Tectonic Processes — CXC CSEC Geography Revision Notes

What you'll learn

This revision guide covers all testable content on tectonic processes for the CXC CSEC Geography examination. You will understand the structure of the Earth, the theory of plate tectonics, and the formation of major landforms including volcanoes, earthquakes and fold mountains. Caribbean examples feature prominently, as students must be able to apply tectonic concepts to the region.

Key terms and definitions

Plate tectonics — the theory that Earth's lithosphere is divided into large plates that move slowly over the asthenosphere, driven by convection currents in the mantle

Convergent boundary — a plate boundary where two tectonic plates move toward each other, causing subduction, collision, or mountain building

Divergent boundary — a plate boundary where two plates move apart, allowing magma to rise and create new crust

Subduction zone — an area where one tectonic plate descends beneath another, typically where oceanic crust meets continental crust

Fold mountains — large mountain ranges formed when two continental plates collide and compress sedimentary rock layers into folds

Focus (hypocentre) — the point within the Earth where an earthquake originates

Epicentre — the point on the Earth's surface directly above the focus of an earthquake

Viscosity — the thickness or resistance to flow of magma or lava, which determines the explosiveness of volcanic eruptions

Core concepts

Structure of the Earth

The Earth consists of four main layers, each with distinct properties:

The crust is the outermost solid layer, varying in thickness from 5-10 km under oceans to 30-70 km under continents. Oceanic crust is denser (basaltic) while continental crust is lighter (granitic).

The mantle extends to approximately 2,900 km depth and contains the asthenosphere, a semi-molten layer where convection currents occur. These currents are the driving force behind plate movement. The upper mantle, combined with the crust, forms the rigid lithosphere.

The outer core is liquid iron and nickel, approximately 2,200 km thick, creating Earth's magnetic field through its movement.

The inner core is solid iron and nickel due to extreme pressure, with temperatures exceeding 5,000°C.

Plate tectonics theory

Continental drift, proposed by Alfred Wegener in 1912, suggested that continents were once joined as Pangaea and have drifted apart. Evidence includes:

Matching coastlines of South America and Africa
Identical fossil species (Mesosaurus, Glossopteris) on now-separated continents
Similar rock formations and mountain chains across continents
Glacial deposits in now-tropical regions

The modern plate tectonics theory explains HOW plates move through seafloor spreading and mantle convection. Heat from radioactive decay in the core creates convection currents in the mantle. Rising currents spread beneath the lithosphere, dragging plates along. Where currents descend, plates are pulled downward (slab pull).

The Caribbean Plate is a small tectonic plate bounded by the North American Plate, South American Plate, Cocos Plate, and Nazca Plate. Its complex boundaries create significant tectonic activity throughout the region.

Types of plate boundaries

Divergent (constructive) boundaries occur where plates move apart. Magma rises to fill the gap, creating new crust. Features include:

Mid-ocean ridges (Mid-Atlantic Ridge between North American and Eurasian plates)
Rift valleys on land (East African Rift)
Shallow-focus earthquakes
Volcanic activity with low-viscosity basaltic lava

Convergent (destructive) boundaries occur where plates collide. Three types exist:

Oceanic-continental convergence: The denser oceanic plate subducts beneath continental crust, creating deep ocean trenches, volcanic mountain ranges, and powerful earthquakes. The Nazca Plate subducting under the South American Plate formed the Andes Mountains.

Oceanic-oceanic convergence: One oceanic plate subducts beneath another, forming island arcs like the Lesser Antilles (Montserrat, St. Vincent, Dominica). These volcanic islands result from magma rising through the overriding plate.

Continental-continental convergence: Neither plate subducts due to equal density. Instead, collision crumples and uplifts sedimentary rocks into fold mountains like the Himalayas.

Transform (conservative) boundaries occur where plates slide past each other horizontally. No crust is created or destroyed, but friction causes shallow earthquakes. The Enriquillo-Plantain Garden Fault in Haiti demonstrates this boundary type, responsible for the devastating 2010 earthquake.

Volcanic activity

Volcanoes form where magma reaches Earth's surface. The nature of eruptions depends on magma composition and viscosity.

