help identify these rocks

June 11, 2012 8:12 AM

I'm visiting family and we want to know more about these rocks. What are they? How are they formed? When they break, what causes the breakage? We are in the skirts of San Jose.
posted by bleary to science & nature (11 answers total)

Professional geologist here. It's really hard to tell without looking closely at the rock and chipping away a fresh surface. The rock can be many things seen from a distance (I expect there'll be some answers later saying the rock most definitely, undeniably, without a doubt Rock A, B or C! But a proper scientist or geologist will tell you that it's really hard to tell without investigating it up close. Be careful of people who have "definite" answers!).

There are minerals and there are rocks. And there are three basic types of rock in the world: sedimentary, igneous and metamorphic. However, it's hard to tell what type of rock at it because of weathering, where rain, sun and snow can alter the surface until the outside becomes chemically altered and unrecognisable. Kinda like the Statue of Liberty, which is made out of bronze but now is rusted and looks green on the outside.

My guess is that rusty brownish-red colours on the surface suggests that it is a type of iron staining. It is possibly some sort of sedimentary rock with a high iron content, whose outsides have been weathered so much. Either that or it's a type of ore - an iron nodule? This occurs when the iron ions have been carried along with the groundwater, swirled around in the rock, until the iron ions met with a change in condition to precipitate out and concentrate in this bit of rock.

The rocks are round-ish due to weathering.
posted by moiraine at 8:32 AM on June 11, 2012 [2 favorites]

Those are almost certainly serpentine which is relatively rare throughout the world, but common in the Bay Area. Lots of very cool endemic(found nowhere else) plants are found in serpentine soils because they are so hard to live in. I would write more, but I have to do work. Most museums and botanical gardens in the bay area will have great serpentine exhibits.
posted by rockindata at 8:36 AM on June 11, 2012

Another professional geologist here. To tell you more we need more context. Where did you find these? In a field, in an outcrop, in your driveway? The rounded cobbles will have been transported by a river or other water source at one point.
posted by cakebatter at 9:46 AM on June 11, 2012

"These rocks are endemic to the gravely soil, and the native soil is pale reddish clay, kind of terracotta clay color."

I'm visiting my mom-in-law. She lives on a small farm with a creek out back. When I told her about my post on the "question and answer" site, she had me go outside so that I could take more pictures. She didn't have a rock hammer, but we used a little sledge hammer to break one of the rocks open.

We don't think any of these are serpentine. They aren't soapy, she says.
posted by bleary at 10:18 AM on June 11, 2012

I think your first move is to think about what's upstream from you. Does that creek come down from a mountain or pass through an old landslide? You can find a geologic map of your area and find out what rocks are at the the surface on that mountain - which will give you a good first guess about what these might be.

Here's a text description of a USGS map of the San Jose area, which shows a huge number of very different kinds of rocks at the surface in different parts of the area - obviously, it makes a big difference where you are in that area. A bit of searching will let you reach a huge file (17 megs) with the map itself.

Why do they have that round outside shape?
The nice round shape comes from weathering - having water, ice, or wind run over it for a long time. The softer the rock, the faster it will become round.

Why do they have similar colored outsides?
As said above, chemical reactions can change the outside color and look.

What minerals are they made of?
What process caused them to be made of that?
It looks like you have a bunch of different kinds of rocks there - some with light interiors, some dark gray, some with very uniform smooth fine-grained texture throughout and some with bigger chunks embedded in a smoother matrix. Your first step for these two questions is to narrow down the likely source of the rocks, where they came from, and find out from the geologic map what kind of rocks are available for transport and weathering.

Why do some of them have such nice clean breaks?
Many possibilities.
First, a rock can have a natural tendency to break in a clean plane. In a sedimentary rock, there are often natural breakage lines where the layers of sediment meet, so that's one way you can get clean breaks. In some metamorphic rocks, such as slate or mica, there are natural breakage lines where the pressure causes minerals to re-knit themselves in flat sheets. In an igneous rock, the breakage ("cleavage") tendencies have to do with the crystal shape of the minerals and how quickly the crystals formed.
Second, some event that caused the breakage - could be a hit from the outside, or possibly something to do with forces within the rock (for an extreme example: heat a rock fast enough and - if it has trace amounts of water in it - the water can steam and expand and break the rock).
posted by LobsterMitten at 11:33 AM on June 11, 2012

Looks like chert nodules weathered out of Great Valley Sequence. Do the rocks exhibit conchoidal fracture when broken?
posted by Big_B at 11:54 AM on June 11, 2012

Where in San Jose? If you're near Alum Rock Park perhaps they're Thenardite?
posted by hampanda at 1:38 AM on June 12, 2012

It would really help if you could say exactly where in San Jose you picked up these rocks. They are called cobbles - meaning they are rounded from the action of water in a river or stream. I'm pretty sure they are not all the same type of rock. If you picked them up next to an existing creek then most likely they came from wherever the creek comes from. Here's the thing though, they could have been rounded in the creek where you found them, or they could have been rounded in a river 80 million years ago, formed into a conglomerate rock, then eroded out again and washed down a creek for you to find them.

One clue is that most of the rocks look granular - meaning you can see different type of grains within the rock. The shape of the grains seems to be angular not round. You can also see that the grains are different sizes. There are two common types of rock that look like this, granite and volcanic tuff (volcanic ash compressed into rock). Some of these rocks look like tuff to me (the big light grey one for example).

A second clue is that some of the broken rocks show conchoidal fracture in a light colored material which is almost certainly quartz grains within the rock. These could be a granite.

Some of the very light & rose colored rocks seem to be mostly quartz and perhaps chert. I would call this probably metachert. This is a common rock in the east bay hills and the Diablo Range.

Taken all together these rock types were probably not formed locally. If I'm correct that you have some granite and volcanic cobbles, and if you are located on the southeast side of San Jose then there is a good chance that these rocks weathered out of the Berryessa Formation, which is part of the great valley sequence. It is a conglomerate made up of cobbles that washed down from the proto-sierra mountains during the Cretaceous period. If you go to Alum Rock Park in east San Jose you can see the formation right at the entrance to the park. The cobbles within it look similar to your pictures.
posted by Long Way To Go at 4:25 PM on June 12, 2012 [2 favorites]

The place is in the South East area of San Jose. On google maps there are areas labeled The Villages, and Silver Creek. It's in that area. I'm at a conference in Walnut Creek at the moment, so I can't break any more rocks to check if they all break similarly.
posted by bleary at 5:41 PM on June 12, 2012

Perfect. The geo map of San Jose (17MB PDF) shows that The Villages is located on recent alluvial fan deposits originating from a couple of creeks that flow down from the hills to the east that are mapped as Cretaceous Berryessa Formation Conglomerate described as:

Kbc Conglomerate -- Thick, indistinct beds of pebble, cobble, and less common boulder conglomerate interfingered with coarse-grained mica-quartz-lithic wacke. Clasts include silicic to intermediate volcanic rocks, black chert and argillite, quartz, mica schist, semi-gneissic meta-andesite, granodiorite and granite, black hornfels, and rip-up clasts of mudstone and lithic wacke.
posted by Long Way To Go at 7:43 PM on June 12, 2012 [1 favorite]

Thanks all.

btw if any of you are in San Jose or Walnut Creek I would be delighted to meet you. If you are in San Jose I would love to introduce you to my mom-in-law who has the farm with the rocks. I'm only in the area until Sunday. She lives here, so I will be here again. (I live in Chicago.)
posted by bleary at 9:03 AM on June 13, 2012