Pulling apart

Model: Phil Prince

Words: Lisa Whalen


Most of the time in this blog we discuss collision which makes us thing about compressional forces. This time we’re going the opposite direction and investigating what we  can replicate with sandbox models in terms of extension, or when the continents are under tensional forces. This gets me thinking about pull-apart bread as geologists love food analogies.


image: www.rachelcooks.com

In this video Phil shows the formation of normal listric faults occur as two continents pull away from each other much like what is occurring in the Basin and Range in the Western United States (or in the bread above).

Normal faults form as the hanging wall moves downward relative to the foot wall. In a listric normal fault the fault plane or where the hanging wall and foot wall touch isn’t plane so much as it’s scoop shaped.

Images: nps.gov and geologylearn.blogspot.com

Bringing down Mountains: More Landslides

Model by Phil Prince

Words by Lisa Whalen


Last time we looked at a model replicating a translational landslide. Continuing with our landslide theme, this time Phil has built a model comparing translational slides with rotational or slump landslides. These are characterized by a more curved plane of weakness that allows the slide to move. The curvature leads to the moving material becoming deformed, first through extension or tensional forces as it initially pulls away to compression as the sliding mass piles up on top of itself.


Going with the Flow

Words by Lisa Whalen

Model by Phillip Prince

Images credit: British Geological Survey


Sandbox models aren’t just for mountain building. In this video Phillip Prince replicates landslides.

A translational landslide is when rock or soil moves down-slope along a plane of weakness like a joint, fault or bedding surface. Best illustrated with chocolate and a weaker layer of caramel:



A rotational landslide on the other hand has a curved surface of weakness and moves large blocks of material that tend to rotate backwards as they move.



Here in southwest Virginia are some of the largest known landslides in the world, with one slide reaching almost 3 miles long. You can learn more about “The Mountain that Moved” on the USGS site about Sinking Creek Mountain. The landslides occurred in the Pleistocene, but weren’t discovered until the 1980’s because the enormous size of the landslides obscured their presence. Today they can be identified partly through different vegetation and the presence of springs and pare visible from many of my favorite hiking areas. They may have been caused by erosion or even by earthquakes.

As an aspiring mountaineer another way to think about translational landslides is to think of slab avalanches. In this case you have layers of snow separated by a weaker layer. Weather, more snow, or people (such as in the video below) can tip the scales of the forces that are keeping the layers of snow coherent and trigger an avalanche.

Two Steps to Triton

Words by L. Whalen
Model by P. Prince

Nothing stays the same very long – and tectonic environments are no exception. Structures from a later event can overprint a previous event. Sorting this all out is both fun and hard work. In this video, Phil recreates two stages of the geologic history of Triton Bay, West Papua (northwest New Guinea).

First, we see convergence forming parallel anticlines. Subsequent extension creates a characteristic rectangular pattern, that when filled in with water matches the local structure.

Ouachita Mountains: Rock Strength in Action

Words by Lisa Whalen

Video/Model: by Phil Prince

image: wikipedia.org


The Ouachita orogeny occurred ~300 million years ago when part of South American collided with the southern part of North America.

image: By R.Q. Foote, L.M. Massingill, and R.H. Wells – Petroleum geology and the distribution of conventional crude oil, natural gas, and natural gas liquids, East Texas basin

In this model Phil Prince takes us to the Ouachita Mountains of Arkansas and Oklahoma. Here you will see how the strength of different rock layers coupled with deep erosion can produced the characteristic shape of this range.  Sinuous ridges formed from chert snake around valleys scooped out of shale abutting a broad sandstone plateau.