Lecture 3 - What is a Volcano Flashcards
Define the meaning of a Volcano.
-A vent in the Earth’s crust where molten rock, gas and heat escape from a magma chamber
What is the difference between Intrusive and Extrusive Igneous Structures?
-Intrusive are formed by magma cooling slowly (hundreds of millions of years), and form coarse crystals
-Extrusive are formed by lava cooling quickly (weeks or seconds), and produce fine crystals or glass
Where did and does the Earth get it’s Heat?
-Initially, asteroid impacts, compaction and core formation (friction of iron moving past the mantle to the core)
-Now it comes from radioactive decay of potassium, uranium and thorium
What 4 Factors determine a Rock’s melting point?
-Temperature
-Pressure
-volatiles (e.g. water, sulphur dioxide)
-Composition
Why does Adding Water make Rocks melt more easily?
-Water acts as a catalyst
-Helps to break crystal structure
-This occurs at subduction zones (where seawater can seep in)
What is the Difference between a Sill and a Dyke?
-Sills follow sedimentary units and bedding planes, travelling horizontally
-Dykes cut across sedimentary units and bedding planes
How does Cooling Rate affect Crystal Size?
-The faster the cooling, the smaller the crystal size and the finer texture a rock is
-E.g. Basalt (mafic and fine-grained) forms nearer the surface and therefore cools more quickly
Describe the Structure of the Oceanic Crust.
-Sediment at the top, followed by pillow basalt underneath, then sheeted dykes, plagiogranite and gabbro above the plutonic mantle
What is the Definition of a Rock?
-A naturally occurring solid composed of at least 1 mineral
What is the Definition of a Mineral?
-A solid, naturally-occurring inorganic substance with a characteristic chemical composition and ordered crystal structure
What is meant by a Porphyritic Structure?
-Large crystals (phenocrysts) surrounded by small crystals (matrix)
What is meant by a Vesicular Structure?
-Rocks that are full of bubbles, e.g. pumice and tephra
-Gas escapes during eruption, however the rock may freeze before all gas escapes which forms a vesicle
-More vesicles means more gas rich magma and therefore a more explosive eruption
What is meant by an Amygdaloidal Structure?
-Vesicles filled in by new crystals
-Form by precipitation of minerals in water, e.g. calcite
-Suggests that rock has been affected by fluids after emplacement
What should you Observe when Describing an Igneous Rock?
-Colour
-Crystalline or fragmented
-Size of crystals (variation?)
-Identify minerals and %
-Draw the rock, including scale
-Texture or particular shape?
