What processes lead to Volcanic Activity ? In other words why do they occur ? 1. There is a link between WHY they occur and WHERE they occur. 2. There is also a link between their LOCATION and the TYPE of volcano..

Volcanoes tend to produce either:- A lot of ash, gas and dust from violent explosions...

Or: Lots of fluid basalt lava which can flow for miles !

To understand where volcanoes occur we need to understand how our planet is made:-

Essentially the thin surface of our planet (the CRUST) is broken into about 12 large pieces called plates. These plates sit on a layer of denser, and hotter rocks known as the MANTLE. The top 60km of MANTLE is solid and behaves in the same way as the CRUST. Together they are called the LITHOSPHERE.

The lower MANTLE rocks are very hot. Rocks close to their melting point become soft or ‘plastic’ and are able to flow even though they are still solid. This layer is called the ASTHENOSPHERE. The vast heat energy from the liquid OUTER CORE generates rock flows in the ASTHENOSPHERE called convection currents.

In some places the LITHOSPHERE is being drawn together and in others it is being pulled apart by these slow moving convection currents

Where the plates are drawn together you get a DESTRUCTIVE MARGIN, where they are pulled apart you get a CONSTRUCTIVE MARGIN.

DESTRUCTIVE PLATE MARGIN !

At a destructive margin one plate is drawn under another. Usually thinner and denser oceanic crust is pushed under lighter continental crust. As it is reabsorbed into the mantle the rocks break up and melt. Oceanic crust contains a lot of water and this lowers the melting point of the rocks and makes the melt ‘sticky’. There is also a lot of steam……...

So volcanoes at destructive margins are... VIOLENT AND EXPLOSIVE !

CONSTRUCTIVE PLATE MARGIN!

At a constructive margin the plates are being drawn apart. This allows hot rocks to rise upwards. As pressure is reduced the rocks melt to form magma. This bursts through cracks to form lava flows. Almost all constructive margins are at the bottom of the Oceans so…..

The lava is cooled very quickly into ‘lumps’ known as Pillow Lava......which heaps up into huge mountain ranges known as Mid Oceanic Ridges.

Because new crust is being created at these ridges the Oceans get wider. This is known as... SEA FLOOR SPREADING The Atlantic Ocean is widening at the rate of 2cm a year !

The Earth differs from an electric power plant in that there is no permanent magnet in the center of the Earth; at the very high temperatures of the core, permanent magnets can't exist, because thermal energy makes atoms vibrate and tumble so fast that their tiny magnetic dipoles can't lock into alignment. Instead, the Earth is a "self-exciting dynamo," meaning that the magnetism is produced by electric currents in the outer core. This is the magnetism that led to the generation of an electric current in the flowing iron alloy in the first place—this system perpetuates itself.

In 1963, F. Vine and D.H. Matthews reasoned that, as basaltic magma rises to form new ocean floor at a mid-ocean spreading center, it records the polarity of the magnetic field existing at the time magma crystallized. As spreading pulls the new oceanic crust apart, stripes of approximately the same size should be carried away from the ridge on each side (Fig. 5). Basaltic magma forming at mid-ocean ridges serves as a kind of "tape recorder", recording the Earth's magnetic field as it reverses through time. If this idea is correct, alternating stripes of normal and reversed polarity should be arranged symmetrically about mid-ocean spreading centers. The discovery of such magnetic stripes provided powerful evidence that sea-floor spreading occurs. Basically, as basalt cools, it ‘records’ the magnetism of the earth. This magnetism switches and so we get alternate stripes. This is called Paleomagnetism.

This helps explain CONTINENTAL DRIFT.

There is one exception………. This is the East African Rift Valley.

This is a constructive margin beneath a continent. Diverging convection currents are splitting Africa in two. Volcanoes form where cracks appear in the crust. Eventually a new Ocean will form ! Such volcanoes (like NYIRAGONGO) will produce lava flows.

So most volcanoes occur at plate margins. Violent explosive ‘grey’ type volcanoes tend to be found at Destructive margins. Volcanoes that produce lava flows are associated with Constructive margins.

BUT…. What about Hawaii ? It has much volcanic activity but it is not near a plate margin !

Scientists believe that there is a very hot plume or convection current in the mantle below Hawaii. This ‘blow torches’ its way through the overlying crust to make volcanoes. This is called a HOT SPOT.

The ‘hot spot’ remains fixed but the plate moves over it. This means that there is a linear chain of volcanic islands stretching many hundreds of miles to the north west of Hawaii. The further away the older the island ! A brand new volcanic island is now forming just to the south east of Hawaii.

Random facts - Measuring Water Depth Today's oceanographers use sonar instruments to generate a sound signal that is bounced or "echoed" off the sea floor and then recorded on board the ship. The speed of sound in water is 1,500 m per second, four times faster than the speed of sound in air. By carefully measuring the round-trip time of the sound waves and taking into account the variables of temperature and salinity, the depth of the water and the distance to another object can be measured accurately