Crater Lake Earth Cache
This beautiful small lake occupies the crater of a long-extinct volcano - one of several formed in this area as a result of tectonic activity which started some 20 million years ago. This is related to the development of a divergent tectonic boundary whereby the African plate is in the process of stretching, thinning and eventually splitting into two new tectonic plates – the Somali Plate (carrying the Horn of Africa) and the Nubian Plate (carrying the rest of the continent).
This process has created the East African Rift Valley a giant trough bordered by escarpments to the east and west. The floor of the Rift Valley is broken by volcanoes, some still active, and a series of soda-lakes and fresh-water lakes such as Naivasha.
A volcano is an opening, or rupture in a planet's surface or crust which allows hot magma, volcanic ash and gases to escape from the magma chamber below the surface (see below for an illustration of the key features - using the example of a stratovolcano).
Basic types of volcano (see here for more info):
1. Composite or Stratovolcanos: Mount Longonot (which can be seen some 25km SE of here @ S 00 54.878 E 036 27.385) is an good example (see Gallery & illustration below). Typically large steep-sided, symmetrical cones built up of alternating layers (strata) of lava flows, ash, cinders, blocks, and bombs resulting from successive eruptions. 
They may rise up to 8,000 feet above their bases. Typical examples include Mt Fuji (Japan), Mt Mayon (Philippines) and Mts Vesuvius & Stromboli (Italy)
2. Cinder Cones: (see graphic left) the simplest type, built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a 1,000 feet from their base.
3. Shield Volcanos: consist of thousands of highly fluid basaltic lava flows which pour out in all directions from a central summit vent, or group of vents, spreading widely over a large area and then cooling and slowly building up (by accretion) into thin gently dipping sheets which form a broad, gently sloping cone of flat, domical shape (ie. like a shield). Lavas also often erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some of the largest (but not highest!) volcanoes in the world are shield volcanoes. Mount Suswa 40km to the south of here @ S 01 09.017 E 036 21.350 is an example of this type (see Gallery).
4. Lava (Volcanic) Domes: (right) formed by relatively small, bulbous masses of lava too viscous to flow far. On extrusion, the lava piles over and around its vent and a dome grows mainly by internal expansion. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides.
Some domes form craggy knobs or spines over the volcanic vent, others form short, steep-sided lava flows known as 'coulees'. They typically occur within the craters or on the flanks of large composite volcanoes.
Types of Lava
Volcanoes may also be classified by the composition of ejected material (lava) as this affects the shape of the volcano. Lava can be broadly classified into 4 different compositions, depending on silica content as follows:
1. Felsic: (also known as dacites or rhyolites) which contains a high percentage (>63%) of silica.These are usually highly viscous and are erupted as domes or short, stubby flows. Such lavas tend to form stratovolcanoes or lava domes and trap volatiles (gases) causing magma to erupt catastrophically.
2. Andesitic: which contains 52–63% silica. Such volcanoes generally only occur above subduction zones, like Mt Merapi (Indonesia), and are typically formed at convergent boundaries/margins of tectonic plates.
3. Mafic (basaltic): which contains 45-52% silica - so called as it has higher percentages of magnesium (Mg) and iron (Fe). These lavas are usually much less viscous and are hotter than felsic/rhyolitic lavas, depending on the eruption temperature, and occur in a wide range of settings: at mid-ocean ridges, where two oceanic plates are pulling apart, basaltic lava erupts as pillows to fill the gap; in shield volcanoes like the Hawaaian Islands and Mt Suswa) on both oceanic and continental crust; and as continental flood basalts.
4. Ultramafic: which contains <=45% silica. These lava flows (aka askomatiites) are very rare and very few have been erupted on Earth since the Proterozoic period when the planet's heat flow was higher. They are the hottest and most fluid lavas.
Tephra and Tuff
Tephra is fragmental material ejected in an eruption regardless of composition, fragment size or emplacement mechanism. Airborne fragments are also called pyroclasms. 
Once clasts (rock fragments or grains resulting from the breakdown of larger rocks) have fallen to the ground they remain as tephra unless hot enough to fuse together into pyroclastic rock or tuff (see photo) which usually consists mainly of volcanic ash with larger embedded particles like cinders. There are many types of tuff depending on its composition and the type of lava involved in its formation. (see here for more information on this)
Obsidian
Obsidian is a naturally occurring volcanic glass ( extrusive igneous silicate tock) produced when felsic lava cools rapidly with minimum crystal growth. It is often found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces the formation of a lava with a high viscosity and degree of polymerization. 
The resulting prevention of atomic diffusion through this lava explains the lack of growth of pure quartz crystals. With the rapid cooling any impurities are 'frozen in'. Common impurities include iron oxides which give the rock a very dark, often black colour. Obsidian (like glass) is hard and brittle and therefore fractures with very sharp edges, which have been used in the past for cutting and piercing tools (eg. stone tools used by early man) and has been used experimentally as surgical scalpel blades! (see here for more information on this fascinating rock).
(Cache description material adapted from information found on various Wikipedia pages and USGS web page. For graphics sources see Gallery images notes)
Questions to Answer:
1. Using the Gallery images and/or Google Earth, observe and compare aerial images of Mount Longonot and Crater Lake volcanoes. What obvious striking differences can you see?
2. Considering your observations in 1. and the typical features of the 4 main types of volcano (eg. size, area, shape, etc) what type of volcano is the Crater Lake volcano? Mention two features which led you to this conclusion.
At ground zero you will find a very interesting area of partially lichen-covered tuff just off the trail towards the crater which immediately came to my notice because of the two fascinating inclusions sticking prominently out of the surface . . .
3. What are these inclusions?
4. What is the approximate diameter of the circular dimension of the larger piece?
5. Given the composition of the answer to 3. and the way it is formed, what kind of lava do you think was most likely erupted from this volcano?
Note the appearance and texture of the rock/earth material around the tuff outcrop:
6. What is the colour of the stain?
7. What mineral causes this? (It is the same one that produces the colour of the tuff inclusions)
8. How would you describe the texture of this surrounding material? Why is it like this?
9. Optional: take a photo of yourself + GPSr and the crater/lake in the background