Constructive Margins

Question 1

What type of convection currents dominate a constructive margin

Answer 1

Rising convection current

Question 2

Describe the process of plate upwelling

Answer 2

Heat escapes from the core
This heat travels through the mantle and heats rock in asthenosphere
This heat flows upwards towards lithosphere
Rising part of convection current reaches bottom of lithosphere
Lithosphere is heated, becomes less dense, stretches and deforms upwards

Question 3

What is sea floor spreading

Answer 3

The plates are pushed apart
Caused by the ascending convection currents
Causing the stretched plate to separate, creating new oceanic plate material

Question 4

Ocean ridge formation

Answer 4

As lithosphere is heated by rising convection currents below, it becomes more dense and is stretched and deformed upwards to form an OCEAN RIDGE

An ocean ridge is a RAISED portion of the sea bed

Thousands km length, 1-3km height, very wide

Mid Atlantic Ridge 2000km wide

As ridge begins to RISE above the ABYSSAL PLAIN it becomes increasingly RUGGED as you move towards its CENTRE as it has a series of ERODED SUB RIDGES running PARALLEL to the main ridge

Question 5

Central rift valley formation

Answer 5

The centre of the OCEAN RIDGE often features a CENTRAL RIFT VALLEY

A central rift valley is a linear valley that runs along the length of the ridge and can be as much as 1.5km deep and 30km wide

This has been created as updooming of the crust, which causes fractures to form.

These fractures are widened by TENSIONAL FORCES pulling the crust apart.

Two PARALLEL FAULTS create a TROUGH where a section of crust has subsided, leaving a valley with steep inner slopes

INNER slopes of the rift valley are STEEP and relatively STRAIGHT

OUTER slopes of the rift valley have GENTLER gradient and are marked by a series of STEPPED TERRACES parallel to the main ridge

Stepped terraces are edges of former rift valleys

Overtime, the raised sides of the ridge tend to slide down sideways from the centre of the plate margin under the influence of gravity as they cool slightly and become denser: ridge push

Question 6

What is ridge push

Answer 6

Overtime, the raised sides of the ridge tend to slide down sideways from the centre of the plate margin under the influence of gravity as they cool slightly and become denser

Question 7

What activity is there at constructive margins

Answer 7

Volcanic activity and shallow earthquakes

Question 8

Why is there volcanic activity at a constructive margin

Answer 8

The FRACTURING of the rock creates FAULTS and allows basaltic magma to intrude into the lithosphere

This basaltic magma comes from DECOMPRESSION MELTING in the asthenosphere

As the magma intrudes into the lithosphere, it may COOL and HARDEN to form DYKES which are vertical intrusions of igneous rock

This intruding magma can help DISPLACE the rock LATERALLY away from the margin (sea floor spreading)

New magma can also make its way to the surface as FRESH ERUPTIONS pour out lava which will SOLIDIFY and create NEW OCEANIC CRUST

Question 9

What are dykes

Answer 9

Vertical intrusions of igneous rock

Question 10

Why are shallow earthquakes associated with constructive margins

Answer 10

Because they are caused by the movement of magma rising towards the surface

Question 11

What landforms are found at constructive margins

Answer 11

Ocean ridge

Central rift valleys

Question 12

What activities are there at constructive margins

Answer 12

Volcanic and shallow earthquakes