Getting to GZ: 

This earthcache is within the boundaries of Monash University. It’s best to visit the campus on a weekend or during semester break. During semester school hours on a weekday, parking is very difficult. The majority of the carparks are geared towards staff and student parking and not visitors. If you are here on a weekday day it’s best to park off campus in a nearby street and walk in. I’ve parked here in the past: S37° 54.581' E145° 07.723' It is one of the closest points of entry.

On the weekends and during holidays the carparks are empty, however don't assume no one is watching you. There are security cameras everywhere. Campus security are aware of the earthcache but try not to give them a reason to come have a chat! They do also block off and lock certain access points during the holidays....so in the interest of you making your way safely in and out without getting lost....
The easiest, best and closest entry & exit point is along Bayview Ave & drive to the entrance here: S37° 54.435' E145° 07.753' Drive almost all the way around the roundabout at the entrance and turn left onto Boundary Rd, turn left into the second, smaller carpark and go to the far side of it. Then it is a very short walk to GZ. Make your observations and leave the way you came in…we have been told it's very easy to get lost and drive around and around, which does attract the attention of the security staff!

Lava Bomb

The Questions:

1.) The largest tephra type is called (A) lapilli; (B) bombs or (C) ash

On the south side of the black lava bomb at GZ

2.) Examine the outer layer of the side of the rocks. This side appears to be the outer crust that solidified first. Is it cracked or smooth?

Now look at the rock from above, this appears to be the core of the lava bomb. Notice the texture of the rocks, would you say this bomb was the result of lava with a lot of dissolved gases in it or not? Give your reason for your answer.

3.) Based on the what you have learnt here, can you determine what type of lava bomb you are looking?

Then post a photo of you at the location with your log, (please do not show the subject of the questions in your photo). Of course, if you do not want to appear in the photo, a personal item in the photo is enough proof of your presence. You may log the cache as soon as you submit your answers to us via messenger.

Logs without accompanying answers sent or without a photo uploaded may be deleted without notice. Sending the answers is a requirement not a request. Please note, answering the logging tasks is based on your observations at the site and using your understanding of  content of the cache page, there’s no need to do any further research. 

The Lesson:

Welcome to the Monash Earth Sciences Garden, officially opened in 2015, it is the first of its kind in Australia. Inspired by the geology and physical geography of Victoria, this ‘living’ geological map comprises of a stunning arrangement of nearly 500 rock specimens, some weighing up to 14 tons, laid out to represent a pattern of rock outcrops and set amongst beautiful native plants representing each geographical region.

*At GZ; is a garden of red scoria. The Large black volcanic ‘bombs’ - approximately 1 metre in diameter - from an 8,000-year-old volcano near Colac. This volcano is located in the Newer Volcanics Province (NVP), which stretches from Melbourne’s CBD to Mount Gambier in South Australia and contains at least 437 volcanoes ranging in age from 8 million years to just 5,000 years, some are still considered to be active.

Both scoria and lava bombs are a type of pyroclastic material, known as tephra.

Volcanoes erupt more than just lava, eruptions can include a variety of fragmentary material and volcanic gases (water vapor is most common), especially during explosive events. The rapid exsolution of gases from magma is the main driver of explosive eruptions, flinging clots of lava into the air in some eruptions, and shattering it into tiny bits (ash) in columns that may rise tens of thousands of feet into the atmosphere.

Collectively, the fragments ejected during explosive eruptions are termed Pyroclastic tephra, they may be ejected while still molten, or may consist of solidified magma or of other rock fragments.

Pyroclastic tephra are classified by size and shape:

• Ash: diameter less than 2 mm
• Lapilli: diameter between 2 and 64 mm
• Block: diameter greater than 64 mm and has an angular shape. Blocks are solid during eruption and transport.
• Bomb: diameter greater than 64 mm. Specimens of up 6 metres in diameter are known.

