Volcanics in outcrop: Lava flows

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Columnar jointing in Edziza volcanics, northern British Columbia

A look at lava flows in outcrop

This is part of the How To… series  on describing rocks – in this post describing and interpreting lava flows in outcrop. Although the series focuses on sediments and sedimentary rocks, I have added these posts on volcanics because of their importance to sedimentary basins and basin stratigraphy.

Volcanic eruptions catch our attention, even the least violent. One of the more spectacular  recent effusions saw the east rift zone of Kilauea Volcano spew highly mobile basaltic lava from May 3 to September 4, 2018, destroying property and infrastructure. Terrifying for residents, morbidly thrilling for scientists.

Eruptive centres within sedimentary basins or along their margins act as dynamic loads that contribute to basin subsidence, effect geothermal gradients that can alter the conditions for organic and inorganic diagenesis, and contribute sediment. Lava flows are common additions to the stratigraphy of many sedimentary basins. The erosion of lava flows also produces debris that is reworked and redeposited by subaerial and submarine sediment gravity flows, and fluvial systems.

The images that follow illustrate some of the outcrop characteristics of lava flows and pillow lavas, organized thus:

  • flow-base structures
  • flow top structures
  • internal structures
  • pillow lavas

Fragmented debris such as autobreccias and peperites are classified as Primary Volcaniclastics, but are included here because their formation is directly associated with flowing lava. The first diagram, cobbled together from White and Houghton (2006) summarizes this association of structures and processes.

Terminology for primary volcaniclastics proposed by White and Houghton, 2006

 

Additional photos of volcanic rocks and volcaniclastic facies are gathered in the Atlas of Volcanoes and Volcanic Rocks.

 

Related links in this series on outcrop descriptions

Accretionary aggregates and accretionary lapilli

Ignimbrites in outcrop and thin section

Block and ash flows

Volcanics in outcrop: Pyroclastic density currents

Volcanics in outcrop: Pyroclastic fall deposits

Mount St. Helens: 40th Anniversary

Volcanics in outcrop: Secondary volcaniclastics

Sedimentary structures: Alluvial fans

Sedimentary structures: coarse-grained fluvial

Sedimentary structures: fine-grained fluvial

Sedimentary structures: Mass Transport Deposits

Sedimentary structures: Turbidites

Sedimentary structures: Shallow marine

 

Other useful links

Describing sedimentary rocks; some basics

Measuring a stratigraphic section

 

Lava flows: base of flow structures

flow base pipe vesicles

 

 

flow base peperite

 

Lava flows: flow-top structures

modern and ancient ropey lavas

 

 

lava flow top breccia

 

 

lava autobrecciation

 

Lava flows: flow interior

tiered columnar jointing

 

 

entablature in lava flows

 

 

colonnade jointing in lava flows

 

flow banding in rhyolite

 

Pillow lavas

pillow lava piles

 

 

lobate pillow lavas

 

 

inter pillow voids

 

 

pillow lava feeder tube

References

J.D.L White and B.F. Houghton, 2006. Primary volcaniclastic rocks. Geology, vol. 34, Issue 8.

Many Volcanology text books are oldish, but no less excellent for all that. And as far as book prices go, not too expensive. Here are three excellent titles. Of course there are newer more expensive titles…!

L. Parfitt, and L, Wilson, 2008. Fundamental of Physical Volcanology.John Wiley & Sons, 256 p.

H-U, Schminke, 2004. Volcanism. Springer, 324 p.

Williams, and A.R. McBirney, 1979. Volcanology. Freeman, Cooper & Co, 397 p.

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