Constructive Margin

Question 1

Sub plate processes

Answer 1

dominated by ascending and diverging convection currents

-heat in core moves up through mantle, heating rock in asthenosphere. This causes it to flow upwards towards the lithosphere

-pressure reduces and melting point within the mantle is reduced. Melted mantle =Magma (DECOMPRESSION MELTING)

-TECTONIC EXTENSION- rising convection current reaches bottom of lithosphere, heats it, solid plate becomes less dense and pressure forces it to warp upwards, stretching the crust and breaking along fault lines.

Question 2

Plate processes

Answer 2

-sub plate convection currents spread sideways, forcing plates apart and creating rifts where molten basalt comes to the surface.

-material slides down from the ridge under gravity (rift faulting)

-pattern of emergence, solidification, and outward movement is called SEA FLOOR SPREADING

most notable feature is a mid ocean ridge eg mid Atlantic ridge

Question 3

Formation of rift valley

Answer 3

-lateral movement causes tectonic extension of the lithosphere and causes the ridge to widen further. This results in rift faulting (cracks) in the lithosphere.

-The central block of crust then subsides (down faulting) forming a rift valley which runs the length of the ocean ridge.

Question 4

Dykes

Answer 4

-Over time raised sides of the ridge slide down away from the plate margin under gravity (Ridge-Push) as they cool and become more dense.

-fracturing of rock during this deformation allows magma to intrude into lithosphere where it may cool and harden to form dykes -> can help displace the rock further, away from the central ridge.

Question 5

Formation of ocean basin

Answer 5

-As plates continue to diverge, magma continues to rises through the faults, fills the gaps and spreads out onto the rift valley and solidifies, creating new oceanic crust.

-As this process further continues, the stretching plate and rift valley may allow a nearby ocean to spill in and water to flow into the valley, initiating the formation of a new ocean basin, e.g. North Atlantic as Europe and North America were slowly forced apart.

Examples of Constructive Plate Margins and their features
1. Iceland and the Mid-Atlantic Ridge North American and Eurasian plates moving apart