Sunday, 27 February 2011

sedimentry rocks (introduction)

Sedimentary rocks are rocks compossed of fragments that have been deposited compacted and cemented

Sedimentary rocks can be:

chemically weathered-   Through carbon dioxide reacts with rainwater and pore water in soil to form carbonic acid

physically weathered-  also known as mechanically weathering, exfoliation sometimes reffered as 'onion skin weathering'. in hot desserts there is marked difference beetween hot day time and cold night time temperatures. different minerals expand and contract by different amounts during heating and cooling, causing the rock to disintegrate.

biologically weathered-  caused by plant roots braking rocks and from burrowing from animals such as rabbits and worms.  

if there is any questions i would more than happy to answer 

Saturday, 26 February 2011

Types of volcanoes: shield volcanoes and strato

      There are two types of volcanoes these are shield volcanoes and strata volcanoes, I will write about the two seperatly.

Shield volcano  

Shield volcanoes have gentle slopes, which are less than 10 degrees, and have entirly basalt lava flow.  They have fissure eruptions which means magma reaches the surface along , linear cracks or fissures. these fissures erupt almost constantly and are not explosive due to the low pressures.

Strato Volcanoes 

     Strato volcanoes form the largest percentage, over 60%, of the earths individual volcanoes. they are very explosive and look like your typical volcanoes with steep sides. The lava that is erupted has high viscosity as it slightly cooler than fissure eruptions. strato volcanoes also contain lots of felsic minerals such as feldspar and contains high amounts of sillica. 

products of volcanoes igneous rocks

There are various products from volcanoes not just lava as you may think.

products are:

. volcanic gasses - from the volcanoes these include sulphur and co2

. lava flows

. pyroclastic materials-  this happens when magma is forcefully or explosively ejected into the atmosphere as particles called pyroclasts .   these include ash blocks of magma and lapilli which are beetween 2mm and 64mm

where do you find pyroclasts? 

distabution by grain size - bombs and blocks of rock close to volcanoes and ash further away

distrabution by wind - the prevaling wind means the products of a volcanoe are mostly in that direction

intrusive igneous rocks

intrusive means the rocks have been formed in the earth.

Minor intrustions - cool at the hypabyssal depth below the surface and includes sills and dykes

major intrusions - are plutonic and cool depth below the surface and include batholiths

chilled margins are where the ingneous rock has colled rapidly so have few chrystals, a baked margin is the country rock where it was heated by the intrustion and altered

classification of rocks

sedimentry rocks - a rock compossed of fragmants that have been deposited compacted and cemented

igneous rocks - rocks formed from magma within the earth

metamorphic rocks - formed by the recrystallisation of other rocks in a solid state

classification of igneous rocks 

chrystal grain size - bigger longer taken to cool

silica percentage % - the more silica a mineral contains the lighter it is

characteristics of rock forming mineral

you can characterise a rock/mineral by several ways;

These are:

Habit-  shape of minerals/rock

Colour - can be used for some minerals but is inacurate as mineral can be found in many colours

Hardness- mohos scale of hardness measures ressistance to scratching, the scale ranges from 1-10

Lustre- ability to reflect light

Specific gravity- ratio of mass of a mineral compared with the mass of an equal volume of water