2.2 - Tectonic Plates
The structure and composition of tectonic plates
Tectonic plates form the Earth's outer layer, known as the lithosphere, and are crucial in shaping the planet's surface through their movement. These plates consist of the crust, which varies in type and characteristics, influencing geological activities across the globe.
Types of Earth's crust in tectonic plates
- Continental crust - This type of crust is thicker, ranging from 35 to 70 km, and is composed of older rocks, often exceeding 1500 million years in age. It is lighter, with a density of approximately 2.6, and appears paler in colour. The rocks are diverse, predominantly granite, and rich in silica and oxygen.
- Oceanic crust - Thinner than continental crust, it measures about 6 to 10 km in thickness. The rocks are younger, typically less than 200 million years old, and denser with an average density of 3.0, appearing darker in colour. Basalt is the primary rock type, with fewer variations compared to continental crust.
Major and minor tectonic plates on Earth's surface
The Earth's surface is divided into several large and small tectonic plates that float on the semi-fluid mantle beneath. These plates are not static and their interactions create significant geological features and events.
Identification of tectonic plates
- Major plates - There are seven primary tectonic plates that cover vast areas of the Earth's surface. These include the Eurasian, North American, South American, African, Indo-Australian, Pacific, and Antarctic plates.
- Minor plates - Alongside the major plates, numerous smaller plates exist, such as the Caribbean, Juan de Fuca, Cocos, Aegean, Adriatic, and Turkish plates. These smaller plates often lie between larger ones and contribute to regional geological activity.
- Boundary uncertainties - While maps depict plate boundaries as clear lines, in reality, these edges are often vague and not well-defined. Many regions show uncertain boundaries due to complex interactions and overlapping zones.
Different types of plate boundaries and their processes
Tectonic plates interact at their boundaries, known as plate margins, where various geological processes occur. These interactions are classified into four main types, each with distinct movements and outcomes.
Categories of plate margins
- Destructive or convergent boundary - Occurs when an oceanic plate moves towards a continental plate and is forced beneath it due to higher density. This process, called subduction, results in the formation of deep sea trenches and island arcs. Volcanic activity is frequent in these zones.
- Constructive boundary - Happens when two plates diverge or move apart, allowing magma to rise and form new oceanic crust. This creates mid-ocean ridges and is often accompanied by volcanic activity.
- Collision boundary - Takes place when two continental plates meet and, unable to sink due to similar density, are pushed upwards to form fold mountains. This results in significant uplift and mountain ranges.
- Conservative boundary - Involves two plates sliding past each other horizontally. No land is created or destroyed in this process, but it often leads to friction and seismic activity along fault lines.
Landforms and activities associated with plate margins
The type of plate boundary directly influences the geological features and events that occur in a region. Each boundary type is associated with specific landforms and activities that shape the Earth's landscape.
Features and events at different boundaries
- Destructive/convergent boundary landforms - Deep sea trenches form where the oceanic plate sinks, and island arcs emerge due to volcanic activity. For instance, along the boundary of the Pacific and Philippine plates, a deep trench has developed.
- Constructive boundary landforms - Mid-ocean ridges are prominent features, formed by the upwelling of magma as plates separate. An example is the East African Rift, where the continental plate is splitting into two separate plates.
- Collision boundary landforms - Fold mountains are the hallmark of this boundary, resulting from the compression of continental crust. A notable case is where the Arabian plate has collided with the Eurasian plate, forming a mountain range.
- Conservative boundary activities - These boundaries are marked by fault lines where plates grind past each other, often causing earthquakes. The North Anatolian Fault exemplifies this, with frequent seismic events due to lateral movement.