Melbourne sits on the distal reaches of the Yarra River before it flows into the northern shores of Port Phillip Bay

Port Phillip Bay and the surrounding lands are surrounded by hills

the north-western regions are dominated by basaltic rock from relatively recent volcanic laval flows - see Victoria's volcanoes

north-east and east is dominated by the Kinglake and Mt Dandenong hills

the south-eastern regions - the Mornington Peninsula - is dominated by sandy soils - hence called the “sand belt”

the You Yangs to the west are the eroded remnants of a granite landscape

1324-1150mya Musgravian Orogeny in SW Australia in the Grenville era (formed gneiss and granitoids) ¹⁾

1100-750mya, a single global supercontinent Rodinia

c750mya, the Terra Australis Orogen, from the Neoproterozoic to late Palaeozoic, records the beginning of rifting when the Pacific Ocean opened and the initiation of subduction associated with ocean closure. Prior to dispersal the orogen was about 18,000 km long and about 1600 km wide, and incorporated in it were the Tasman Orogen of Australia, the Ross Orogen of Antarctica, the Tuhua Orogen of New Zealand and the Andean Cordillera of South America.

600mya, southern part of the Australian craton is related to the East Antarctic craton

up until the Silurian period around 400 mya, although major submarine volcanic activity was occurring, Victoria had been submerged under ancient oceans and sediment from primeval mountain ranges near the current Flinders Ranges was to form Ordovician shale deposits embedded with graptolite fossils - best seen in the Bullengarook shale quarry

in the Silurian and Devonian periods (446-359mya), the oceans were retreating and half of Victoria became to be above sea level due to the Lachlan Fold Belt orogeny ⁴⁾ which formed much of eastern Victoria and NSW as the Great Dividing Range (in Victoria, the western limit of the LFB is defined by the Stawell-Ararat Fault and to the west of this is the Moornambool Metamorphic Complex which formedc440mya), and this period produced Victoria's most explosive volcanoes - in some parts of central Victoria, the volcanic layers are 1km thick. The remains of this peak vulcanic era now form Mount Macedon, the Dandenong Ranges, the mountainous region between Healesville, Warburton and Eildon (including Mount Donna Buang and the Cerberean Ranges) and near Violet Town, in the northern part of the Strathbogie Ranges.

During the late Devonian period, at 380mya, there was extensive plate movement (the Tabberabberan orogeny) which buckled and folded the Ordovician sediments in a north-south direction as evidenced in Werribee Gorge, while molten magma melted through them to form granite which formed the Ingliston Granites (at the west end of Werribee Gorge), the You Yangs, Anakies and Dog Rocks near Geelong. Over subsequent millenia, further ocean sediments covered these granite formations until sea levels fell around 200mya and they were eventually eroded again to reveal the granite.

300-250 Ma the Terra Australis Orogen ended, the termination being associated the assembly of Pangaea

in the Triassic and Jurassic periods (251-145mya), Victoria was above sea level but covered by glaciers in an ice age (eg. Werribee Gorge). Eruptions were mostly in western and north-east areas. The far majority of dinosaur fossils laid down in this period has long since been eroded.

in the Cretaceous period (145-65mya), earth had a mainly tropical climate due to high CO₂ levels, flowering plants, primitive birds and mammals including marsupials evolve, and Australia started to tear away from Antarctica causing depression between Victoria and Tasmania where lakes and swamps formed

the Tasman Sea's mid-ocean ridge developed between 85-55mya as Australia and Zealandia broke apart during the breakup of supercontinent Gondwana

in the Cenozoic Era (65mya to now), earth's climate began to cool allowing Antarctica's ice caps to again form (~40-50mya), but as the Australian continent drifted further north and became warmer, the formation of the Tasman Sea allowed magma from the mantle to reach the surface starting a new period of vulcanism and the formation of the basaltic lava plains outlined below and in many places the lava flowed down ancient valleys (forming the deep leads in Victoria's gold mining regions which resulted from several generations of fluvial placer deposits of sedimented mineralisation). In many cases, when the softer walls of these valleys became eroded over millenia, the former laval valley deposits became the spurs of the local hills creating an inverted topography.

the cool climate resulted in thick forests of the ancestors of Antarctic Beech and Myrtle Beech Nothofagus cunninghamii but in lower lying regions, these became flooded with the Tasman Sea waters 15-50mya, and resulting compression from layers of sedimentation over the peat resulted in the extensive brown coal deposits in Victoria which in some places is up to 330m thick (Australia has 19% of the world's recoverable brown coal)

around 6.25mya, stiff magma pouring from a vent and congealing in place formed the mamelons of solvsbergite, a form of trachyte, and what we currently know as Hanging Rock, Camel's Hump and Brock's Monument around the Mount Macedon region.

around 2.5mya-4mya, there were further major plate movements and the Rowsley, Hanover and Meredith Faults uplifted the Brisbane Ranges above the “sunk-land” plains to the east (Port Phillip), while vulcanism from Mt Anakie covered the Werribee plains with lava ⁵⁾

during the Ice Ages, Victoria and Tasmania were connected by land

some 10,000yrs ago, at the end of the last Ice Age (which peaked 30,000yrs ago), the ocean waters rose and Tasmania became cut off from Victoria with the formation of Bass Strait, and the sunk-lands of Port Phillip around the Yarra River became inundated to form Port Philip Bay

the south-west regions of Victoria are dominated by relatively recent volcanic activity over the past 6 million years, up until around 8,000 yrs ago (the most recent at 8,000yrs ago is the Red Rock Volcanic Complex at Lake Corangamite, west of Melbourne)

the laval structures in the Organ Pipes NP in Keilor on Melbourne's outskirts are from early in this period of volcanism and are around 2.5-2.8 million years old, dating from the Pleistocene and these were exposed by 1-2 million years of erosion from the waters of the Jacksons Creek, the base of which also exposes a prehistoric buried creek valley, which is cut into 400 million year-old (Silurian) mudstones and sandstones which contain marine fossils indicating they were formed under an ancient ocean

basalt is a fine grained volcanic rock which accounts for 90% of all volcanic rock on earth and is formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava). Mafic magma - as lava is called before erupting on the surface - forms by partial melting of peridotite (a magnesium-iron-silicate-rich rock) found in Earth's upper mantle.