dense continental plate, the oceanic plate is forced underneath into the asthenosphere in a process called subduction. Water trapped in the subducting crust lowers the melting point of the mantle, causing partial melting. This produces andesitic/granitic magma, which is sticky, highly viscous, and gas-rich, leading to explosive volcanic eruptions.

Subduction zones create deep ocean trenches and steep-sided composite volcanoes (also known as stratovolcanoes). If two oceanic plates collide, they form curved chains of volcanic islands called island arcs (e.g., Japan). The highest magnitude earthquakes occur here along the Benioff Zone, an inclined path of seismic activity reaching depths of $700\,\text{km}$.

If two continental plates meet, a collision boundary is formed. Because continental crust is too light to subduct, neither plate sinks. Instead, the intense pressure forces the rock layers to crumple upwards in a process called orogeny, creating fold mountains like the Himalayas.

This geology features complex folds, including anticlines (upward peaks) and synclines (downward troughs). Crucially, there are no volcanoes at collision boundaries because there is no subduction to melt the crust into magma.

Conservative (Transform) Plate Boundaries

You can easily slide two smooth blocks of wood past each other, but try sliding two rough bricks—they catch, grind, and build up pressure. At conservative plate boundaries, plates slide horizontally past one another, either in opposite directions or in the same direction at different speeds.

The main landform here is a transform fault, such as the San Andreas Fault, which is often visible as a linear valley with displaced surface features. At these boundaries, crust is neither created nor destroyed. Importantly, there are absolutely no volcanoes here because there is no subduction to melt rock and no divergence to create a gap for magma to rise.

Instead, these margins are known for extremely powerful, shallow-focus earthquakes (occurring less than $70\,\text{km}$ below the surface). Friction causes the rough edges of the plates to lock together, building up massive pressure over time. When the rocks finally break, the plates "slip" or "jerk" forward, releasing stored energy as destructive seismic waves.

Hotspots and Mantle Plumes

Not all volcanoes form at the edges of tectonic plates; some sit directly in the middle of them. A mantle plume is a stationary column of superheated rock rising from the core-mantle boundary.

As the plume rises, decreased pressure causes decompression melting. This hot magma burns through the middle of the moving plate above it, causing intra-plate volcanism at a surface area known as a hotspot. These eruptions feature runny basaltic lava and create large shield volcanoes.

Because the tectonic plate keeps moving while the mantle plume remains stationary, a chronological chain of islands is formed. The oldest islands are carried furthest away from the hotspot and eventually erode into underwater extinct volcanoes called a seamount (such as Loihi near Hawaii). The youngest, active islands remain directly above the plume. By measuring the distance and age of islands in the chain, geologists can calculate the exact speed and direction of the plate's movement.