Geographic location:

Latitude:  12° 31′ 43.068144″ N

Longitude: 107° 46′ 8.4″ E

Administrative location: Phuc Duy Petroleum station, Tân Lợi village, Đắk Gằn ward, Đắk Mil district, Đắk Nông province.

Basalt Separation and Phúc Duy basalt
Basalt Separation and Phúc Duy basalt

On the horizontal section in the northern area of Phuc Duy petroleum station, it is obvious to see the 2 separate layers. The lower layer is from an eruption earlier than the upper layer. The surface of the older basalt layer was eroded and intensely weathered into kaolin clay. Subsequently, the lava flow of the later eruption spilled over the older layer like a flood, sweeping all the matter on the way and finally led to the formation of eluvi-deluvi boulders of the older basalt.

The obvious separation between the two layers by their differences in physical features and weathering sufferance levels indicates the long-time gap between the two eruptions. The upper layer has just been half-weathered while the lower is completely weathered.

At the lowest part of the upper layer and the uppermost of the older layer, there is a thin (1-2cm) brown/tawny/penny rock layer which is relatively hard and stable. This is due to the contact of the lava of the later eruption to the basalt layer of the previous eruption.

Within the internal parts of the older basalt layer, a thin sub-layer of Gotit and Limonite can be observed. It proves the existence and the fluctuation of the groundwater level during the weathering process of this layer.

Dislike the basalt of the later eruption in the northern area of Phuc Duy petroleum station which is not columnar, the basalt in the southern area formed in the columnar block. This means the southern area had a thicker lava flow and the more consistent environment to cool down the lava than the northern area.