geologictimepics

Geology and Geologic Time through Photographs

Archive for the tag “faults”

Where rocks touch: geologic contacts

Geologic contacts are the surfaces where two different rocks touch each other –where they make contact. And there are only three types: depositional, intrusive, or fault. Contacts are one of the basic concerns in field geology and in creating geologic maps –and geologic maps are critical to comprehending the geology of a given area. For those of you out there who already know this stuff, I’ll do my best to spice it up with some nice photos. For those of you who don’t? This post is for you!

Depositional contacts are those where a sedimentary or volcanic rock was deposited on an older rock (of any type). Intrusive contacts are those where igneous rocks intrude older rock (of any type). Fault contacts are… faults! –surfaces where two rocks of any type have moved into their current positions next to each other along a fault.

In a cross-sectional sketch they may look like this:x-sxnlr

And here are some photos. Click on the image to see it at full size.Depositional contact and windows,  Jurassic Entrada Fm (red) ove

So how do you tell them apart in the field? If the actual contact surface isn’t exposed –which is usually the case– you have to use some indirect observations. Here are some general rules that can help. Of course, each “rule” has exceptions, described later. Read more…

Countertop Geology: Desperate for rocks? Visit a “granite” countertop store!

Where can you see some rocks? It’s winter and everything’s covered in snow –or you’re visiting family in some place where there’s virtually no bedrock exposed anywhere –or you’re simply stranded far from any good rocks in the center of a big city.IP18-0957c

Take yourself on a field trip to a granite countertop store! You might not see very much real granite, but you will see some other types: folded gneiss, pegmatite, amphibolite, quartzite, maybe even some granite… and a lot of amazing metamorphic and igneous features and faults –and they’re all polished and none are covered by vegetation.

I needed a rock fix the other day while visiting my mother in SW Florida –so I drove to a granite countertop store. And wow— I saw all sorts of great stuff, a lot of which related to faulting and fracturing, and a lot of it could go right into a geology textbook. In Florida!

IP18-0947e

Red garnet along with quartz and feldspar in gneiss -a metamorphic rock.

Read more…

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