Shield volcanoes have gentle slopes built from low-viscosity basaltic lava that flows easily. Eruptions are relatively gentle with lava fountains. These form at divergent boundaries or hotspots (Hawaii).

Composite (stratovolcanoes) have steep slopes built from alternating layers of lava and ash. High-viscosity andesitic or rhyolitic magma traps gases, causing explosive eruptions. Most Caribbean volcanoes are this type, including:

La Soufrière, St. Vincent (erupted April 2021)
Soufrière Hills, Montserrat (ongoing activity since 1995)
Mount Pelée, Martinique (1902 eruption killed 30,000)

Caldera volcanoes form when magma chambers empty during eruptions, causing collapse and creating large depressions.

Volcanic hazards include:

Lava flows destroying property and vegetation
Pyroclastic flows (dense, fast-moving mixtures of hot gas, ash, and rock)
Ash fall disrupting air travel and agriculture
Volcanic gases (sulfur dioxide, carbon dioxide)
Lahars (volcanic mudflows when ash mixes with water)
Tsunamis from submarine eruptions

Benefits of volcanic activity:

Fertile volcanic soils support agriculture (bananas in St. Vincent, coffee in Jamaica's Blue Mountains)
Geothermal energy potential
Tourism revenue (volcano tours)
Mining valuable minerals

Earthquakes

Earthquakes result from sudden energy release when rocks break or slip along faults. Most occur at plate boundaries where stress accumulates over time.

The focus is the actual point of rupture within the Earth. The epicentre is the surface point directly above. Shallow-focus earthquakes (0-70 km depth) cause the most damage.

Energy travels as seismic waves:

Primary (P) waves: fastest, compressional, travel through solids and liquids
Secondary (S) waves: slower, shear waves, travel only through solids
Surface waves: slowest, most destructive, travel along Earth's surface

Earthquake measurement:

The Richter Scale measures magnitude (energy released) logarithmically from 0-10. Each whole number increase represents 10 times more ground motion.

The Mercalli Scale measures intensity (observed effects) from I (not felt) to XII (total destruction).

Caribbean seismic activity is concentrated along plate boundaries:

Haiti (2010, magnitude 7.0): 230,000 deaths, capital Port-au-Prince devastated
Jamaica lies along transform boundary with frequent small earthquakes
Eastern Caribbean islands experience earthquakes from subduction

Earthquake effects:

Ground shaking causing building collapse
Liquefaction where saturated soil behaves like liquid
Landslides in mountainous terrain
Tsunamis from submarine earthquakes
Fires from ruptured gas lines
Economic losses and displacement

Preparation and management:

Building codes requiring earthquake-resistant construction
Early warning systems (limited effectiveness given short warning times)
Public education and earthquake drills
Emergency supplies and evacuation plans
Land-use planning avoiding high-risk zones

Fold mountains

Fold mountains form at convergent boundaries where continental plates collide, compressing sedimentary rock layers into folds over millions of years.

Anticlines are upward-arching folds where rock layers bend upward. Synclines are downward-arching folds where layers bend downward. Complex folding creates parallel ridges and valleys.

Examples include the Himalayas (Indo-Australian and Eurasian plates), Alps (African and Eurasian plates), and Andes (Nazca and South American plates, though this involves oceanic-continental convergence).

Fold mountains typically feature:

High elevation with snow-capped peaks
Steep slopes and deep valleys
Young, geologically active landscapes
Continued uplift and earthquake activity
Resistant rock types (metamorphosed sedimentary rocks)

Economic activities in fold mountain regions include:

Tourism (skiing, mountaineering)
Hydroelectric power from steep rivers
Mining valuable minerals
Limited agriculture in valleys
Forestry on lower slopes

Fold mountains affect climate by creating rain shadow effects and temperature variations with altitude, influencing settlement patterns and agricultural potential.

Worked examples

Example 1: Explain how convection currents in the mantle cause plate movement. (4 marks)

Mark scheme approach: 1 mark per valid point, maximum 4

Model answer: Radioactive decay in Earth's core generates intense heat [1 mark]. This heat causes the mantle to warm and rise as it becomes less dense [1 mark]. As the mantle material reaches the lithosphere, it spreads horizontally, dragging plates along with it [1 mark]. The cooled mantle material then sinks back down at convergent boundaries, creating a circular convection cell [1 mark].

