Page 133 - Fundamentals of Geomorphology

5

SMALL-SCALE TECTONIC

AND STRUCTURAL LANDFORMS

The folding, faulting, and jointing of rocks creates many large and small landforms. This chapter looks at:

How molten rocks (magma) produce volcanic landforms and landforms related to deep-seated (plutonic)
processes
How the folding of rocks produces scarps and vales and drainage patterns
How faults and joints in rocks act as sites of weathering and produce large features such as rift valleys
Geological forces in action: the birth of Surtsey

On 8 November 1963, episodic volcanic eruptions began to occur 33 km south of the Icelandic mainland and
20 km south-west of the island of Heimaey (Moore 1985; Thorarinsson 1964). To begin with, the eruptions were
explosive as water and magma mixed.They produced dark clouds of ash and steam that shot to a few hundred metres,
and on occasions 10 km, in the air above the growing island. Base surges and fall-out of glassy tephra from the volcano
built a tuff ring. On 31 January 1964, a new vent appeared 400 m to the north-west. The new vent produced a new
tuff ring that protected the old vent from seawater. This encouraged the eruptions at the old vent to settle down into
a gentle effusion of pillow lava and ejections of lava fountains. The lava, an alkali-olivine basalt, built up the island
to the south and protected the unconsolidated tephra from wave action. After 17 May 1965, Surtsey was quiet
until 19 August 1966, when activity started afresh at new vents at the older tuff ring on the east side of the island
and fresh lava moved southwards. The eruptions stopped on 5 June 1967. They had lasted three-and-a-half years.
Thus was the island of Surtsey created from about a cubic kilometre of ash and lava, of which only 9 per cent breached