MORE DIKES FOR PHILLIP ISLAND
Dikes come in many different shapes and sizes. These are two pictures showcase the different shapes and sizes.
It seems Phillip Island is lucky to have more than one dike. I thought it worthy of highlighting this one as well. It is a different area to the other one and has some stunning views of the coastline. To access it you will find parking at the Berry's Beach Car park and will then have to walk about a 1km to GZ. Don't forget to check the tide and take a camera.
Igneous Intrusion
What is that I hear you say. Well Igneous intrusions are any igneous rock which hardens underground. There are three common types of intrusion are sills, dykes, and batholiths.
WHAT IS A SILL?
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means that the sill does not cut across preexisting rocks, in contrast to dikes, discordant intrusive sheets which do cut across older rocks. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source. The rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related to foliation in metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations.
Sills parallel beds (layers) and foliations in the surrounding country rock. They can be originally emplaced in a horizontal orientation, although tectonic processes may cause subsequent rotation of horizontal sills into near vertical orientations. Sills can be confused with solidified lava flows; however, there are several differences between them. Intruded sills will show partial melting and incorporation of the surrounding country rock. On both contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed (contact metamorphism). Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will typically show evidence of vesicles (bubbles) where gases escaped into the atmosphere. Because sills generally form at shallow depths (up to many kilometers) below the surface, the pressure of overlying rock prevents this from happening much, if at all. Lava flows will also typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock, typically do not.
WHAT IS A DIKE?
A dike or dyke in geological usage is a sheet of rock that formed in a fracture in a pre-existing rock body. However, when the new rock forms within and parallel to the bedding of a layers rock, it is called a sill. It is a type of tabular or sheet intrusion, that either cuts across layers in a planar wall rock structures, or into a layer or unlayered mass of rock.
Dikes can be either intrusive or sedimentary in origin. For example, when molten rock intrudes into a crack then crystallizes, it is an igneous dike. When sediment fills a pre-existing crack, it is a clastic dike.
WHAT IS BATHOLITH?
A batholith is a large emplacement of igneous intrusive (also called plutonic) rock that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite. Although they may appear uniform, batholiths are in fact structures with complex histories and compositions. They are composed of multiple masses, or plutons, bodies of igneous rock of irregular dimensions (typically at least several kilometers) that can be distinguished from adjacent igneous rock by some combination of criteria including age, composition, texture, or mappable structures. Individual plutons are crystallized from magma that traveled toward the surface from a zone of partial melting near the base of the Earth's crust.
Traditionally, these plutons have been considered to form by ascent of relatively buoyant magma in large masses called plutonic diapirs. Because the diapirs are liquified and very hot, they tend to rise through the surrounding native "country" rock, pushing it aside and partially melting it. Most diapirs do not reach the surface to form volcanoes, but instead slow down, cool, and usually solidify 5 to 30 kilometers underground as plutons. It has also been proposed that plutons commonly are formed not by diapiric ascent of large magma diapirs, but rather by aggregation of smaller volumes of magma that ascended as dikes.
A batholith is formed when many plutons converge to form a huge expanse of granitic rock. Some batholiths are mammoth, paralleling past and present subduction zones and other heat sources for hundreds of kilometers in continental crust
THIS PICTURE BELOW GIVES YOU AN IDEA HOW IT ALL FORMS
NOW TO THE LOGGING REQUIREMENT OF THIS EARTHCAHE.
NO1 Roughly how long do you think this dike is?
NO2 Is the dike Black Basalt ?
NO3 You will need your GPS Compass for this one. Do you think this dike runs more to the East or the North?
NO4 This is an easy one is the Dike running in a straight line or wiggly line?
Hope you have fun in the area. Please email me your answer.