Example 2: Using a named Caribbean example, describe the effects of a volcanic eruption. (6 marks)

Mark scheme approach: Named example required; 1 mark per effect with development

Model answer: The 2021 eruption of La Soufrière volcano in St. Vincent had multiple effects. Ash fall covered crops and farmland, destroying agricultural production and threatening food security [1+1 marks]. Approximately 20,000 people were evacuated from the Red Zone, causing displacement and social disruption [1+1 marks]. Volcanic ash contaminated water supplies, creating health hazards for the population [1 mark]. Air travel was disrupted as ash posed risks to aircraft engines, affecting tourism and connectivity [1 mark].

Example 3: Compare the characteristics of earthquakes at convergent and transform boundaries. (6 marks)

Mark scheme approach: Comparative answer required; marks for identifying differences

Model answer: At convergent boundaries, earthquakes occur at varying depths, including deep-focus earthquakes up to 700 km depth where subduction occurs [1 mark], whereas transform boundaries produce only shallow-focus earthquakes (0-70 km) as plates slide horizontally past each other [1 mark]. Convergent boundary earthquakes often trigger tsunamis when they occur under the ocean and involve vertical displacement [1 mark], while transform boundary earthquakes rarely generate tsunamis as movement is horizontal [1 mark]. The strongest earthquakes (magnitude 9+) occur at convergent boundaries due to the larger area of plate contact [1 mark], whereas transform boundaries typically produce moderate earthquakes (magnitude 6-7) as seen along the Enriquillo Fault in Haiti [1 mark].

Common mistakes and how to avoid them

Confusing focus and epicentre: Remember the focus is INSIDE the Earth where the earthquake starts; the epicentre is the point on the surface ABOVE the focus. Draw a simple diagram to visualize this relationship.
Mixing up divergent and convergent boundaries: Divergent = plates DIVERGE (move apart), creating new crust. Convergent = plates CONVERGE (come together), destroying crust. Use the word roots to remember.
Listing effects without developing them: CXC examiners award marks for developed points. Instead of writing "people died," write "over 230,000 people died due to building collapse and lack of earthquake-resistant construction in Haiti 2010."
Forgetting to name Caribbean examples: Questions often specify "using a named Caribbean example." Always name the specific location (Montserrat, Haiti, St. Vincent) and give dates where relevant.
Using incorrect terminology: "Destructive margin" is acceptable, but the CXC syllabus uses "convergent boundary." Similarly, use "transform boundary" rather than "conservative margin." Be consistent with CSEC terminology.
Confusing volcanic types: Shield volcanoes have LOW viscosity lava and gentle eruptions. Composite volcanoes have HIGH viscosity magma and explosive eruptions. Most Caribbean volcanoes are composite volcanoes at subduction zones.

Exam technique for "Tectonic Processes"

Command words matter: "Describe" requires characteristics and features (what it looks like). "Explain" requires reasons and processes (why/how it happens). "Compare" requires identifying similarities AND differences. Award yourself marks only when you address the command word properly.
Use annotated diagrams: Simple, labeled diagrams of plate boundaries, Earth's structure, or volcanic cross-sections can earn marks. Ensure labels are clear and relevant to the question. Diagrams should complement, not replace, written explanations.
Structure longer answers systematically: For 6-8 mark questions, write in clear paragraphs with one main idea per paragraph. Use connectives like "furthermore," "additionally," and "consequently" to show development. Always use a named example when asked.
Manage your time by marks available: Spend approximately 1-1.5 minutes per mark available. A 6-mark question deserves 6-9 minutes. Don't write a full page for a 2-mark question.

Quick revision summary

Tectonic processes result from Earth's internal heat driving convection currents that move lithospheric plates. At divergent boundaries, plates separate and create new crust. At convergent boundaries, plates collide, causing subduction, volcanic island arcs, fold mountains, and earthquakes. Transform boundaries produce earthquakes as plates slide past each other. The Caribbean Plate experiences significant tectonic activity, with volcanic eruptions in the Lesser Antilles and earthquakes along transform faults. Understanding plate boundaries, volcanic hazards, earthquake effects, and fold mountain formation is essential for CSEC success.