Constructive Boundaries

Definition of the Lithosphere: The Earth's crust, known as the lithosphere, is not a solid shell but is split into large slabs referred to as tectonic plates.
Plate Floating: These tectonic plates float on top of semi-molten rock located in the upper mantle.
Mechanism of Movement (Convection Currents): * Heat emanating from the Earth's core generates slow-moving convection currents within the upper mantle. * Convection currents are defined as the circular movement of magma in the mantle. * These currents drag the overlying plates, causing them to move in various ways.
Types of Plate Interactions: The movement driven by convection currents leads to three primary types of plate boundaries: * Destructive (Convergent): Where plates collide. * Passive (Transform): Where plates slide past one another. * Constructive (Divergent): Where plates separate.

General Process: At a constructive plate boundary, plates move apart or diverge. As they separate, new land and crust are created.
Variation in Outcomes: Different geological situations occur depending on the specific type of plates separating (oceanic or continental) and the geological processes involved.

Formation Process: * Constructive boundaries form on land when plates are pulled apart by tensional forces. * These tensional forces are driven by mantle convection currents moving in opposite directions.
Structural Results: * As the continental crust splits, a steep-sided valley called a rift valley is formed. * Geological activity in these regions includes shallow earthquakes and volcanic activity.
Case Study: The East African Rift Valley: * Timeline: Formation began approximately $25 \text{ to } 30 \text{ million years}$ ago. * Cause: Rising magma plumes caused the continental African crust to stretch and eventually split. * Scale: The rift currently extends over $6,400\,km$ down East Africa. * Activity: Magma rises through fissures (cracks) in the crust, leading to the formation of volcanoes, such as Mount Kilimanjaro, and significant earthquake activity.
Geological Features of Rift Valleys (based on Figure 10): * Fault Scarps: Steep cliffs formed by faulting. * Parallel Faults: Multiple cracks in the crust running alongside each other. * Central Block (Horst): Raised blocks of crust (though the valley floor itself is a dropped block or graben). * Multiple Fracturing: The process involves the crust breaking into many segments. * Lakes on Valley Floors: Water often collects in the dropped sections of the rift, such as Lake Tanganyika.

Process of Separation: This occurs when two oceanic plates separate due to tensional forces, causing the crust to thin.
Fissure Formation: Large fissures form along the plate boundary as the plates move apart.
Magma Characteristics: * Basic lava rises from the mantle through these fissures. * This lava has a low silica content, defined as being less than $55\%$ .
Solidification: Upon contact with cold ocean water, the lava cools and solidifies rapidly into solid igneous rock, creating new oceanic crust.
Continuous Process: This ongoing cycle of separation and new crust formation is known as sea floor spreading.

Origin: The theory was proposed by Harry H. Hess, an American geologist, in 1960.
Core Principle: Hess stated that new oceanic crust is continuously formed and spreads away from the mid-oceanic ridge in a "conveyor belt-like" motion.
Supporting Evidence: * Rock Age: The age of the seabed rock is youngest specifically along the mid-oceanic ridges. * Progression: Rocks become progressively older and are found at deeper levels the further one moves away from the ridge point.
Underwater Topography: Over time, the accumulation of new rock builds up to form massive underwater volcanic mountain ranges known as mid-oceanic ridges.

Formation: Created by the divergence of the American Plates and the Eurasian Plate.
Rate of Divergence: The plates separate at a rate of approximately $3\,cm/year$ .
Volcanic Islands: While the ridge is primarily underwater, it occasionally rises above sea level to form islands.
Iceland: Iceland is a prime example of the Mid-Atlantic Ridge appearing above sea level. * The ongoing process of sea floor spreading contributes to the physical growth of Iceland. * Other volcanic islands formed by this process include Surtsey.