Note: This is an Earthcache. There is no physical cache to find. Logging this Earthcache requires that you undertake an educational task relating to the specific Earth Science at the site. You can log the find immediately, but such logs which are not followed up with answers will be deleted after a reasonable period of time, say two weeks, has elapsed.
Geocache Description:
Click on ‘Message this owner’, or send an email here with answers to the following logging tasks:
Logging Tasks:
- At the posted coordinates, you will be standing in front of a basaltic lava flow. A section of this flow about a half to a metre squared, is significantly different to its surrounds, please describe this section.
- During your examination of this section, take note of its texture. What is the texture of that you see here? (Please note: Igneous Rock Texture is described in the cache description)
- Please include a photo of the Merri Creek from this location that shows an identifying personal item.
Eruption points for the basalt flows of the Merri Creek, within the Newer Volcanics Province, arise at Mt Ridley, between Mickleham and Craigieburn to the north of Melbourne.
Mount Ridley is a broad lava hill with two vents approximately 600 m apart, the southern vent apparently erupting first. Both vents are flat domes without craters or conspicuous eruption features, the northern dome being the younger as its lava overlies that from the southern dome. The vents are now lava filled and are recognizable only from the concentration of blocks of spatter and vesicular lava. Most of the loose blocks have been cleared from the paddocks. The lava from the northern vent of Mount Ridley is the only known example of a nephelinite (analcite) in the Newer Volcanics Province. Nephelinite is a fine-grained or aphanitic (see Texture of Igneous Rocks further down) igneous rock made up almost entirely of nepheline and clinopyroxene.
The age of these eruptions is 4.6 to 0.8 million years ago (Mya). It is estimated that the last volcanic eruption of Mount Ridley was approximately one million years ago.
The cliffs above the lake displays a number of primary features and basalt lava flows. These include ropey lava, massive basalt, strings of vesicles and differing orientations of joints. Detailed analysis of lava features indicates there are at least two flow units in this area. Weathering features and basalt pavements are exposed in the channel of the Merri Creek at the head of the lake.
Lava Flows
The content of a lava – which elements are within the liquid and whether there are dissolved gases – gives the lava certain characteristics, such as how fluidly it flows. From observations of the Hawaiian volcanos, Mauna Loa and Mauna Kea, volcanologists have distinguished two very different types of basalt magma flows.
Pahoehoe (Pa-hoy-hoy) is a wrinkled ropey looking lava, and eventually a basalt rock, with a smooth surface. As this lava, with low amounts of silica and dissolved gases, flows, the outer surface exposed to the air cools and solidifies. However, the inner lava is still flowing, which creates the twists and wrinkles in the crust of the flow.
Another type of flow produced is a’a (ah-ah) lava. A’a lava is also composed of basalt but although chemically the same, is very different to pahoehoe. Instead of fluidly flowing, a’a is blocky, rough and jagged. As it flows forward, a’a looks and sounds like an advancing pile of rubble. A’a usually has a high amount of dissolved gases in it. As the a’a cools, the gases escape leaving sharp spiney protrusions of basalt on the surface.
These two types of lava were first observed and named in Hawaii, but now scientists use the terms pahoehoe and a’a to describe lava flows with these characteristics no matter where they occur in the world.
Texture in Igneous Rocks
Texture is the appearance of the rock, and to some extent how it feels when you touch it. It describes the size of the individual minerals in the rock. The texture of an igneous rock gives clues to how and where the rock was formed, including how long the rock took to cool from the magma and the individual mineral content of the magma. As a general rule, igneous rocks that form underground (Intrusive) have larger minerals than ones that form above ground (Extrusive).
Following are the common terms used to describe the texture of igneous rocks with an indication as to whether the term is associated with an intrusive (I) or extrusive (E) rock.
- Phaneritic (I): An igneous rock with crystals that are large enough to be seen without a microscope is called phaneritic. This rock forms deep underground allowing the magma to cool very slowly. This slow cooling allows the crystals to grow.
- Aphanitic (E,I): An igneous rock with aphanitic texture forms when the rock cools very quickly, so quickly that the crystals don’t have time to grow and are very tiny. A microscope is needed to see individual crystals.
- Porphyritic (I,E): A porphyritic igneous rock is formed by slow then quick cooling. A magma begins to cool slowly, allowing crystals to form but then its environment changes. The resultant rock will have some crystals large enough to see but they will be in a matrix of smaller crystals not seen without a microscope.
- Vesicular (E): A rock full of holes is described as vesicular. As the rock cools, gas bubbles are trapped in it, eventually the gas escapes leaving holes called vesicles in the rock.
- A few other terms used which I won’t go into are Pillow (E), Pegmatite (I), Glassy (E) and Pyroclastic (E).
Although not required to complete the logging tasks, further interesting information can be found at the link below, including two videos presented by geoaware (Gary Lewis), describing the two common lava flows, pahoehoe and a’a.
https://sway.cloud.microsoft/8MhNBri1etONuNf7?ref=Link
Glossary
- A'a: A term of Hawaiian origin. Used in reference to a basaltic lava that occurs in flows with a fissured, rough and jagged surface. As it flows forward, it sounds like an advancing pile of rubble.
- pahoehoe: A type of basaltic lava (material) with a characteristically smooth, billowy or rope-like surface and vesicular interior.
- Mya: Million years ago
- weathering: All physical disintegration, chemical decomposition, and biologically induced changes in rocks or other deposits at or near the earth's surface by atmospheric or biologic agents or circulating surface waters with essentially no transport of the altered material. These changes result in disintegration and decomposition of the material.
- lava dome: A steep-sided rounded extrusion of highly viscous lava squeezed out from a volcano and forming a dome-shaped or bulbous mass above and around the volcanic vent. The structure generally develops inside a volcanic crater.
- vesicular: A textural term applied to an igneous rock containing abundant vesicles, formed by the expansion of gases initially dissolved in the lava.
- vesicle: A cavity in a lava, formed by the entrapment of a gas bubble during solidification of the lava.
- lava flow: A solidified body of rock formed from the lateral, surficial outpouring of molten lava from a vent or fissure, often lobate in form.
- Igneous Texture:
- Phaneritic (I): An igneous rock with crystals that are large enough to be seen without a microscope is called phaneritic. This rock forms deep underground allowing the magma to cool very slowly. This slow cooling allows the crystals to grow.
- Aphanitic (E,I): An igneous rock with aphanitic texture forms when the rock cools very quickly, so quickly that the crystals don’t have time to grow and are very tiny. A microscope is needed to see individual crystals.
- Porphyritic (I,E): A porphyritic igneous rock is formed by slow then quick cooling. A magma begins to cool slowly, allowing crystals to form but then its environment changes. The resultant rock will have some crystals large enough to see but they will be in a matrix of smaller crystals not seen without a microscope.
- Vesicular (E): A rock full of holes is described as vesicular. As the rock cools, gas bubbles are trapped in it, eventually the gas escapes leaving holes called vesicles in the rock.