Category: Rock Specimens

Identifying obsidian.  This is a new kind of rock identification guide for you.  A plain language, no silly business, get your rocks sorted in no time guide to “what rock is that?”

Igneous rocks: Obsidian

If it’s dark, sharp and looks like glass it’s obsidian.

Obsidian is Extrusive, felsic & glassy.  Have a look back at this post if you’ve forgotten what those terms are.

Obsidan comes from the same lava as rhyolite .  In this case it comes from a very, very viscous lava with exceptionally slow flow and cooling produces its characteristic glassy texture.  Obsidian forms on the edges of a rhyolite flow like the watery bits on the edge of scrambled eggs as it cooks. It is not unusual to find pieces of pumice and perlite in obsidian.  These are produced in a chemical reaction in the lava when it touches groundwater.

Right now you’ll find granite forming at the Puyehue-Cordón Caulle in Chile.

This guide is designed to help you identify the classic rocks that are listed in most textbooks and found in most geology kits.  No curly examples, no red herrings – just the bog standard basics that you need to teach geology in the classroom. Up next: Diorite

 

Identifying pumice.  This is a new kind of rock identification guide for you.  A plain language, no silly business, get your rocks sorted in no time guide to “what rock is that?”

Igneous rocks: Pumice

If it’s light grey, light weight, full of holes and floats it’s pumice.

Pumice is extrusive – explosively extrusive, intermediate/felsic, glassy.  It is extremely fine grained from very fast cooling on exposure to air.  Have a look back at this post if you’ve forgotten what those terms are.

Pumice comes from rhyolitic lava that had a high water content.  So the sister rocks of pumice are granite, rhyolite, obsidian and perlite.

Pumice is used for many industrial and household applications including toothpastes and stonewashing jeans.

Right now you’ll find pumice forming at the Havre submarine volcano 800km NE of NZ.

 

This guide is designed to help you identify the classic rocks that are listed in most textbooks and found in most geology kits.  No curly examples, no red herrings – just the bog standard basics that you need to teach geology in the classroom. Up next: Obsidian

Identifying rhyolite.  This is a new kind of rock identification guide for you.  A plain language, no silly business, get your rocks sorted in no time guide to “what rock is that?”

Igneous rocks: Rhyolite

If it’s very light coloured, very hard and fine grained it’s rhyolite.

Rhyolite is extrusive, felsic & aphanitic.  Have a look back at this post if you’ve forgotten what those terms are.

Rhyolite comes from the same lava as granite but with smaller crystals because it has cooled quickly on the surface.  The crystal size is one of the keys to identifying it as an extrusive igneous rock.

It is a viscous lava so it is slow flowing and often displays flow banding from solidifying as it moves.  Rhyolitic lava also forms pumice and obsidian and perlite.

Right now you’ll find rhyolite forming at the Taupo Volcano in NZ.

This guide is designed to help you identify the classic rocks that are listed in most textbooks and found in most geology kits.  No curly examples, no red herrings – just the bog standard basics that you need to teach geology in the classroom. Up next: Pumice