Nobbys Tuff EarthCache

Go for a popular local walk to examine some of the geological features of Nobbys Head.

The exposed hillside at Nobbys Head reveals an eroding cliff of a rock known as Nobbys Tuff. Tuff (from the Italian tufo) is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is compacted into a solid rock in a process called consolidation.

As tuff is produced volcanically, but laid down in layers, it can be classified as either sedimentary or igneous rock.

"Tuff" is a name that is used for a broad range of materials. The only requirement is that the materials are ejecta produced by a volcanic eruption. Tuff can contain fragments of dust-size particles to boulder-size particles and be composed of many different types of material.

Many tuff deposits contain fragments of bedrock that are unrelated to volcanic activity. These materials are involved when the volcanic explosion occurs below the ground. The subsurface explosion crushes the overlying bedrock and launches it into the air mixed with tephra and volcanic ash produced from the magma source below.

Different volcanoes are supplied with magma of different compositions. Many tuff deposits form from magma with a rhyolitic composition, but andesitic, basaltic, and other types of magma might contribute to the tuff.

Tuff also varies by particle size. Near the vent, a tuff might consist mainly of large blocks of material in a volcanic ash matrix. With distance from the vent, the clasts (fragments) will be smaller in size. At the edges of the rock unit, the tuff might be mainly composed of very fine ash.

About 250 million years ago, numerous massive eruptions from a volcano located 30 km to the east of Newcastle blasted volcanic ash over the Hunter region, which at this time consisted of a low-lying alluvial plains covered by vast wetlands and swamps. The shoreline lay further to the southeast than it does today.

The volcanic ash smothered the swamps and lakes. After the eruptions ceased, the volcanic ash layers were buried by more swamps and river deposits and compacted to form the rock tuff. This layer of rock extends from sea level up to almost the top of the cliff, with a thickness of 25 metres. The layer of Nobbys Tuff is found in outcrops out to Stockrington, and has been identified in coal mine drillings in Lake Macquarie and as far away as Broke.

The swamp on both sides of the layer of tuff became seams of coal.

Nobbys Head also showcases a prominent basalt dyke, clearly visible in the cliff face from the beach to the south of the breakwall (at the published coordinates). The vertical feature formed when magma intruded upwards into the sedimentary layers.

To log this earthcache:

Proceed to the published coordinates on the beach in front of the cliffs. From the Nobbys Beach carpark the walk is approximately 1km each way, and is a level concrete path, bikes are OK. Low tide is preferable, as you will have much more beach! Take note of the signs regarding the danger of falling rocks at the cliff base. You do not need to climb up to the cliff face to complete this cache.

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