Here’s an overview of how Lava bombs form:

1. Magma Composition: is viscous (thick and sticky) enough to resist breaking into small particles. Viscosity is influenced by factors like silica content.
2. Gas Content: dissolved gases in magma are primarily water vapor & carbon dioxide. As magma rises to the surface pressure decreases & force dissolved gases to exit the solution, forming bubbles. Accumulated gas bubbles within the magma increases internal pressure. This leads to an…..
3. Explosive Eruption: pressure from expanding gas within the magma begin to exceed the strength of the surrounding rock, it can lead to fragmentation of the magma. This forms a mixture of smaller particles, volcanic ash, and gases known as a pyroclastic surge. This creates the force behind the next step….
4. Ejection of Molten Fragments: expelled from the vent; fine ash, rock fragments and larger, semi-liquid or plastic globs of magma. These globs are volcanic bombs. The bombs are shaped by their aerodynamic interaction with the air during ejection, giving them their form.
5. Solidification: now expelled into the atmosphere, the bombs start to cool rapidly. The outer layer solidifies, forming a crust, while the interior can remain partially molten.
6. Landing: The solidified outer crust helps it retain its shape as it hits the ground.

Diagram: Summary of models in the theory of formation of different shaped bombs. The blue dot at the center of the bombs in Fig. a & b depict the probable occurrence of a central core.

Volcanic bombs can travel significant distances away from the vent of a volcano before landing. Lava bombs can spend up to 10 seconds soaring through the air before landing. Most lava bombs land within 5km of an eruption but some have travelled as far as 10km.

Lava Bombs are named according to their characteristic shape, which is determined by the viscosity of the magma from which they are formed.

Spherical bombs: form from high to moderately fluid magma. In the case of spherical bombs, surface tension plays a major role in pulling the ejecta into spheres.

Spindle bombs: Spinning during flight leaves these bombs looking elongated or almond shaped. The bombs are characterized by longitudinal fluting, one side slightly smoother and broader than the other.

Photo: Spindle lava bomb, Cinder Cones region of the Mojave National Preserve, California

Ribbon or cylindrical bombs: Elongated, ropy-shaped bombs form from highly to moderately fluid magma. The strings break and fall to the ground intact and look like ribbons. These bombs are circular or flattened in cross section, are fluted along their length.

Photo: Ribbon bomb, Kīlauea volcano, Hawaii

Cow pie bombs: when very fluid magma falls from a moderate height it flattens or splashes on landing. These bombs form irregular roundish disks, which resemble cow dung.

Photo: Cow pie bomb, Krakatoa, Java, Indonesia

Breadcrust bombs: Bombs with a cracked surface from a quickly solidified crust has fractured due to continued expansion of gases creating vesicles (bubbles of gas) within a molten core. Breadcrust bombs vary in size, shape, and degree of cracking. Some may have a dense glassy crust, while others may be more pumiceous, ie, internally they have a porous structure with numerous gas bubble pockets.

Cored bombs: Bombs that have a nucleus of solid material. The solid core may be fragments of previously solidified magma from the volcano, or a xenolith (foreign rock fragment from either the crust or the mantle).

Photo: Cored bomb, olivine core, Razas Grand Quarry, Auvergne, France

**There are other rocks here that look like cored lava bombs with a light coloured centre surrounded by a darker exterior but don’t be fooled, they are fake. The designers of the garden; Landscape Architects RushWright & Associates designed the “bombs” in consultation with staff of the School of Earth, Atmosphere and Environment at Monash, and artists from Open Spatial Workshop and named them “Anthropocite”. The name comes from the word Anthropocene, an informal term referring to the current geological epoch, characterized by significant human impact on the Earth's geology and ecosystems.

Art in the Garden: Anthropocite 2015 by Open Spatial Workshop (Terri Bird, Scott Mitchell, Bianca Hester).

Presented within the Monash Earth Sciences Garden, Anthropocite (2015) tells a story of material transformation and parallels the multiple geological narratives evident in the surrounding rocks. The artwork explores how the Anthropocene may be evident in rock formations of the future. The fascinating part is the art studio went through a whole process of experimenting to produce their ‘lava bombs’. This involved them both the production of sedimentary rock and volcanic magma. The sedimentary rocks were produced through the layering of graded material, including recycled concrete, asphalt, glass, metal oxides, sands and soil. These layers were stabilized with refractory cement and fired to 1,200°C. The volcanic magma was produced through the melting of crushed scoria within a mold around the created “sedimentary” ceramic core. When combined, these two processes formed a new ‘volcanic bomb’. The “Anthropocite” sedimentary rock is encapsulated in magma. The cut face of the constructed volcanic bombs reveal the sedimentary layers and magma shell.

Resources:

https://geologyscience.com/rocks/volcanic-bomb/

https://www.monash.edu/news/articles/9288

https://osw.com.au/Anthropocite-2015