True granite: an unusual granite erratic in Lake Stevens

The True Granite Erratic, 99th Avenue NE, Lake Stevens, Washington

By Dave Tucker  July 21, 2011

Thanks to Craig Valvick and Matt McCourt for reporting this nice rock.

The True Granite erratic draws a crowd. Photo by Bob Mooers. Click to enlarge.

An unusual granite erratic can be visited in Lake Stevens. I mean an honest-to-goodness true granite, with big (1 cm +) pink orthoclase and smoky gray quartz crystals. Sure, we have plenty of granitic erratics around the Salish Sea, but these are typically granodiorite, quartz diorite, or tonalite, the salt-and-pepper speckled mix of black and white minerals, not granite. Granite is a plutonic igneous rock with a specific ratio of three silica-rich minerals: quartz, and two feldspars, orthoclase and plagioclase. Granite is characterized by a predominance of quartz and the potassium-rich feldspar orthoclase, which is often pink. Granodiorite, on the other hand, is much richer in plagioclase feldspars (white or clear) and quartz; orthoclase does not exceed 20%. Both have dark mafic minerals, which are iron- or magnesium-rich minerals, usually hornblende or biotite mica. Granite tends to have biotite, while granodiorite and related rocks may have more hornblende and less biotite. It all has to do with the chemistry of the magma. If the magma that cooled within the crust to form these plutonic rocks had managed to erupt, the equivalent volcanic rocks would be: granite = rhyolite, and granodiorite = dacite. Rhyolite volcanism is very rare in the Cascades while dacite is much more common (e.g Mount Saint Helens 1980).

This is a “Q-A-P” triangle use to classify granitic rocks based on relative proportions of Q (quartz), P (plagioclase) and A (alkaline minerals rich in sodium or potassium).

Here is how a ternary diagram is read. A rock with composition '1' has 60% mineral A, 20% mineral B, and 20% mineral C.Test yourself: how about the rock with composition '2'? If A-B-C represented Q-A-P, what rock would this be in the IUGS diagram? Answer at the bottom of the page.

The 99th Ave NE erratic is rich in cm-sized pink orthoclase and smoky-gray quartz.

This is granodiorite of the Shasta Court erratic in Coquitlam BC (see elsewhere in this website). The dark rods are hornblende; the bronze minerals are biotite (look real close).

Step across the roadside ditch- the line of trees and power poles mark the edge of the street right of way, so you can walk up and touch the street-side of the boulder without trespassing. The True Granite Erratic is 10’ high x 20′ x 10′. Note a 1-cm-wide vertical dark vein on the edge nearest the street. This is a late stage intrusion into a fracture that had developed in the cooling, solid granitic magma.

The source for this glacial erratic is unknown to me. Can anyone propose a likely spot up in BC? The only pink orthoclase-bearing granite I know of in the North Cascades is the Golden Horn batholith at Washington Pass (Liberty Bell Mountain, etc.) and that location doesn’t seem at all reasonable as a source for this boulder.

Getting there:

The True Granite Erratic is on 99th Ave NE . It  is not far from the Lake Stevens Monster [read about it here], so they could be visited at the same time.

The yellow pins mark the Lake Stevens Monster (west) and True Granite erratic (east). Google Earth. Click to enlarge.

• I-5 to US 2, east.
• North on WA 204 East.
• 2.3 mi right at Market Placet [signal]; cross Highway 9 [signal]
• 0.8 mi left on 99^th Ave NE (aka ‘Swalwell Ave’) The erratic is in the 200 block of 99th Ave NE
• 0.1 erratic on the left.
• Latitude 47° 59.856’ N, Longitude  122° 5.905’ W

Answer: the rock with composition ‘2’ in the Q-A-P diagram has ~25% mineral A, 40% mineral B, and ~35% mineral C. It falls into the domain for ‘granite’ on the IUGS classification.