Enquiry Question 2

Extrusion
Extrusion is the upward movement of magma through the Earth's crust and onto its surface as lava. Extrusive landforms form once the lava has cooled. 
(Byrne et. al. p163)

Volcanic Cones
The central vent of the volcano is connected to a store of magma below the surface, which is known as the magma chamber. The extrusion of lava from the vent leads to the creation of a volcanic cone. with every eruption new layers are added to the cone. These layers are initially added by pyroclastic flows and then by volcanic ash which settles from the air.

Fissures
Fissures are cracks or openings within rock through which magma escapes on to the surface of the Earth. They usually form because of weaknesses that exist within rock. Fissures can be connected back to the magma chamber below a volcano or to a secondary source of magma, such as the vent of the volcano itself.

Geysers
Water in the lower crust is heated by rocks and turns into steam; pressure increases and the steam and water explode onto the surface.


Different types of lava

Basaltic Lava
Lava that has a low viscosity with a heat of 1200 degrees centigrade. The basaltic lava has a low viscosity which means it has a very low silica content. The lava and steam flow quickly and slowly cool and this slowly forms gentle landforms. Basaltic lava is commonly found at constructive plate margins where magma rises from the mantle.

Andesitic Lava
Lava that has a higher viscosity than basaltic lava with a heat of 800 degrees centigrade. The andesitic lava has a higher viscosity than which means it has a higher silica content. The lava and steam flow are ejected at a quicker rate meaning steeper landforms are created. Andesitic. lava is commonly found at destructive margins where oceanic crust is subducted, melts and rises

Rhyolitic Lava
Rhyolitic lava is the lava with the highest viscosity with a heat of 650 degrees centigrade. The lava has a very high silica content. The lava and steam erupt out of the volcano this therefore makes steep landforms such as composite volcanoes. Rhyolitic lava is commonly found at destructive plate margins.

(Geography An Integrated Approach, Waugh, 2002) 

Different types of volcanoes

Cinder
This is the most common type of volcano. These volcanoes tend to have very straight sides. They are mainly composed of mostly lapilli-sized volcanic material, with a cinder like feel.

Composite
This is a volcano that is composed of different types of volcanic material including lava, pyroclastics and ash 

Caldera
A caldera volcano is a volcano usually formed by the collapse of land following a volcanic eruption. Caldera volcanoes are commonly found at destructive plate margins.
Krakatoa

Shield
This is a volcano that is the morphology of these volcanoes is thought to resemble a shield resting on the ground in an upright position

Types of Eruption

Hawaiian Eruption
Semi-permanent upwelling of free-flowing lavas to create shield volcanoes.

Strombolian Eruption
Infrequent violent eruptions, largely made up of lavas.

Vulcanian Eruption
Infrequent ejections of lava fragments and ash which settles as cinders.

Plinian Eruption
Explosive ejections of relatively viscous lavas and ash which rise high into the air before settling, as well as pyroclastic flows, forming steep composite volcanoes.

Pelean Eruption
Explosive eruptions of viscous lavas, gas and ash which roll downslope as rapidly moving glowing clouds, like an avalanche.


Hotspots
Mantle plumes are areas of hot, upwelling mantle. A hot spot develops above the plume. Magma generated by the hot spot rises through the rigid plates of the lithosphere and produces active volcanoes at the Earth's surface. As oceanic volcanoes move away from the hot spot, they cool and subside, producing older islands, atolls, and seamounts. As continental volcanoes move away from the hot spot, they cool, subside, and become extinct.
(http://volcano.oregonstate.edu/what-is-a-hot-spot)

Intrusion
The action or process of forcing a body of igneous rock between or through existing formations, without reaching the surface.

Intrusive Volcanic Landforms:

Batholith
A batholith is a large emplacement of igneous intrusive rock that forms from cooled magma deep in the Earth's crust.

Stock
An irregular igneous intrusion, usually an offshoot of a batholith.

Laccalith

Mass of igneous rock that has been intruded between rock strata causing uplift in the shape of a dome.

Dike
Dike is a sheet-like intrusive igneous rock body which cuts across adjacent rock layers.

Sill
A sill, is a similar body of intrusive igneous rock which intrudes along adjacent rock layers.

Neck
A column of igneous rock formed by congelation of lava in the conduit of a volcano and later exposed by the removal of surrounding rocks.

San Andreas Fault

The San Andreas Fault, which passes through coastal California, is one of the world's best known and most studied plate boundaries. It is a broad fault zone of many faults, about 1300 kilometers long, up to 100 kilometers wide and 20 kilometers deep. The San Andreas Fault is a transform boundary where the continental North American plate and the Oceanic Pacific plate are grinding slowly sideways past each other, creating massive strain in the earth's crust. This stress is released as earthquakes. Most of the fault zone is slowly creeping forward at about 39 millimeters per year—too small to be felt by people and to be measured as earthquakes. What is actually happening is that the rocks on either side of the main faults are locked together for years at a time, but the crust between the faults is moving slowly ahead of the crust along the main faults. Eventually the stress and strain becomes too great along the fault and the stress is suddenly released.

Africa's Great Rift Valley

The Great African Rift Valley is the south branch of the Afar Triangle, the only one remaining which has not yet been filled by the sea. This is therefore an example of continental rifting. The diverging of the African and Arabian plates and also the magma plume beneath the plates allowed the rift valley to be formed. The plume pushed up the land creating bulges and cracks allowing flood basalts to rise through fissures and add to the gradient and create long mountain ranges. As a result of the divergent plates a small section of land in between these plates breaks away and sinks leaving faults.

The Rift Valley stretches over approximately 5000km from Syria to Mozambique. The width of the rift varies between 30-100kms and the depth also varies from a few hundred to a few thousand metres. The rift is 35million years old and is still growing meaning that eventually East Africa including the horn of Africa: Ethiopia, Somalia and Eritrea will break away and form its own continent.

Fold Mountains
Fold mountains are mountains formed from the folding of the earth's crust. The extreme pressure forces the edges of the plates upwards into a series of folds.