Dumb as a rock? A lumpy bit of earth reveals a geological timeline in Seymour

Editor’s note: Today’s blog comes to us from paleontologist and field volunteer Neal Immega.

You all know by now that the museum has a dig in Seymour where we are finding fabulous Permian fossils, including the toothy Dimetrodon and the weird boomerang-headed salamander Diplocaulus. We don’t dig in just one place; lots of people go prospecting for new sites (or maybe they are just looking for a private spot to do their business).

A previous blog topic was on a weird lump that turned out to be a caliche ball. Today we have another lumpy rock to look at. This specimen was collected by geologist Gretchen Sparks, who brought it in just to plague me.  Let’s see just how much information we can squeeze out of it:

CrossBeds

At first glance, it’s just a rock showing cross-bedded sandstone with low-angled bedding, doubtless caused by water deposition in the Permian creek that crossed our digging area. This is pretty normal stuff. We see cross-bedding everywhere at the dig site, because the sandstone is durable and stands in relief.

But why is it lumpy? The bulge in this picture is not exactly standard:

Profile

It gets better. The rock is too heavy to be only quartz. A heavy, light-colored sandstone is likely to be cemented by barite (barium sulfate). Let’s cut the rock in half and polish the face.

cutLumpLabeledThis is turning out better than I expected. You can see a seam of barite cutting the nodule vertically in half. The sandstone shows horizontal layers which correspond to the cross-bedding.

What about the red-colored area? The area we are working in North Texas is called the Permian Red Beds because everything got oxidized from prolonged exposure to the atmosphere. It was a really dry time, and the critters stayed close to the Permian creek which deposited these sediments. It is good for us because the fossil remains are concentrated in a small area (our dig site is just about the size of a tennis court).

Let’s consider this possible sequence of events.

1.    250 million years ago, sandstone was deposited in a creek. It is all cross-bedded.

2.    The sand grains were likely coated with iron oxide and thus turned red.

3.    The sandstone was buried by maybe 1000-plus feet of additional rock.

4.    Shales deeper in the geological section were heated by the normal geothermal gradient to hundreds of degrees and adsorbed water was squeezed out, taking with it the barium that was also adsorbed (from the ocean) on to the clay surfaces. The water moved vertically along cracks in the rock.

5.    When the barium reached the rocks we are digging in, the barium precipitated because the pore water is very “hard” with dissolved gypsum. The barium reacts with the local sulfate, producing barium sulfate which is essentially insoluble in water. It is the ultimate “hard water” scum.

6.    The barite precipitated as the vertical seam and filled the pore space in the red rock.

7.    A whole lot of rock was eroded in the next 250 million years to bring us to the present and the rocks back to the surface.

8.    10 to 100-thousand years ago or so, North Texas was in an Ice Age, and it was really wet with lots of vegetation. Decomposing vegetation created a reducing environment which dissolved iron right out of the rock. Barite is very chemically resistant, and this lump could have been at the surface through part of the ice age. The iron could have partially leached out of the lump at that time. You can see that the leaching went deeper into the lump where there are horizontal fractures in the rock.

All this from a lump of rock. Thanks, Gretchen!

Gems and Minerals

The Houston Museum of Natural Science is currently hosting a special exhibition, The Nature of Diamonds, so this month our booklist features Gems and minerals.

According to www.rocksforkids.com a mineral is the same all the way through, and  there are about 3000 known minerals on earth.  A rock, on the other hand, is made from two or more minerals. 

For young children, Let’s Go Rock Collecting by Roma Gans is a great introduction to rocks.  The book begins by saying that people collect many things, and that the oldest thing you can collect is rocks.  In simple terms, with wonderful illustrations and photographs, Ms. Gans explains the three types of rocks—igneous rocks, sedimentary rocks and metamorphic rocks.

20071804 157
Creative Commons License photo credit: Walter Rodriguez

You will learn how magma becomes lava which becomes igneous rocks. You will also see photographs of granite, quartz and basalt –  all igneous rocks.  Sandstone and limestone are examples of sedimentary rocks.  The Egyptian pyramids were made from limestone. In modernt times, limestone is mixed to make cement. Metamorphic rock means changed.  Slate is a metamorphic rock that used to be shale before being exposed to intense heat and pressure.

Children are encouraged to collect rocks, and examples of simple rock collections are pictured.  Rocks are everywhere, so collecting rocks is an inexpensive introduction to science.  And, who knows?  You might grow up to be president of the Houston Museum of Natural Science!

Byrd Baylor’s Everybody Needs a Rock begins “Everybody needs a rock.  I’m sorry for kids who don’t have a rock for a friend.”  Baylor gives the reader rules for finding a special rock, something you might keep forever.  The rules say you can find a rock anywhere, but make your choice when things are quiet.  You need to look the rock in the eye to make sure it is the perfect size, color, shape and smell.  Do not let anyone help you make the choice — the decision is yours alone. 
Baylor’s words paired with Peter Parnall’s simple black and bronze drawings work together to create quite a book that will make children anxious to begin the search for their own rock.

Gemstones by Ann O. Squire is a nonfiction introduction to gems.  You learn that deep within the earth, high temperatures and pressure transform minerals into crystals which can be cut, polished and sold for thousands of dollars.  A crystal must pass 3 tests to be considered a gemstone:  it must be rare; it must be beautiful; and it must be hard enough to resist scratching or breaking.

bariteSquire says that gemstones began forming millions of years ago up to 100 miles beneath the earth’s surface.  The intense heat caused the rocks to become magma which contains tiny mineral crystals.  Pressure caused the magma to erupt from the earth as a volcano or flowing between layers of rock.

Diamonds, rubies, sapphires and emeralds are inorganic minerals, meaning they have never been alive.  A pearl, however, comes from a living source — an oyster.  Amber comes from the sap of trees that lived long ago and coral is made from the skeletons of tiny sea creature.
Squire briefly explains some of the superstitions involving gemstones and tells how the idea of birthstones began.

Don’t miss the Cullen Hall of Gems and Minerals including the Lester and Sue Smith Gem Vault on the second floor of the Houston Museum of Natural Science.  You will see the most incredible collection of gems and minerals in the world.